Nutritional Aspects and Safety of Modified Food Starches

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Nutritional Aspects and Safety of Modified Food Starches Otto B. Wurzburg The term modified starches covers a wide variety of starch-derived products.

1 Nutritional Aspects and Safety of Modified Food Starches Otto B. Wurzburg The term modified starches covers a wide variety of starch-derived products. A number of techniques, such as those involved in bleaching, enzymic digestion, dextrinizing, and hydrolysis, have either no effect on the starch or are used to reduce the size of the starch molecule. The latter process is similar to the digestive process, as well as such common foodprocessing techniques as toasting or baking bread. Starch modifications in these categories are widely accepted as being similar to unmodified starches for use in foods. For this reason, no further attention will be given to this type of modification. Chemical Starch Modification This paper is devoted to those modified starches made by treating starch with agents that introduce substituent chemical moieties into the starch granule through reaction with hydroxyl groups in the starch molecule. These chemically modified starches have been used as food ingredients for over 45 years. All are cleared for use in the US, Canada, and the UK, and have been accepted by the Joint Expert Committee on Food Additives of the FAO/ WHO. They impart to food systems valuable functional properties that cannot be realized with unmodified starches. They provide texture, thicken, suspend solids, stabilize emulsions, facilitate processing by protecting foods during processing, or protect finished foods during distribution and storage. Within the broad category of chemically modified starches there are wide variations in Mr Wurzburg is past Vice President of Natural Polymer Research at National Starch and Chemical Company, Bridgewater, NJ, USA. the extent to which the starch has been chemically altered. Many such starches contain only infinitesimal amounts of substituent groups and might be considered 99.9 percent pure starch. The presence of the added chemical groups is not readily detected except by sophisticated analytical methods or by physical characterization. Others contain sufficient amounts of substituent chemical moieties to be easily detected. There are two major ways to modify starch chemically: crosslinking, and introduction of monosubstituent groups. Most of the modified food starches used in the food industry are made either by crosslinking or by combining crosslinking and introducing monosubstituent groups. Limited amounts of starches containing only monosubstituent groups are used in foods. Unmodified or native starch occurs in the form of minute granules that are insoluble in cold water. They owe their integrity to crystallites formed by hydrogen bonds between hydroxyls on adjacent molecules. When heated in the presence of water, the hydrogen bonds holding the granule together weaken, permitting the granules to imbibe water, swell, and thicken to form colloidal dispersions. Such dispersions have limited value in most food systems because of the sensitivity of the hydrogen bonds responsible for the integrity of the swollen granule to thermal and mechanical breakdown. Crosslinking as a Chemical Modification Crosslinking reinforces starch granules with intermolecular chemical bonds that keep the swollen starch granules intact after the hydrogen bonds have been ruptured. The technique 74 NUTRITION REVIEWSNOL. 44, NO. ZFEBRUARY 1986

