147 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, Institute of Food Technologists

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1 Nucleation and Expansion During Extrusion and Microwave Heating of Cereal Foods C.I. Moraru and J.L. Kokini Dept. of Food Science and Center for Advanced Food Technology, Rutgers Univ., New Brunswick, NJ Direct inquires to author Kokini ( ABSTRACT: Expansion of biopolymer matrices is the basis for the production of a wide variety of cereal foods. A limited number of manufacturing processes provide practical solutions for the development of an impressive variety of expanded products, just by changing process variables. It is therefore essential that the mechanisms involved in expansion are well known and controlled. This paper summarizes the knowledge of nucleation and expansion in extruded and microwaved products available to date. The effect of processing conditions and properties of the biopolymeric matrix on nucleation and expansion are discussed. Moisture content enables the glassy polymeric matrix to turn into rubbery state at process temperatures, which allows superheated steam bubbles to form at nuclei and then expand, expansion being governed by the biaxial extensional viscosity of the matrix. Nucleation and expansion theories are presented along with quantitative data that support them. Keywords: nucleation, expansion, extrusion, microwave, cereals Introduction As life continues to get busier in the contemporary cash-rich, time-poor society, snacking has increased tremendously in the recent years, making the American and global snack market one of the most dynamic in the food sector. According to the State of the Industry Report of the Snack Food Assn. (SFA), sales of savory snack foods in the U.S. increased by 6.2% between 1999 and 2000 and by 5.1% between 2000 and 2001, and the growth seems to continue. In terms of sales, this translates into $19.38 billion for the year 2000 and $21.8 billion for These figures include, among other snacks, potato and tortilla chips, popcorn, cheese snacks, and corn snacks (IFT 2002). Many snack products are expanded, as expansion results in textures that make them appetizing and crisp. A good understanding of the mechanisms involved in the expansion of food biopolymers is extremely important, since it can help improve the texture and palatability of existing snacks and create the platform for developing new snack foods. Foods are usually expanded by extrusion, hot-air puffing, deepfat frying, baking, and more recently by microwave heating. Extrusion expansion is relatively well addressed in literature, but most of the available studies are usually focusing on a particular food system and do not offer a full coverage of the phenomenon. The other expansion methods, particularly microwave expansion, are addressed to a lesser extent, which creates a need for studies and publications in this field. The objective of this paper is to provide a comprehensive review of the work currently available in the domain of extrusion and microwave expansion of cereal foods and to make a critical analysis of the factors that affect air nucleation and expansion during extrusion and microwave heating. The paper also offers a review of the most representative practical applications of the two expansion processes. Factors that Affect Extrudate Expansion Extrudate expansion is a complex phenomenon which occurs usually during high-temperature, low-moisture extrusion cooking. It is the consequence of several events such as biopolymer structural transformations and phase transitions, nucleation, extrudate swell, bubble growth, and bubble collapse, with bubble dynamics dominantly contributing to the expansion phenomenon (Chang 1992). The factors that have a significant influence on extrusion expansion are summarized in a diagram in Figure 1. Material Parameters Over the years, numerous studies have investigated extrusion expansion, particularly of cereal foods. Chinnaswamy (1993) reviewed most of the work on extrusion expansion of starches and cereal flours up to that date. In the last decade, many other researchers continued to study expansion and the factors that affect it, aiming to create the basis for new or improved extruded foods. 147 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, Institute of Food Technologists

2 Nucleation and expansion during extrusion... Their work focused on the extrusion of cereal flours, such as corn flour and grits (Desrumaux and others 1998; Singh and others 1998a,b; Zhang and Hoseney 1998; Ali and others 1996; Garber and others 1997; Fan and others 1996; Parsons and others 1996; Faller and others 1995; Sokhey and others 1994); rice (Martinez- Bustos and others 1996; Grenus and others 1993); wheat flour (Koh and others 1996; Sutheerawattananoda and others 1994); rice flour (Bhattacharya and others 1994); various starches (Bhatnagar and Hanna 1996; Chinnaswamy 1993); bean flour (Czarnecki and others 1993; Avin and others 1992); amaranth flour (Chavez-Jauregui and others 2000), or of various other formulations: Morini and Maga 1995 (chestnut flour); Arora and others 1993 (potato peel); Iwe 1998, 2000; Iwe and Ngoddy 1998 (soysweet potato); Mathew and others 1999a,b; Rhee and others 1999; Gogoi and others 1996b (starch-meat); Matthey and Hanna 1997 (protein concentrate-corn starch); Hanna and others 1997 (starch-xanthan gum); Bhattacharya 1997 (rice-green gram); Patil and others 1992a,b (soy-sorghum blends); Zasypkin and others 1994 (potato starch-soy protein); Kim and Maga 1993 (starchprotein); Bhattacharya and Prakash 1994 (rice-chick pea flour); Park and others 1993 (soy flour-starch-beef blends); Suknark and others 1999 (tapioca-fish and tapioca-peanut); and Ferdinand and others 1992 (starch-sucrose). Bhatnagar and Hanna (1995, 1996) and Fang and Hanna (2001) have also addressed nonfood applications of starch extrusion. Several authors have reported on the effect of minor ingredients on expansion (Onwulata and others 2001a,b; Zazueta-Morales and others 2001; Carvalho and Mitchell 2000; Singh and others 2000; Alavi and others 1999; Desrumaux and others 1999; Martinez-Bustos and others 1998; Onwulata and others 1998; Pan and others 1998; Singh and others 1998a,b; Hanna and others 1997; Parsons and others 1996 ; Bhattacharya and others 1996; Fan and others 1996; Li and Lee 1996; Koh and others 1996; Prinyawiwatkul and others 1995; Chinnaswamy 1993; Czarnecki and others 1993; Ryu and others 1993). Effects of Starch on Expansion Most studies recognize that starch as the dominant polymer in most cereal systems plays a major role in expansion, while other ingredients such as proteins, sugars, fats, and fiber act as diluents. Maximum expansion has been observed with pure starches (500% increase in product dia), followed by whole grains (400%), various pet foods with added starch (200 to 300%), and oilseeds (150 to 200%) (Horn and Bronikowski 1979). According to Conway (1971), the lower limit of starch content for good expansion is 60 to 70%. Starch is made of linear amylose and branched amylopectin, which impact expansion differently. Mercier and Feillet (1975) reported that a high amylopectin content leads to light, elastic, and homogeneous expanded textures, while a high amylose content leads to hard, less expanded extrudates. According to several authors, maximum expansion of starch is reached at 50% amylose content (Mercier and Feillet 1975; Launay and Lisch 1983; Chinnaswamy and Hanna 1990; Chinnaswamy 1993). Della Valle and others (1997) reported that at 20.5% moisture content and a temperature of 165 C, starches with various amylose contents exhibited similar volumetric expansion, whereas starches without amylose showed much lower expansion values. At higher moisture contents (24.5 to 27.5%) and 156 C, volumetric expansion was higher for high-amylose starches. An increase in expansion with amylose content increasing from 23 to 48% was reported for extrudates formulated from partially defatted peanut flour and different starches (Suknark and others 1997). Amylopectin-rich starches expand more than amylose-based starches because the linear amylose chains align themselves in the shear field and thus are difficult to pull apart during expansion. At the same time, amylopectin starches are not as hard as amylose starches at the same moisture content, which also favors expansion (Kokini and others 1992; Della Valle and others 1996). In a very interesting study on the viscous behavior of starches, Della Valle and others (1996) attributed the lower