2 imparts to the dispersions of starch resistance to thermal, chemical, and mechanical breakdown. Two types of crosslinked starches are used in foods: distarch adipates, made by treating starch with a mixed anhydride of adipic and acetic anhydride; and distarch phosphates, made by treating starch with either phosphorus oxychloride or sodium trimetaphosphate. Since the granule contains a tremendously large number of anhydroglucose units in the molecules, relatively few chemical crosslinks are required to reinforce the granule and produce a marked effect on the physical properties of the starch dispersion. In most crosslinked food starches there is one substituent crosslinking group per 1,000 or more anhyd rog I ucose units. Since the substituent groups present in crosslinked food starches may be metabolized, and in view of the extremely low levels at which they are present, the impact of crosslinking on the nutritional value of starch and on physiologic behavior is minimal or undetectable. Numerous in vitro studies indicate that low levels of crosslinking have little effect on enzyme digestibility. Thus, in vitro digestibility studies on distarch adipates containing low levels of acetyl groups and on distarch phosphates indicate that starches crosslinked within the limits set by regulation approach native starches in digestibility. In vitro amyloglucosidase digestibility of acetylated distarch adipate, made by treating waxy maize starch with 0.15 percent adipic acid in a 1:3 adipic-acetic mixed anhydride, was 98 percent of the digestibility of unmodified waxy maize starch. I2 Under these conditions; the level of adipic treatment was 20 percent higher than the maximum approved for use in making modified food starch. In vitro enzyme digestibility studies on distarch phosphate also show in vitro digestibilities approaching or comparable to those obtained with unmodified starch. Thus, in vitro amyloglucosidase digestibility of waxy maize starch, crosslinked by reaction with 0.035, and percent phosphorus oxychloride, ranged from 98 to 96 percent that of unmodified waxy maize starch. I2 In vitro digesti- bility by pancreatin of corn or potato starch crosslinked with 0.05 or 0.1 percent phosphorus oxychloride was similar to that of unmodified starch.l While in vitro studies indicate the susceptibility of modified starches to digestion by specific enzymes, the digestion and absorption of carbohydrates is an integrated rather than sequential process. As a result, in vivo studies provide a better picture of the digestibility and nutritive value of modified food starches. The caloric value of acetylated distarch adipate, prepared by treating an acid hydrolyzed waxy maize starch with a mixed anhydride containing 0.2 percent adipic acid and 5.5 percent acetic anhydride in an adipic-acetic mixed anhydride, was equal to that of control starch in 28-day feeding studies. Groups of ten weanling male albino rats were fed a basal diet containing 1.5 or 3.0 g of modified or control starch, or 0.75 to 4.5 g of sucrose supplement.ls2s4 In these studies, the level of adipic acid treatment was 67 percent higher than that permitted in making crosslinked distarch adipate for food use. Studies of the caloric value of distarch phosphates prepared by treating waxy maize starch with 0.03 or 0.1 percent phosphorus oxychloride, as well as of distarch phosphate made by treating milo starch with trimetaphosphate, showed that distarch phosphates are comparable to unmodified control starches in caloric value. s2s5 Since the level of crosslinking substituents is extremely low, it is questionable whether caloric determinations reflect the presence of substituents even if they are indigestible. To determine the metabolic fate of the adipate crosslink, young male adult rats were given by stomach tube a suspension of acetylated distarch adipate prepared using 14C-labeled adipic acid. A physical mixture of 14C-labeled adipic acid and unmodified starch was administered as a control. After 25 hours, 99.3 percent of the activity of the free adipic acid had been recovered in the respired air compared to 70.5 percent of the activity of the labeled starch. Most of the balance of the activity had been recovered in the feces and the rest in the urine.1,2,6 Other studies included investigations by An- NUTRITION REVIEWSNOL. 44, NO. 2/FEBRUARY

derson et a17 in which 3-day-old Pitman-Moore miniature pigs were fed for 25 days formulated diets containing either acid hydrolyzed waxy maize starch as the control, or acid hydrolyzed waxy maize