viscosity of amylopectin to the presence of short chain branches in its macromolecular structure. This reduces the ability of amylopectin molecules to form entanglements, while the linear structure of amylopectin molecules allows them to entangle and thus increase the viscosity. Chinnaswamy (1993) studied the effects of chemical modifications on the degree of starch gelatinization and expansion in the presence of sodium chloride, sodium bicarbonate, and urea. Sodium chloride increased the expansion ratio by 0.5 to 5.5 units, while urea decreased it by 1 to 6 units, depending on the type of starch. The work of Hanna and others (1997) showed that the addition of 1% polyvinyl alcohol had a significant effect on the properties of starch (70% amylose) xanthan gum extrudates, including expansion, due to an increase in the apparent viscosity of the melt. In the same study, irradiation dosages of 10 to 30 kgy increased the apparent viscosity and expansion of starch-xanthan gum mixtures, presumably due to increased crosslinking. Interestingly, in an earlier study on the effects of irradiation modifications of starch samples with 0 to 70% amylose contents performed by Chinnaswamy (1993), starches treated with gamma-irradiation dosages of 10, 20, and 30 kgy had lower expansion volumes than their native counterparts, although between the irradiated starches expansion tended to increase with dosage. The authors hypothesized that debranching of starch molecules occurred at high irradiation dosages, which reduced significantly the molecular sizes and thus the viscosity, and this affected expansion. These differences point out that the effect of irradiation on expansion depends both on the composition of starch and on the level of irradiation, because of the complex effects triggered by irradiation such as crosslinking, decomposition, or radical formation. Figure 1 Factors that influence nucleation and extrudate expansion (Chang 1992) Effects of Other Ingredients on Expansion While starch is the major component of the cereal flours used in manufacturing expanded extrudates, minor components such as proteins, fat, and fibers are also present. Most extruded foods are actually made out of complex formulations which include, besides starch, protein, fat, sugar, fibers, and so on. All these ingredients have different effects on extrudate expansion. Faubion and Hoseney (1982) reported that the effect of proteins on expansion depends on their type and concentration. Soy protein isolate increased the expansion ratio of wheat starch upon increasing its concentration from Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 148

3 CRFSFS: Comprehensive Reviews in Food Science and Food Safety 1 to 8%, while wheat gluten reduced it when used in concentrations of up to 11%. Mathew and others (1999b) found that the decrease in protein content in corn meal benefited extrudate expansion, due to the increase in starch content. Alavi and others (1999) found that the addition of egg-white proteins reduced shrinkage of high-moisture starch extrudates expanded with supercritical carbon dioxide, and reported subsequent increases in the expansion ratio by 140 to 341%. They attributed this effect to an increase in extrudate viscosity caused by protein crosslinking. According to Onwulata and others (1998, 2001a), a maximum of 25% whey protein concentrate (WPC) added to corn, rice, and potato flour improved expansion through changes in extrusion shear and moisture, while concentrations beyond 25% reduced expansion. Incorporation of chickpea into rice flour decreased product expansion (Bhattacharya and Prakash 1994). Proteins have an effect on expansion through their ability to affect water distribution in the matrix and through their macromolecular structure and conformation, which affects the extensional properties of the extruded melts. They also contribute to extensive networking through covalent and nonbonding interactions that take place in extrusion (Madeka and Kokini 1992; Li and Lee 1996). Oils and fats are frequently used in extruded products in order to improve their eating quality, by modifying both texture and taste. Addition of liquid fats is generally reported to have a negative effect on expansion of various extruded systems, such as wheat starch (Faubion and Hoseney 1982), amylopectin (Thewessen and others 2002), soy protein/corn gluten blends (Bhattacharya and Hanna 1988), soy flour-amylose corn starch-raw beef extrudates (Park and others 1993), rice and tapioca flour (Jomduang and Mohamed 1994). The addition of fat during extrusion decreases significantly the degree of starch gelatinization, due to a decrease of the barrel temperature caused by the lubricating effect of fat (Lin and others 1997). Fat also decreases starch conversion during extrusion by preventing severe mechanical breakdown of the starch granules by shear stress and preventing water from being absorbed by starch. Reduced starch conversion/gelatinization ultimately results in decreased expansion. However, not all fats have the same effect on expansion. When fatty acids were added in corn grits extrusion (0.2 to 0.8% wb), the radial expansion index decreased, the longitudinal expansion index increased, and the cell number increased, which were attributed to the formation of complexes between the fatty acids and amylose (Singh and others 1998b; Desrumaux and others 1999). Singh and others (1998b) reported that wheat germ oil caused the increase of starch expansion and water solubility index of the extrudates. Thewessen and others (2002), comparing the effect of an addition of up to 10% of fats with various melting points on the direct expansion of amylopectin, reported maximum expansion at 6% addition of solid fat with a melting point of about 70 C. At 8%, solid fat expansion decreased due to collapse of the excessively softened matrix. The addition of vegetable oil reduced the degree of expansion of amylopectin extrudates. The positive effect of solid fat on extrudate expansion as mentioned above is consistent with the improvement of gas retention and expansion of bread upon addition of solid fat reported by Gan and others (1995) and Brooker (1996). In their study, Gan and others (1995) suggested that about 5% of the fat must be solid at the proof temperature of bread dough in order for the fat to have a positive effect on expansion. Fan and others (1996) reported that sugars reduced the sectional expansion of maize extrudates, monosaccharides having a more pronounced effect than disaccharides. The reduced expansion was interpreted as a combined effect of a reduction in bubble growth and shrinkage. According to the authors, the reduction in bubble growth was a result of reduced driving force for bubble growth and reduced bubble wall extension before rupture, caused by less starch conversion at high sugar content. Shrinkage, on the other hand, was believed to be due to the reduction in the glass transition temperature of the starch melt upon sugar addition. This results in a softer matrix, which then collapses under the high vapor pressure during expansion, reducing the final degree of expansion. The effect of fibers on extrudate expansion seems to be concentration-dependent. Radial and axial expansion of rice flour extrudates increased at 10% rice bran and decreased at higher levels (20% and 30%) in the study of Grenus and others (1993). Hsieh and others (1988) reported that increasing the wheat or oat bran content in cornmeal up to 30% and, respectively, 20% increased the longitudinal expansion and bulk density, but decreased the radial expansion. Similar results were reported for extruded cornmeal with added beet fiber (Lue and Huff 1991). Onwulata and others (2001a) found that 12.5% fiber increased the expansion and breaking strength of expanded corn and also improved the physical characteristics of expanded corn with added milk proteins. It is likely that at small concentration the long and stiff fiber molecules align themselves in the extruder in the direction of flow, reinforcing the expanding matrix and increasing its mechanical resistance in longitudinal direction. The structural anisotropy becomes detrimental to the biaxial extensional properties of the extrudates, lowering their radial expansion. Above a critical concentration, the fiber molecules disrupt the continuous structure of the melt, impeding its elastic deformation during expansion. Fibers are also able to bind some of the moisture present in the matrix, thus reducing its availability for expansion. Minor ingredients are sometimes