3 derson et a17 in which 3-day-old Pitman-Moore miniature pigs were fed for 25 days formulated diets containing either acid hydrolyzed waxy maize starch as the control, or acid hydrolyzed waxy maize starch crosslinked with 0.08 percent phosphorus oxychloride as the test starch. Starch provided 24 percent of the calories in the diet. Body weight gains of the animals fed the distarch phosphate were similar to those fed the control starch. There were no statistically significant differences in the organ weights expressed as percentage of body weight. Serum cholesterol, triglyceride, calcium, phosphorus, alkaline phosphatase, urea nitrogen, total protein, albumin, and globulin levels were similar in both groups of animal^.^ Starches resistant to attack by digestive enzymes are subject to increased microbial activity in the gut when fed to rats at high dietary levels, resulting in flatulence and enlarged caeca. When waxy maize distarch phosphate containing levels of crosslink within the legal range was fed to rats at a dietary level of 16 percent there was no evidence that such levels of crosslinking caused caecal enlargement.* In view of these and other studies, it appears reasonable to conclude that crosslinking starch within the limits allowed by regulation provides a modified food starch comparable to unmodified starch in nutritive value and safety. Substitution of Monosubstituent Groups The second major approach to chemical starch modification is through the introduction of monosubstituent groups of acetate, phosphate, octenyl succinate half ester, and hydroxypropyl groups. These may represent the sole modification or, in the case of all but the octenyl succinate half ester, part of a dual modification involving introduction of adipate or phosphate crosslinks. Since crosslinks per se have no significant effect on nutrition and safety, and since most of the commercial food applications for modified food starches containing monosubstituent groups also require a crosslinking treatment, nutrition and safety studies on acetates, phosphates, and hydroxypropyl derivatives have frequently been based on monosubstituent modifications that have also been crosslinked. The level of monosubstitutent groups intro- duced in the production of modified food starches is relatively low. In the case of acetates, the maximum level of substitution by regulation is 2.5 percent. This corresponds to one acetate group for roughly every 10 to 11 anhydroglucose units. The maximum level for phosphates is 0.4 percent phosphorus which corresponds to one phosphate group for roughly 47 to 48 anhydroglucose units. The treatment level of propylene oxide permitted in making hydroxypropyl starches varies among regulatory agencies. However, the typical maximum level is 10 percent, corresponding to a maximum of one hydroxypropyl substituent for about every 5 anhydroglucose units. In the case of octenyl succinate half esters, there is at most about one substituent group for every 50 anhydroglucose units. There are two objectives in modifying starch by introducing monosubstituent groups. One is to stabilize the starch so its sols will not thicken or gel on cooling (in the case of cereal starches), and to maintain hydrating ability, syneresis and gelling ability when holding at low temperatures or repeatedly freezing and thawing (waxy maize, tapioca, and potato starches). This is accomplished by interrupting the linearity of the amylose component or of the outer branches of the amylopectin, reducing the tendency for intermolecular association via hydrogen b~nding.~ The other objective is to introduce new or added functions into the starch molecules. This is the primary objective in making octenyl succinate half esters of starch. These substituents impart emulsion stabilizing properties to starch. Substituent groups introduced at significant levels will affect the in vitro enzymic digestibility of starch.l0s1l An approximation of the influence of the degree of substitution on the in vitro digestibility of cooked cereal starch with pancreatin or amyloglucosidase is illustrated in Figure 1. The nature of the substituent group and type of starch will vary this pattern. An in vitro study showed that acetate ester linkages are cleaved by pancreatic juices6 This finding was based on the isolation and detection of 14C-labeled acetic acid from the pancreatic digest of acetylated distarch adipate containing 14C-labeled acetate groups. The conditions for the in vitro study, however, 76 NUTRITION REVIEWSNOL. 44, NO. 2/FEBRUARY 1986

looti 0 005 01 015 02 D.S. Figure 1. Percentage of in vitro enzymic digestibility relative to that of unmodified cereal starch as function of degree of substitution (D.S.).