added in the extrusion process with the purpose of enhancing the nutritional, textural, or sensory quality of the extrudates. The addition of up to 0.2% calcium hydroxide was reported to correlate well with the expansion index of blue maize extrudates (Zazueta-Morales and others 2001). Martinez-Bustos and others (1998) found out the best values for radial expansion of corn meal extruded at an addition of 0.15% calcium hydroxide, although expansion in the presence of calcium hydroxide was lower than for the samples without calcium hydroxide. The formation of a crystalline starch-calcium complex was suggested to be responsible for this behavior. Cysteine was reported to result in a decrease in the expansion ratio of wheat flour extrudates by Li and Lee (1996) and Koh and others (1996). This was attributed to the disruption of disulfide bonds in gluten, which resulted in lower molecular weight of the protein component and inhibition of network formation via disulfide bonds. Formation of carbon dioxide during extrusion was reported to enhance extrudate expansion. If sodium bicarbonate is added during cereal extrusion, this can react with acidulants or acids produced by oxidation of starch during extrusion to form carbon dioxide (Hoseney and others 1992). Lai and others (1989) improved the expansion of wheat extrudates by adding sodium bicarbonate or carbonate to the feed blend, which, however, caused browning and weaker expanded structure. Parsons and others (1996) reported that, by increasing the level of sodium bicarbonate and sodium aluminum phosphate from 0% to 0.45%, the apparent bulk density and the expansion ratio of cornmeal decreased. Sodium bicarbonate increased extrusion expansion of maize grits at 125 C and decreased it at 175 C (Singh and others 2000). The degree of expansion and structural homogeneity of starch or cereal flour extrudates can be controlled using blowing agents. Ferdinand and others (1990) injected carbon dioxide (CO 2 ) into the extruder barrel and kept the die temperature below 100 C to prevent steam expansion, but the density of the extrudates obtained was higher than for water vapor-expanded foams. Mulvaney and Rizvi (1993), Sokhey and others (1996), and Alavi and others (1999) have used similar processes for the extrusion of food materials using supercritical CO 2 as a blowing agent. They 149 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, 2003

4 Nucleation and expansion during extrusion... found this method to produce a more regular structure as compared to steam-expanded products, at lower temperatures, lower shear rates, and higher moisture contents. Such extrusion conditions also resulted in less macromolecular fragmentation of starch. Physical properties of cereal flours, such as particle size, can also impact expansion. Mathew and others (1999c) reported that extrusion of small particle size cornmeal feeds resulted in corn curl and pet food extrudates with a significantly higher volumetric expansion index (VEI) as compared to feeds with coarse or medium sized particles. This is in agreement with the findings of Garber and others (1997), who showed that the larger the particle size in the range 50 to 1622 mm, the lower the degree of expansion of cornmeal extrudates. Zhang and Hoseney (1998) reported that large particle size combined with a large number of opaque particles of cornmeal resulted in poor expansion. The small voids in the opaque endosperm acted as capillaries and lowered the water activity. As a result, less water was available for hydration and the opaque particles were insufficiently plasticized by water and thus did not melt during extrusion, which led to poor expansion. Effects of Extrusion Parameters on Expansion Processing conditions and equipment-related variables also have the ability to influence the degree of expansion significantly, since they dictate the type and extent of physical and chemical modifications that take place during extrusion which, in turn, affect expansion. Following the studies reviewed by Guy and Horne (1988) and Chinnaswamy (1993), several recent studies continued to investigate the effects of process parameters on extrudate expansion: extrusion variables (Giri and Bandyopadhyay 2000; Liu and others 2000; Lee and others 1999; Pan and others 1998; Della Valle and others 1997; Ilo and others 1996; Bhattacharya and others 1994; Kim and Maga 1993; Arora and others 1993; Avin and others 1992; Patil and others 1992b), temperature and moisture content (Cha and others 2001; Mathew and others 1999a; Prinyawiwatkul and others 1995; Bhattacharya and others 1994), and extruder configuration (Choudhury and Gautam 1998, 1999; Bouzaza and others 1996; Gogoi and others 1996a,b; Bhattacharya and others 1994; Sokhey and others 1994). Chinnaswamy and Hanna (1988) observed that expansion ratio of corn starch extrudates increased sharply from 4.5 to 13 as the die nozzle length-to-diameter ratio L/D ratio increased from 2.5 to 3.4, and then decreased to 8.5 as the nozzle L/D ratio increased to In this study, die nozzle L/D ratio was shown to affect expansion more than screw speed and temperature. The authors also suggested that extrusion pressure is a better indicator than nozzle L/D ratio for predicting starch expansion ratio. According to Sokhey and others (1997), the effects of die nozzle L/D on expansion properties are dominated by die nozzle length or diameter separately, rather than by L/D. In their study, an increase in die nozzle dia decreased radial expansion, but had no effect on overall expansion, while an increase in die nozzle length increased overall expansion, but had no effect on radial expansion. The influence of L/D ratio on expansion is due to its effects on starch viscosity, since a smaller die size results in higher shear rates, which leads to lower shear viscosity and thus higher expansion. The effects of extruder L/D ratio on rice flour extrudates were studied by Bhattacharya and others (1994), who reported significant linear as well as quadratic effects of L/D ratio on expansion in the L/D range of 16 to 24. As extrusion shear and temperature increase, the viscosity of the extruded melt is reduced, facilitating high expansion at moderate shear (Lai and Kokini 1990). At very high shear rates and residence times, macromolecular starch degradation is induced, especially at low moisture content (less than 30% w/w), which leads to low expansion ratios (Pan and others 1998, Chinnaswamy 1993). When extruding starchy materials under low shear conditions, some starch granules remain intact and the extrudate shows little expansion (Guy and Horne 1988). Screw speed has generally a positive effect on extrudate expansion due to the increase in shear, and thus decrease in melt viscosity induced by high screw speeds (Kokini and others 1992). This was confirmed by the study of Ali and others (1996), who found that overall and radial expansion, as well as pore volume, increased with screw speed in the range 80 to 200 rpm, whereas axial expansion decreased. In the study of Bhattacharya (1997), screw speeds of 100 to 400 rpm imparted curvilinear effects on the characteristics of rice and green gram extrudates. For that particular formulation, high barrel temperatures combined with low screw speeds were suitable for obtaining expanded products. Other workers have reported, however, that screw speed had no significant effect on the expansion ratio: Liu and others (2000) for extruded oat-corn puff and Giri and Bandyopadhyay (2000) for fish muscle-rice flour extrudates. Such differences may be explained by significant differences in the extrusion conditions, such as type of extruders and screw configuration, temperature, and composition of the feed. The specific mechanical energy input (SME) is a good quantitative descriptor in extrusion processes, since it allows the direct comparison of different combinations of extrusion conditions such as screw speed, feeding rate, and torque. The amount of mechanical energy delivered to the extruded material determines the extent of macromolecular transformations and interactions that take place; that is, starch conversion and, consequently, the rheological properties of the melt. Increased SME leads to lower viscosity, which promotes mobility and thus may lead to an increase in the rate of bubble growth, the latter being observed by Mitchell and Areas (1992). Guy and Horne (1988) reported that an increase in SME input from 100 to 150 Wh/kg favored extrusion expansion of starchy materials, due to advanced loss of granular