4 looti D.S. Figure 1. Percentage of in vitro enzymic digestibility relative to that of unmodified cereal starch as function of degree of substitution (D.S.). ~ were such that some hydrolysis of the starch acetate possibly occurred during the study, so that the observation may have been based on an artifact due to hydrolysis rather than evidence of enzymatic removal. As noted previ~uslyl~~~~ in the discussion on starches crosslinked with adipate groups, in vivo feeding studies of rats fed acetylated distarch adipate made by treating with a mixed anhydride containing 0.2 percent adipic acid in 5.5 percent acetic anhydride showed that the caloric value of the modified starch, which had an acetate degree of substitution of 0.07, was comparable to that of unmodified starch. Thus, even though this starch had an in vitro digestibility of about 80 percent that of the control, its in vivo digestibility was essentially 100 percent that of the control, indicating that in starch stabilized with ester groups there are digestive mechanisms for removing stabilizing ester groups, probably through hydrolysis. The hydroxypropyl substituent groups in hydroxypropylated starches, however, are not susceptible to hydrolysis as are ester groups. Leegwater studied the digestion of a hydroxy- (2-14C) propylated starch having a degree of substitution of 0.12 in rats, and found that over 95 percent of the radioactivity administered to the rats was detected in the feces. * In subsequent studies using hydroxypropyl starches with varying degrees of substitution ranging from to 0.106, the major components were tentatively identified as hydroxypropyl maltose, dihydroxypropyl maltotriose, and dihydroxypropyl malt~tetraose. ~ The digestibility of the series decreased with increasing substitution. Rats fed substituted and substituted crosslinked starches, depending upon the degree of substitution and nature of substituent group, and unlike those fed crosslinked starches treated in accordance with regulations for modified food starches, tended to develop enlarged caeca at high feeding levels. Such enlargements are an adaptive change involving the proliferation within the large intestine of microorganisms capable of degrading food constituents not degraded by enzymes such as amylases, glucosidases, maltases, etc, present higher up in the gastrointestinal tract.* They are associated with feeding slowly digested materials and formation of hyperosmotically active substances in the caecum. They are not of toxicological significance. The caecum returns to normal size when the slowly digestible material is removed from the diet.** Long-term Toxicity Studies of Modified Starches Long-term studies in rats fed various modified starches14-15 gave negative results in respect to carcinogenesis, but rats fed high dietary levels of some substituted or crosslinked substituted modified starches exhibited an increased incidence of mineral deposits in the pelvic region of the kidney along with caecal enlargement. Concern over the etiology of these deposits by regulatory authorities led to extensive studies by Feron et a1,16 Newberne et ai,l7,l8 and Hodgkinson et ai.l9 The conclusions drawn from these studies and literature investigations were that: 1) Rats are far more sensitive than mice to the development of pelvic nephrocalcinosis, and the phenomenon has no apparent human counterpart; 2) pelvic nephrocalcinosis is commonly seen in aging rats fed regular laboratory diets. Its incidence is enhanced by incorporating a wide range of substances other than modified FJC Roe, unpublished data. DC Leegwater, unpublished data. NUTRITION REVIEWSNOL. 44, NO. UFEBRUARY

5 starches in the diets. Lactose in the diet is far more effective in enhancing the incidence of pelvic nephrocalcinosis than are modified starches; 3) all carbohydrates increase calcium absorption in rats, but some sugars, including lactose, increase it markedly.20 The increased absorption of calcium leads to its increased urinary excretion, and it is this increased urinary excretion that predisposes the animals to pelvic nephrocalcinosis; 4) the problem tends to be exacerbated in rats by including too much calcium and phosphorus and too little magnesium in the diet. Of special importance, too, is a low Ca:P ratio; and 5) the effects of excessive calcium absorption in rats are more severe in older than in younger ones, because calcium retention associated with bone growth ceases after maturity is reached. The picture that emerges is that when substituted starches are fed at abnormally high dietary levels, the rate of digestion in the upper gastrointestinal tract is insufficient to handle all the carbohydrate. Consequently, unabsorbable substituted di- and trisaccharides enter the caecum where they accumulate pending possible degradation by bacterial enzymes and, in the meantime, increase the osmotic pressure and cause caecal enlargement. These changes in the gut are associated with increased calcium absorption, and increased calcium levels in renal tissue and in the urine, which lead to calcium deposition in the kidney. These effects are related to the type and level of substitution. Hydroxypropyl derivatives show them to a greater extent than ester derivatives and the effects correlate positively with the level of substitution. The effects observed in rats fed high dietary levels of modified starches are of no relevance to human beings, for two reasons. First, human beings, unlike rats, are not sensitive to the development of any form of nephrocalcinosis as a consequence of dietary mineral imbalance or excessive intake of lactose. Second, the dietary levels of modified starches required to enhance pelvic nephrocalcinosis in rats far exceed those to which humans are exposed. In those foods containing modified food starches with significant levels of monosubstituents, the amount rarely exceeds 5 to 6 percent in the food. In terms of daily intake it is well below 1 percent. In infant foods, modified starches, including simple crosslinked starches, rarely account for more than 10 percent of the caloric intake.21 An unpublished survey in 1970 showed that the ratio of crosslinked to esterified crosslinked, and to esterified crosslinked starches in baby foods was 10:5:1. Since then, esterified crosslinked starches have been dropped from most infant foods. Thus, the level of modified food starches containing monosubstituents rarely accounts for more than about 3.3 percent of the caloric intake in infant foods. In 25-day-long studies on miniature pigs fed modified food starches, including hydroxypropylated crosslinked starch, in formulas in which the modified and control starches were about 25 percent of the diet, no significant treatment-related effects were observed on the growth, biochemical values of blood or serum, or on carcass or liver comp~sition.~ Ninety-day-long studies on rats, using hydroxypropyl distarch phosphate and a control starch fed at dietary levels of 5, 10, and 25 percent, showed no deviations from the control with respect to growth, gains in body weight, food intake, or efficient food utilization at the 5 or 10 percent Controlled human studies have been limited. However, one by Pieters et al involving feeding 12 volunteers a 60-g diet containing starch acetate with 1.98 percent acetyl (degree of substitution of about 0.075) for 4 days, produced no gastrointestinal e ffe~ts.~~,~~ Summary From these observations, it appears that simple crosslinked starches are indistinguishable from unmodified starches in their nutritional characteristics because of the extremely low level of crossbonds in the starch. Modified food starches containing monosubstituent groups alone or in combination with crosslinks, depending upon the level of substitution, may show slightly slower rates of digestion at high feeding levels, but this is of no significance at the levels of daily intake found in the human diet NUTRITION REVIEWSNOL. 44. NO. Z/FEBRUARY 1986