structure. Onwulata and others (2001b) increased extrusion SME by reducing the moisture content of the feed and adding reverse screw elements, which resulted in increased expansion and breaking strength in extruded corn, potato, or rice snacks. In the study of Della Valle and others (1997), an increase in SME from 164 to 184 Wh/kg did not influence volumetric expansion for 70% amylose starches, while an increase in SME from 147 to 265 Wh/kg determined an important drop in volumetric expansion at constant melt viscosity for 99% amylopectin starches. This was due to macromolecular degradation of the high mass molecular weight amylopectin induced by high SME. The extent of amylopectin molecular weight reduction during extrusion has been related to extrudate expansion ratio by Davidson and others (1984a,b) and Tang and Ding (1994). Extrusion temperature plays an important role in changing the rheological properties of the extruded melts, which in turn affect the expansion volume. Kokini and others (1992) found out that there is a temperature range where diametral expansion of starch reaches a maximum, and this optimum temperature depends on the type of starch (Figure 2). Past a critical temperature, which depends both on the type of starch and moisture content, expansion decreases with temperature, most likely due to excessive softening and potential structural degradation of the starch melt, which becomes unable to withstand the high vapor pressure and therefore collapses. Bhattacharya and others (1994) reported a significant quadratic effect of barrel temperature in the range 75 to 185 C on the expansion of rice flour extrudates. An increase in expansion of cereal flours with extrusion temperature was also reported by Chinnaswamy and Hanna (1988), Colonna and others (1989), Bhattacharya and Prakash (1994), Ali and others (1996), and Cha and Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 150

5 CRFSFS: Comprehensive Reviews in Food Science and Food Safety others (2000), Onwulata and others (2001b). Kokini and others (1992) and then Della Valle and others (1997) explained the sharp decrease in volumetric expansion with increasing moisture content by the shrinkage and collapse of the extrudate after maximum expansion, caused by the excessively low viscosity of the melt. Low melt viscosity also leads to low motor torque and SME input, resulting in low product temperature which, in turn, reduces expansion (Moore 1994). Kokini and others (1992) (Figure 3) and Della Valle and others (1997) reported a negative influence of melt viscosity on the volumetric expansion (VEI), confirming the earlier findings of Vergnes and Villemaire (1987). Ilo and others (1996) reported that apparent viscosity affected the radial expansion ratio, but did not affect the extrudate bulk density. Low viscosity of the extruded cereal melt is detrimental to expansion since it allows the cellular matrix to collapse under the high vapor pressure. Figure 2 Diametral expansion of Amioca (98% amylopectin) and Hylon 7 (70% amylose) as a function of melt temperature (Kokini and others 1992) others (2001). Avin and others (1992) found similar effects of temperature for the expansion of red bean extrudates, and Giri and Bandyopadhyay (2000) for extruded fish-rice flour mixtures. Other studies reported a temperature plateau for expansion (Harper and Tribelhorn 1992), while Della Valle and others (1997), performing measurements at constant viscosity, observed the highest volumetric expansion at the lowest temperature used in their study (in the range 159 to 187 C). They attributed the apparent contradiction between their findings and other studies to the fact that the increase in expansion with temperature reported by previous authors is an indirect effect of the decrease in melt viscosity. This study also presented an interesting analysis of the relationships between volumetric (VEI), sectional (SEI), and longitudinal expansion (LEI), explaining them from the perspective of the effect of temperature on structural anisotropy. At high temperatures, the pressure of the saturated vapors exceeds the melt pressure towards the die exit, favoring bubble growth inside the die in the direction of flow and thus longitudinal expansion. The authors concluded that LEI should be positively correlated with melt viscosity and negatively correlated with volumetric and sectional expansion. At low temperatures, bubble growth starts at the die outlet since the pressure of saturated vapors is lower than the melt pressure in the die, and LEI correlates positively with the other expansion indices. Yet, the authors did not provide a clear phenomenological explanation for the negative effect of temperature on VEI. Moisture content during extrusion provides the driving force for expansion and also contributes to the rheological properties of the melt, which in turn affect expansion. Moisture is the main plasticizer of the cereal flours, which enables them to undergo a glass transition during the extrusion process and thus facilitates the deformation of the matrix and its expansion. According to Ilo and others (1996), an increase in moisture content during extrusion decreases the SME, apparent viscosity, and radial expansion ratio during extrusion of maize grits. Parsons and others (1996) reported a decrease in the expansion ratio of cornmeal when the extrusion moisture content was increased from 19.5 to 21.5% (w/ w). The reduction of expansion at high moisture content was later confirmed by the findings of Garber and others (1997), Liu and Mechanism of Extrudate Expansion Many researchers attempted to model extrudate expansion mostly from the perspective of the influence of material and operational variables (Alvarez-Martinez 1988; Bhattachaya and Hanna 1986; Richburg and Whittaker 1988; Srivastava and others 1988). While most of the individual effects of these variables on expansion are, in general, consistent throughout the literature, discrepancies are frequently found due to the complex interactions between material and operational variables. It is therefore crucial to have a basic phenomenological understanding of the complex mechanism that governs expansion of cereal matrices, which incorporates both material properties and processing parameters. While significant work has been done on developing quantitative mechanisms for the extrusion expansion of synthetic polymers (that is, formation of polymer foams), less work is available on the expansion of food biopolymers. This is mainly due to the complexity of such systems, which undergo continuous transformations during extrusion. A few studies have taken the challenge of modeling extrusion expansion of foods, most of them using simplifying assumptions in developing their models. One of the first models was proposed by Alvarez-Martinez and others (1988), who developed an extrudate expansion model based on the Figure 3 Effect of viscosity on expansion of Amioca (98% amylopectin) (Kokini and others 1992) 151 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, 2003

6 Nucleation and expansion during extrusion... dough viscosity model of Harper. Kokini and others (1992), Fan and others (1994), Schwartzberg and others (1995), Della Valle and others (1997), and Huang and Kokini (1993, 1999) proposed more fundamental models, focusing on various aspects of expansion. Kokini and others (1992) approached the various phases involved in extrudate expansion and illustrated the expansion mechanism on the diagram shown in Figure 4. The proposed expansion mechanism includes the following 5 major steps: orderdisorder transformations, nucleation, extrudate swell, bubble growth, and bubble collapse. First, the high shear, pressure, and temperature inside the extruder allow the transformation of cereal flours into viscoelastic melts. The degree of transformation is highly dependent on the extrusion moisture content and extruder operating variables. Nucleation of bubbles within the polymer melt occurs during extrusion, both at sites where small air bubbles or impurities were entrapped during the extrusion process or in the holes that represent the free volume of the polymer. The latter assumption could not be yet verified experimentally. These bubbles grow as the melt leaves the extruder die due to a moisture flash-off process, when the high pressure of the superheated steam generated by moisture vaporization at nuclei overcomes the mechanical resistance of the viscoelastic melt. The bubble growth ceases upon cooling, when the viscoelastic matrix becomes glassy and no longer