6 1. Evaluation of the Health Aspects of Starch and Modified Starches as Food Ingredients. SCOGS-115, Life Sciences Research Office, Federation of American Societies for Experimental Biology, Bethesda, MD, WHO Food Additive Series, No. 5, World Health Organization, Geneva, GJ Janzen, Die Starke 9: , M Oser, Report No 81776, Food and Drug Research Laboratories Inc, New York, NY, BL Oser, Report No a-i, Food and Drug Research Laboratories Inc, New York, NY, K Morgareidge, No 79408, Food and Drug Research Laboratories Inc, New York, NY, TA Anderson, LJ Filer, SJ Fornan, DW Andersen, RL Jensen, and RR Rogers, Food Cosmet TOX~CO~ 11 : , R Walker and EA El Harith, Ann Nutr Alim 32: , OB Wurzburg, Handbook of Food Additives. Second Edition. TE Furia, Editor. CRC Press, Cleveland, OH, M Wootton and MA Chaudhry, Die Starke 31: , M Wootton and MA Chaudhry, Die Starke 33: , DC Leegwater, CIVO-TNO Report R3441, DC Leegwater and Al Speek, Die Starke 24: , AP DeGroot, HP Til, VJ Feron, HC Dreef-van der Meulen, and MI Willems, Fd Cosmet Toxicol 12: , R Truhaut, B Coguet, X Vouillet, D Galland, D Guyot, D Long, and JL Rouaud, Fd Cosmet Toxicol17: 11-17, VJ Feron, HP Til, and HR Immel, ClVO Report No. R5690, PM Newberne and ML Buttolph, Massachusetts Institute of Technology Study 78-4, PM Newberne and ML Buttolph, Massachusetts Institute of Technology Study 79-2, A Hodginson, D Davis, J Fourman, WG Robertson, and FJC Roe, Fd Chem Toxic20: , FJC Roe, Modified Starches and Nephrocalcinosis in Rats: Implications for the AD1 Principle for Food Ingredients. European Toxicology Forum, Geneva, Switzerland, LJ Filer, Jr, Nutrition Reviews 29: 55-59, DE Bailey, GE Cox, and K Morgareidge, Subacute Feeding Studies in Rats with Hydroxypropyl Distarch Phosphate. Food and Drug Research Laboratories Inc, New York, NY, JJL Pieters, NA Van Staveren, and BGAM Brinkuis, Report No. R-3433, CIVO, R Derache, Med et Nutri 19: , 1983 NUTRITION REVIEWSNOL. 44, NO. ZlFEBRUARY

Why study starch fine structure?

1 Fine Structure and Chain Length Distribution Why study starch fine structure? To differentiate starches (and starch derivatives) at molecular levels To define and document unique starches or starch derivatives