allows expansion to take place. These steps are discussed in detail below: 1. Order-disorder transformations during starch extrusion This step occurs inside the extruder and consists of transforming the semicrystalline starchy raw materials into a cohesive viscoelastic mass. As the predominant component of cereal flours, starch plays a major role in extrusion in general and extrudate expansion in particular. Proteins and other minor ingredients (that is, fats or sugars) have their own contribution to expansion, which must not be overlooked. Molecular organization of starch. A clear understanding of the transformations that take place during extrusion and expansion of starchy materials requires a good understanding of the macromolecular organization and properties of starch. This food biopolymer consists of two major molecular components, amylose and amylopectin, which have molecular weights in the range of 10 5 to 10 6 and 10 7 to 10 8 Da, respectively (Young 1984). Amylose is a long and linear polymer, while amylopectin has a highly branched molecular structure. Amylopectin is considered responsible for the structural organization of the starch granule (French 1984) and its semicrystalline character (Zobel 1984; Gallant and others 1997). Of special relevance to the expansion process are the microscopic pores of about 0.5 m in dia, called hilums, localized near the center of the starch granules. The hilums are considered by various researchers to be some of the nuclei at which expansion of vapor bubbles starts (Hoseney 1986; Hoseney and others 1992; Schwartzberg 1995; Cisneros 1999). The starch granule is made up of alternating semicrystalline and crystalline shells, 120 to 400 nm thick, which consist of alternating 9 to 10 nm thick amorphous and crystalline lamellae (Yamaguchi and others 1979; French 1984, Oostergetel and van Bruggen 1989; Jenkins and others 1993, Gallant and others 1997; Parker and Ring 2001). The lamellae are organized into pseudo spherical structures, called blocklets, of 20 to 500 nm dia. Gallant and others (1997), using Atomic Force Microscopy, revealed such structures as protrusions of 10 to 50 nm at the surface of native wheat starch granules and larger protrusions (200 to 500 nm) at the surface of native potato starch granules. Fannon and others (1992a,b; 1993) reported the presence of channels within the starch granules, which was later confirmed by the studies of Baldwin and others (1994) and Gallant and others (1997). The surface Figure 4 Schematic diagram of extrudate expansion (Kokini and others 1992) pores and interior channels are believed to be naturally occurring features of the starch granule structure, the pores being the external openings of the interior channels (Fannon and others 1993; Gallant and others 1997). The information concerning the internal organization of the starch granule is summarized in the diagram shown in Figure 5 (Gallant and others 1997). It is very likely that such pores could also serve as nucleation sites for expansion, although at this moment there is no experimental data to prove this hypothesis. Starch transformations during extrusion. The severe conditions encountered during food extrusion cause various degrees of granular and molecular changes in starch (Colonna and others 1984; Colonna and Mercier 1983; Davidson and others 1984a; Launay and Lisch 1984; Diosady and others 1985; Lin and others 1997; Cisneros 1999; Cisneros and Kokini 2002a,b). When starch is subjected to heating in the presence of water, in a temperature range characteristic of the starch source, its native structure is disrupted and gelatinization takes place (Wang and others 1991; Mitchell and Areas 1992; Parker and Ring 2001; Hoover 2001). Starch gelatinization is a sequential process, which includes the diffusion of water into the starch granule, followed by water uptake by the amorphous region, hydration and radial swelling of the granules, loss of optical birefringence, uptake of heat, loss of crystalline order, uncoiling and dissociation of double helices in the crystalline regions, and amylose leaching (Hoover 2001). Emphasizing the importance of the loss of crystal- Figure 5 Organization of the starch granule structure presenting alternating crystalline (hard) and semicrystalline (soft) shells (Gallant and others 1997) Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 152

7 CRFSFS: Comprehensive Reviews in Food Science and Food Safety line order in starch during the gelatinization process, Parker and Ring (2001) have presented the differences in gelatinization and melting behavior of different polymorphic forms of starch. Wang and others (1991), and then Parker and Ring (2001), demonstrated that the melting temperature of the amylopectin component of starch decreases with increasing water content until a critical moisture is reached; a further increase in water content does not lower further the gelatinization temperature, which instead reaches a plateau (Figure 6). When there is not sufficient water to complete gelatinization, the remaining crystallites simply melt upon heating (Wang and others 1991; Hoover 2001). Donovan (1979) and Wang and others (1991) reported the coexistence of both gelatinized and melted starch in cooked starch at extrusion moisture contents. Figure 7 shows the phase diagram developed by Wang and others (1991), which illustrates the distribution of gelatinized starch and melted starch as a function of moisture content. Kokini and others (1992) studied the effect of extrusion temperature and moisture content on the conversion of amylose-rich starch and amylopectin-rich starch (Figure 8), and found significant differences between the two starches. For instance, at 30% moisture and 200 rpm, 98% amylopectin starch had significantly higher degree of gelatinization as compared to 70% amylose starch, particularly at the lower temperatures used in the study (120 C and 135 C). Starch conversion during extrusion is also affected by the presence of other components, such as fats and proteins. In the study of Lin and others (1997), fat was shown to interfere significantly with starch gelatinization: the degree of starch gelatinization has decreased from 100% for a dry pet food control without fat to 68.4% in the presence of poultry fat and 55% in the presence of beef tallow, at a fat addition of 75 g fat /kg extrusion mix. These values were reported at a screw speed of 200 rpm. At higher screw speeds, the decrease in gelatinization was even more pronounced (Figure 9). One explanation is the lubricating effect of fat during extrusion, which reduces the frictional torque and, conse- Figure 6 DSC peak temperature for Amioca (98% amylopectin) as a function of moisture content (Wang and others 1991) Figure 7 Phase diagram for starch gelatinization and melting (Wang and others 1991) Figure 8 Degree of conversion for (a) Hylon 7 (70% amylose) and (b) Amioca (98% amylopectin) at different extrusion temperatures and moisture contents, at 200 rpm (Kokini and others 1992) 153 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, 2003

8 Nucleation and expansion during extrusion... quently, the mechanical energy input, and lowers the melt temperature due to less viscous dissipation. As a result, less starch conversion takes place. According to Lin and others (1997), it is also possible that fat is coating the starch granules with a hydrophobic layer, preventing moisture from being absorbed and thus interfering with the gelatinization process. Formation of starch-fat complexes (Ho and Izzo 1992) and starch-protein complexes (Madeka and Kokini 1992; Mitchell and Areas 1992; Li and Lee 1996) may be significant during extrusion of cereal flours with complex composition. The presence of such complexes affects the rheological properties of cereal melts, and thus their expansion behavior. Specific mechanical energy (SME) input plays an important role in starch conversion, since it catalyzes the gelatinization process by rupturing intermolecular hydrogen bonds (Wang and others 1992; Gropper and others 2002). Gropper and others (2002) studied the effect of SME on starch conversion in starch-meat extrudates using Scanning Electron Microscopy and observed that starch granules were still present in the extrudates obtained at SME = 142 kj/kg, but were no longer observable at SME = 299 kj/ kg. These results are in agreement with those reported earlier by Gomez and Aguilera (1983, 1984) and Van Lengerich (1990). SME is also considered responsible for fragmentation of starch molecules during low moisture extrusion (Gomez and Aguilera 1983, 1984; Davidson 1984a; Meuser and van Lengerich 1984a,b; Klingler and others 1986; Guy and Horne 1988; Van Lengerich 1990; Wen and others 1990; Lai and Kokini 1991; Politz and others 1994; Della Valle and others 1996). Guy and Horne (1988) reported significant damage of the starch granules at SME > 500 to 600 kj/kg. Experimental data suggests that amylopectin is more prone to macromolecular degradation than amylose, which is mostly due to its high molecular weight rather than to its branched structure (Della Valle and others 1996). As a result of the applied shear, amylopectin molecules are broken mainly at the 1:6 bonds (Gomez and Aguilera 1983, 1984; Davidson 1984a), resulting in degradation products with average molecular weight (M w ) in the range 10 5 to 10 7 (Van Lengerich 1990; Politz and others 1994). Generally, it was observed that the higher the water content in the range 20 to 30%, the less degradation occurs. Kaletunc and Breslauer (1993, 1996) and Barrett and Kaletunc (1998) used the model of starch fragmentation to explain the decrease in the cereal extrudates glass transition temperature with the increase in SME observed in their study, a hypothesis that was recently debated by Gropper and others (2002). The latter studied the extrusion of protein-starch mixtures in a relatively wide range of SME input (344 to 2108 kj/kg) and found that SME did not affect the Tg of the extrudates. This conclusion was supported with a study on dextrans of known molecular weights, which showed an increase in Tg with the weight-average molecular weight (M w ) at low M w, followed by a plateau at M w (Figure 10). This, corroborated with the reports that fragments of M w > to 10 5 are obtained during extrusion of starch (Fennema 1996; Van Lengerich 1990; and Politz and others 1994), allowed the authors to conclude that SME may not have a visible effect on extrudates Tg despite the observed macromolecular fragmentation of starch. Compositional differences between the formulations and extrusion conditions used in different studies were also considered a possible reason for the apparently contradictory data. Rheological properties of extruded starchy melts. During extrusion, the moist cereal flours are transformed into viscoelastic doughs, whose rheological properties depend on the extrusion variables and chemical composition. Even though cereal doughs are viscoelastic, their movement inside the extruder barrel is dominated by shear and thus the viscous character is predominant (Dhanasekharan and Kokini 2003). Therefore, the complicated flow near the die and the extrudate-swell phenomenon, a manifestation of the elastic character of the melt, are typically neglected before the die exit and the polymer melt is treated as a fluid, with the viscosity following an Arrhenius-type temperature dependence (Della Valle and others 1996; Housiadas and Tsamopoulos 2000). However, it has been acknowledged that when extruded melts are in the proximity of Tg, their thermal behavior is better described by the Williams-Landel-Ferry (WLF) model rather than the Arrhenius model, the latter being valid at temperatures at least 100 C above Tg (Fan and others 1994; Della Valle and others 1996; Brent and others 1997b). Other assumptions usually made in extrusion modeling are constant temperature throughout the process and isotropy, which are both oversimplifications of the reality. Both temperature gradi- Figure 9 Effects of fat content and extruder screw speed on the degree of starch gelatinization during extrusion of a complex cornmeal-soybean formulation (Lin and others 1997) Figure 10 The effect of dextran s M w on its measured Tg and the Tg of maltose (measured by DSC) (Gropper and others 2002) Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 154

9 CRFSFS: Comprehensive Reviews in Food Science and Food Safety ents and anisotropy occur during extrusion of polymer melts, the first due to imposed temperature profiles, material inhomogeneities, and uneven distribution of heat transfer coefficients, and the latter because the long polymer chains are stretched by the flow gradients, which determines their backbones to align in the direction of flow and acquire a nonisotropic orientational distribution. Using such simplifying assumptions, various authors developed viscosity models to characterize the rheological properties of extruded cereal melts, which in many cases agreed well with the experimental data for certain ranges of temperature and moisture content. In general, the only non-newtonian effect considered in such models is the shear thinning behavior of the melts. Viscosity of extruded starch melts was reported to depend on shear rate, temperature, moisture content, degree of starch conversion, and macromolecular fragmentation (Colonna and others 1984; Vergnes and Villemaire 1987; Lai and Kokini 1990; Mackey and Ofoli 1990; Kokini and others 1992; Akdogan and others 1997, Della Valle and others 1996). Lai and Kokini (1990), improving the viscosity model of Harper, included these factors of influence in the model used to predict the viscosity of starchy cereal melts during extrusion: Figure 11 Validation of predicted viscosity of Amioca (Kokini and others 1992) (1) where:. is the shear rate, T is the temperature, C is the moisture content, DG is the degree of starch gelatinization, m and n are the power law constants, H 0 is the activation energy, k 10 is a moisture dependence parameter, and is the conversion effect index. The accuracy of this model when compared to experimental results for Amioca (98% amylopectin) was very good, as demonstrated by Figure 11. Following the same idea, Akdogan and others (1997) developed a semiempirical-modified Harper model to predict the apparent viscosity of high-moisture rice starch melts during extrusion. Their model incorporated the shear rate, temperature, moisture content, activation energy, and screw speed, and correlated well with experimental data. Mackey and Ofoli (1990) and Della Valle and others (1996) have also proposed viscosity models that took into account both the effect of extrusion variables and material parameters. A particular merit of the power-law model of Della Valle and others (1996) is that it captured the different viscosity behavior of amylose-rich starches and amylopectin-rich starches in the numerical coefficients. In the proposed model, the coefficients that reflect the effects of macromolecular degradation and, respectively, moisture content are negligible for starches with more than 70% amylose, and maximum for starches without amylose, which is in agreement with experimental observations. Since viscoelasticity can be a significant non-newtonian effect in polymeric melts, several recent attempts have been made to incorporate it in the rheological modeling of extrusion (Brasseur and others 1998; Dhanasekharan and Kokini 2000). Viscoelasticity is relevant in processes where the relaxation time of the polymer molecules is of the same order of magnitude to the characteristic time of the process, which is, at values of the Deborah number (De) close to 1, where De is the ratio of the characteristic fluid time to the process time (Bird and others 1987). Accurate estimates of De cannot be obtained without having data on relaxation times of the starchy melts under the temperature, shear, and pressure conditions encountered during extrusion. Such data is difficult to obtain with existing rheometers, particularly due to the significant moisture losses that would take place during the measurements. Brasseur and others (1998) studied time-dependent asymmetric incompressible Poiseuille and extrudate-swell flows of an Oldroyd-B fluid and demonstrated that viscoelasticity combined with nonlinear slip acts as a storage of elastic energy generating oscillations of the pressure drop similar to those observed experimentally in extrusion instabilities. Dhanasekharan and Kokini (2000) used the Phan-Thien and Tanner viscoelastic model to simulate the rheological properties of wheat flour dough during extrusion (Dhanasekharan and others 1999). Their work demonstrated that, although the pressure buildup was lower when viscoelasticity was taken into account as compared to a Newtonian case, the velocity profiles inside the extruder were very similar for the two different rheologies. The authors concluded that it is the leakage flow that is mainly responsible for the viscoelastic effects in extrusion, due to the localized high shear rates. Such flows are minimal and can be ignored, which supports the idea that the use of simple viscous models to characterize the rheology inside the extruder may be acceptable. However, one has to take into account that these results were obtained by using a De value of 0.001, which means that the polymeric liquid taken into consideration was very close to a Newtonian fluid, while most of the extruded polymeric melts are not. Another reason for which these conclusions cannot be generalized for the extrusion of biopolymers is the fact that the viscoelastic properties of such materials cannot be accurately predicted using existing rheological models, and in most cases have to be determined experimentally. For instance, Brent and others (1997b) reported that various cereal melts, such as yellow degermed corn meal, pregelatinized corn starch, pregelatinized waxy maize starch, and oat flour, have different values of the storage modulus in the rubbery state, due to structural differences caused by temperature and moisture content. The effects of viscoelasticity on extrusion warrant further investigation, and therefore, experiments need to be performed for a wider variety of materials and extrusion conditions. It is expected that, as experimental and computational capabilities evolve, clarification of such intricate issues will become possible. 2. Nucleation The gas cell structure of expanded extrudates seems to be directly related to the number of bubbles nucleated in the starchy melt. Understanding the formation of extrudates cellular matrix requires a good understanding of the nucleation phenomenon, which is responsible for the formation of individual bubbles that give rise to this structure. Shafi and others (1996, 1997) concluded that bubble 155 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, 2003

10 Nucleation and expansion during extrusion... nucleation and growth affect the cell size distribution in polymer foams, with nucleation having the strongest effect. Despite the importance of this step in extrudate expansion, a relatively small number of studies on nucleation phenomena during extrusion of food biopolymers are available. One obvious reason is the complexity of such systems and the continuous transformations they undergo during extrusion. This is why any nucleation model has to use simplifying assumptions in developing quantitative predictions of nucleation rates in extrusion of biopolymeric melts. Kokini and others (1992) used the bubble expansion concept of Amon and Denson (1986), originally developed for soap bubbles, to understand the factors that control nucleation and expansion. At saturation, the pressures in the liquid and vapor in the immediate vicinity of a vapor bubble interface are equal and at equilibrium (Figure 12). The force equilibrium established Figure 12 Spherical gas bubble in a liquid medium at the interface of the spherical vapor/gas bubble in a liquid medium can be expressed by the Laplace equation: Figure 13 Barrel fill length versus extrudate porosity showing air bubble entrapment region (Cisneros and Kokini 2002a), P v >P L (2) where P v is the pressure of the vapor phase in the interior of the bubble, P L is the pressure of the surrounding liquid phase, is the interfacial or surface tension of the liquid-vapor interface, and R is the radius of the sphere. Nucleation of bubbles consists of the formation of small, thermodynamically unstable, gaseous embryos within the liquid metastable phase. Once an embryo reaches a critical size, it grows spontaneously into a stable and permanent bubble, called nucleus (Cisneros 1999). As superheating increases, (P V P L ) becomes larger and thus R, the nucleus radius, becomes smaller, approaching molecular dimensions at very high degrees of superheat (Han and Han 1990a,b). Embryos can form as a result of either homogeneous or heterogeneous nucleation. Homogeneous nucleation is caused by localized thermal and density fluctuations of the metastable liquid phase, which lead to the formation of molecule clusters with vapor-like energies. These clusters further develop into critical vapor embryos, or nuclei. Heterogeneous nucleation takes place when the liquid phase comes in contact with another phase or foreign bodies, such as entrapped noncondensable gas bubbles, suspended nonwetted solids, or container surfaces. Typically, the number of heterogeneous nucleation sites is less than that of homogeneous nucleation sites (Colton and Suh 1987). Guy and Horne (1988) noted that increases in temperature of an extruded melt did not increase the number of gas cells found in expanded extrudates, which would happen if homogeneous nucleation were the dominating mechanism. While the model of Kokini and others (1992) does not take into consideration any viscoelastic effects and assumes a constant pressure difference during the process, it makes a valuable contribution to the phenomenological understanding of bubble nucleation and expansion. Nucleation sites in expansion of cereal matrices may be constituted by trapped air, particles present on the surface of the raw materials, or even the free volume that exists in the polymeric matrix. A few attempts have been made to determine the size and origin of nucleation sites in order to validate existing nucleation theories, but they encountered experimental difficulties. Kumagai and others (1991, 1993) estimated the critical bubble radius for expansion for extruded wheat and rice flour doughs with limited success. Hoseney and others (1992) examined nucleation in third-generation wheat starch extrudate rods that had a few visible bubbles. Their study indicated that the starch hylum appeared to be the main nucleation site, while air bubbles in the extrudates were the nucleation sites for the large cells. According to Hoseney and others (1992) and Cisneros and Kokini (2002a,b), the presence of entrapped air bubbles in the melt may also favor extrudate expansion, since they can act as nuclei for water vapor bubble formation. Based on their observation that porosity of unexpanded amylopectin extrudates changes with the extruder barrel fill length, according to a master curve (Figure 13), Cisneros and Kokini (2002a) formulated a theory to explain the relationship between extrusion processing parameters and air bubble entrapment. According to this theory, when a granular starchy material is conveyed through the extruder and reaches a certain barrel section, it undergoes melting, and some of the air-filled pores in the granular feed are converted to air bubbles in the melt. The entrapment of air is related to the existence of a mass-filled section in the melting region, which forms a mass barrier between the feeder port and the melting region, and blocks the escape of the air back to the feed port (Figure 14). The unexpanded extrudate that results under such conditions will have high porosity and high bubble number density. When the barrel section between the feeder port and the melting region is starved, the air will be able to escape and the unexpanded extrudates will have low porosity and bubble number density. Cisneros and Kokini (2002b) have also studied the effect of extrusion processing parameters on air bubble formation in unexpanded high amylopectin cornstarch, making sure that the bubbles they analyzed originated solely from air bubble entrapment and not from water vapor vaporization. The porosity of unexpanded amylopectin extrudates decreased with screw speed in Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 156

11 CRFSFS: Comprehensive Reviews in Food Science and Food Safety Figure 14 Proposed air entrapment mechanism using the barrel fill length mechanism (Cisneros and Kokini 2002a) the range 150 to 500 rpm for all extrusion conditions studied (Figure 15), and the bubble number density followed a similar trend. The effect of screw speed can be explained through the effect of bubble breakup and coalescence, phenomena controlled by the Capillary number (Ca), which is the ratio of viscous to interfacial forces (Favelukis and others 1995). The extruded starch melts have very low surface tension (Krycer and Pope 1983; Odidi and others 1991; Lawton 1995) and high viscosities (Lai and Kokini 1990), which result in very high Ca (Cisneros and Kokini 2002b). As a result, bubble breakup occurs and the average bubble size decreases until a steady state bubble size is achieved at a critical value of the Capillary number (Ca crit ). This critical value is a function of the flow type and the ratio p between the droplet viscosity and the continuous phase viscosity (Grace 1971). For shear flow, the dominant type of flow in an extruder, Ca crit can, according to Khakhar and Ottino (1986), be calculated as: Figure 15 Effect of screw speed on porosity of unexpanded extrudates (Cisneros and Kokini 2002b) (3) At a given Ca crit, as the shear rate increases (that is by increasing the screw speed), the steady state bubble size decreases, leading to a large number of small bubbles (Favelukis and others 1995). Experiments performed by Cisneros and Kokini (2002b) in a narrow critical screw speed range showed a dramatic change in bubble number density as a result of achieving the critical barrel fill length. In the same study, increasing the mass flow rate from 1.67 to 3.33 g/s resulted in increased air bubble entrapment. Average bubble radius values decreased with mass flow rate, due to higher melt viscosity. For those extrusion conditions that favored high porosities, unexpanded extrudates from pregelatinized amylopectin showed higher porosities as compared to extrudates from native amylopectin, while for those conditions that gave low porosities, the opposite was generally true. The use of pregelatinized amylopectin increased significantly the bubble number density as compared to native amylopectin, due to the higher porosity of the feed. No clear effect of moisture content (in the range 32 to 35%) on porosity and barrel fill length was found (Cisneros and Kokini 2002b). This work gives valuable insight in the complex issue of nucleation during extrusion of starch melts. Yet there are still many questions to be answered, such as, is there a contribution of the free volume of polymers to nucleation and, if so, how significant is this contribution? This is particularly difficult to prove experimentally, because it is extremely difficult to totally avoid the inclusion of air in the extruded melt. Despite the practical challenges, it is expected that, due to the importance of such issues to the fundamental understanding of nucleation and expansion and to the practical need to control extrudate expansion, future nucleation theories will be developed that take into account the complexities of the system, such as chemical transformations, nonuniform structure, temperature distribution, and shear. 3. Extrudate swell As the starchy polymer melt emerges from the die and starts cooling down, the contribution of elastic forces to the rheology of the melt increases significantly, which gives rise to the phenomenon of die swelling (Padmanabhan and Bhattacharya 1989; Chang 1992; Housiadas and Tsamopoulos 2000). Qualitatively speaking, elasticity affects extrudate diametral expansion. Hayter and others (1986) have shown that an increase in the die opening size results in decreased expansion and increased bulk density, which they attributed both to a reduction in the normal forces at the die (that is, reduced swelling) and to a higher viscosity caused by lower shear rate and temperature. Guy and Horne (1988) correlated the extrudate length/dia ratio and the bubble number density, and suggested that die swell controls the overall expansion of the extrudate, especially at small number density of gas bubbles (approx. 600 g/l); as the number density of bubbles increases, the effect of die swell decreases. Park (1976), cited by Della Valle and others (1997), suggested that as the number of bubbles increases, longitudinal expansion is favored. Della Valle and others (1997) suggested that elastic properties may actually affect expansion through the elongational viscosity, which is the variable included in the bubble growth models. While most of the existing growth models assume low elongational strain, Della Valle and others (1997) suggested that, since expansion is a fast process, it is likely that the elongational strain rate is actually very high. Die swell is relatively well characterized for synthetic polymers, but data is scarce for biopolymers. One method that can be used to measure elastic properties (that is, first normal stress difference) is slit die rheometry (Dealy and Wissbrun 1989; Kokini and others 1992). The validity of first normal stress difference measurements performed using exit pressure has been debated, the main issue being considered the accuracy of the exit pressure measurement 157 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY Vol. 2, 2003

12 Nucleation and expansion during extrusion... (Chan and others 1990; Chang 1992). In the case of starchy matrices, additional challenges such as water vaporization, nonfully developed flow at the die outlet, and starch modifications during the measurement may cloud the measurement of elastic properties slit die rheometry (Chang 1992; Della Valle and others 1997). Chang (1992) used the exit pressure method for the estimation of the first normal stress difference during starch extrusion. Since the data obtained at material temperatures > 140 C was erratic, only values obtained at temperatures below 120 C were used for developing first normal stress difference models. For this, the generalized expression developed by Lai and Kokini (1990) was used: where: N 1 = first normal stress difference;. = shear rate; T = temperature ( C); C = moisture content; k 1, k 2, k 3 = constants. Good correlations between the values predicted using this model and experimental data were obtained for extruded Amioca (98% amylopectin), Hylon 7 (70% amylose) and cornmeal (Figure 16). In the same study, Chang (1992) observed that the first normal stress difference increased with shear rate and decreased with material temperature and material moisture. The recoverable shear strain of Amioca was a positive function of temperature, while for cornmeal the recoverable shear strain was a negative function of temperature (Chang 1992). The fact that elastic properties can vary significantly from one material to another was also confirmed by the findings of Brent and others (1997a), who reported different behaviors of cornmeal and pregelatinized waxy maize, the latter showing no elastic recovery in the pressing operation after extrusion. The contribution of die swell on extrudate diametral expansion was calculated by Chang (1992), using the expression of Tanner (1970): (4) (5) Table 1 Predicted extrudate swell values for Amioca Moisture Screw content speed Extrusion temperature (%) (rpm) 110 C 120 C 140 C 160 C 170 C 20% 50 14% 19% 19% 46% 54% % 18% 18% 44% 52% % 18% 18% 44% 52% 25% 50 19% 23% 33% 42% 46% % 22% 31% 40% 44% % 22% 31% 40% 43% 30% 50 25% 29% 35% 39% 39% % 28% 33% 36% 37% % 28% 33% 36% 36% 35% 50 35% 37% 38% 35% 33% % 36% 36% 33% 31% % 35% 36% 33% 31% Bolded values: Chang 1992 Table 2 Experimental extrudate swell values for Amioca Moisture Screw content speed Extrusion temperature (%) (rpm) 110 C 120 C 140 C 160 C 170 C 20% 50 26% 28% 23% 140% 196% % 21% 20% 134% 190% % 15% 22% 86% 131% 25% 50 31% 26% 755% 530% 164% % 22% 379% 835% 146% % 25% 59% 251% 113% 30% 50 77% 168% 429% 1370% 424% % 400% 226% 254% 120% % 153% 203% 227% 253% 35% % 187% 709% 215% 971% % 992% 806% 711% 98% % 367% 18000% 253% 110% Bolded values: Chang 1992 where De is the extrudate dia, D 0 is the dia of the die opening, ( ) is the first normal stress difference, and 21 is the shear stress. Predicted and experimental values of extrudate swell for Amioca (98% amylopectin) are shown in Table 1 and 2 (Chang 1992). One should note that satisfactory correlations were obtained only at moisture contents < 30%, at temperatures up to 120 C (for m.c. = 25%) and, respectively, 140 C (for m.c. = 20%) (the numbers shown in bold font in the tables). 4. Bubble growth Extrudate expansion is governed by the biaxial extension of individual bubbles, and the driving force for bubble growth is the pressure difference between the inside and the exterior of the matrix (Lai and Kokini 1990; Mohamed 1990; Kokini and others 1992; Huang and Kokini 1993, 1999; Fan and others 1994). It is generally accepted that surface tension has an overall negligible effect on expansion of polymer melts (Krycer and Pope 1983; Odidi and others 1991; Fan and others 1994; Lawton 1995), with little effect on initial expansion (Fan and others 1994). The rheological properties of the polymeric matrix have instead the leading role in expansion, since they determine the resistance of the bubble wall to the pressure difference between the inside and the outside of the bubble (Lai and Kokini 1990; Kokini and others 1992; Fan and others 1994). Figure 16 Prediction of first normal stress difference of extruded corn meal (Chang 1992) Vol. 2, 2003 COMPREHENSIVE REVIEWS IN FOOD SCIENCE AND FOOD SAFETY 158