Magdalena Zielinska LDRT #991403, VOL 0, ISS 0 QUERY SHEET TABLE OF CONTENTS LISTING

Transcription

1 LDRT #991403, VOL 0, ISS 0 Drying Kinetics and Physicochemical Characteristics of Laboratory-Prepared Corn/Wheat Distillers Grains and Solubles Dried with Superheated Steam and Hot Air Magdalena Zielinska QUERY SHEET This page lists questions we have about your paper. The numbers displayed at left can be found in the text of the paper for reference. In addition, please review your paper as a whole for correctness. Q1: Au: Please supply publisher for Ref 8. Q2: Au: Please supply volume and pages for Ref. 23. Q3: Au: Please supply city for Ref. 49. TABLE OF CONTENTS LISTING The table of contents for the journal will list your paper exactly as it appears below: Drying Kinetics and Physicochemical Characteristics of Laboratory-Prepared Corn=Wheat Distillers Grains and Solubles Dried with Superheated Steam and Hot Air Magdalena Zielinska

2 3b2 Version Number : 7.51c/W (Jun ) File path : p:/santype/journals/tandf_production/ldrt/v0n0/ldrt991403/ldrt d Date and Time : 05/03/15 and 19:20 Drying Technology, 0: 1 16, 2015 Copyright # 2015 Taylor & Francis Group, LLC ISSN: print= online DOI: / Drying Kinetics and Physicochemical Characteristics of Laboratory-Prepared Corn/Wheat Distillers Grains and Solubles Dried with Superheated Steam and Hot Air Magdalena Zielinska 5 Department of Systems Engineering, University of Warmia and Mazury in Olsztyn, Olsztyn, Poland The effect of superheated steam (SS) drying and hot air (HA) drying on drying kinetics and changes in the color, crude protein, and amino acid concentrations (in particular, lysine content) of corn/wheat wet distillers grains (WDG) and centrifuged solubles 10 (CS) was evaluated. An inversion temperature was reached at 139 C for WDG and 132 C for CS, above which moisture evaporation rate and qualitative changes under SS drying conditions exceeded the values noted in HA, and below which the reverse was observed. A significant decrease (from 8 to 50%) in the lysine 15 content of WDG and CS was reported during SS and HA. The overall changes in the color (DE ) of corn/wheat WDG and CS ranged from to during SS drying and from to during HA drying. The observed deterioration in color was attributed mainly to changes in lightness (L ) and yellowness 20 (b ) of dried samples. The values of L and b were reliable predictors of the lysine content of corn/wheat distillers co-products. Keywords Color; Crude protein; Distillers co-products; Drying; Lysine 25 INTRODUCTION In ethanol production, starch is fermented to obtain ethyl alcohol, and the remaining non-fermentable components of grain kernels retain much of their original nutritional value. Ethanol plants recover and recombine 30 these components into a variety of animal feed ingredients. Conversion of these residues into valuable animal feeds begins with whole stillage which contains non-fermentable materials in both dissolved and suspended form. [1] The current standard in the dry-grind ethanol industry is to use a 35 decanter-style centrifuge to split the whole stillage into two fractions: wet distillers grains (WDG), which contain suspended solids that are removed from the whole stillage, and thin stillage, which contains high concentrations of water-soluble solids. [1] After separating centrifugation, thin 40 stillage is passed through the evaporator to remove the remaining moisture, which produces condensed distillers Correspondence: Magdalena Zielinska, Department of Systems Engineering, University of Warmia and Mazury in Olsztyn, ul. Heweliusza 14, Olsztyn, Poland; m.zielinska@ uwm.edu.pl solubles (CDS). [2] The whole stillage can also be filtered to produce distillers grains, and solubles can be separated from the remaining slurry by condensed matter centrifugation. [3 5] WDG and CDS may be sold locally in wet form or supplied to the feed industry as distillers dried grains (DDG), distillers dried solubles (DDS), or distillers dried grains with solubles (DDGS). The marketability of DDG, DDS, and DDGS as feed ingredients is a key consideration which determines the economic viability of ethanol plants. However, the variability in the physicochemical properties of distillers dried by-products could prevent accurate formulation of feed rations. [1] Significant variability in the physicochemical properties of samples has been noted in corn- and wheat-based distillers co-products and attributed to grain itself, [6 10] particle size distribution of grains, [11] fermentation, [12] blending ratio of WDG and CDS, [13 15] as well as drying. [12,16] Various drying processes are used to dehydrate distillers co-products, and their choice is dictated by the type of dried material, the amount of water that needs to be removed, and the final quality or functionality of the dried products. The most popular drying technique involves the use of hot air (HA) at a temperature of C, depend- ing on the ethanol plant. [17,18] The effects of HA drying [19] and WDG and CDS blending ratios [20] on the quality characteristics of corn and wheat distillers co-products were evaluated in several studies. Drying temperature and blending ratios significantly affected various physico- chemical properties of corn [19] and wheat [21] DDGS. The loss of lysine during drying reduced the nutritive value of feed material and, ultimately, influenced animal growth performance. [10] HA drying may lead to lysine degradation, whereas many of the emerging drying technologies are free of such drawbacks. The application of microwaves (MW) instead of hot air may greatly reduce drying time and ensure higher quality of the final products. [22,23] The influence of MW power, microwave convection settings, and WDG to CDS blending ratios on drying kinetics [25] and physicochemical characteristics [24 26] of laboratory-prepared wheat

3 2 ZIELINSKA DDGS was studied. Solubles had a greater influence on the physicochemical characteristics of dried material, including protein quality [24] and lysine content [25] and color, [26] than 85 on MW power, microwave convection settings, and=or their interactions. Microwave and microwave-convection drying resulted in products with satisfactory protein quality [24] and showed potential for minimizing lysine loss, [25] but microwave-assisted drying is also characterized by 90 several major drawbacks, such as high initial cost, uneven heating, limited penetration of microwaves into the product, or quality losses. [27] Superheated steam (SS) drying has emerged as a popular method in agri-food processing due to its low environmen- 95 tal impact, higher drying efficiency, elimination of oxidation, no explosion risk, and recovery of volatile compounds. [28,29] SS drying is used to dehydrate various foods and agricultural products on a laboratory to industrial scale, including potatoes, [30] beet pulp, [31,32] 100 pasta, [33,34] beef, [35] and shrimp. [36] The effect of SS drying on drying kinetics and quality of wheat, [37,38] corn, and wheat distillers grain [39,40] has been investigated by several studies. SS drying has been successfully used to preserve WDG without exerting adverse effects on â-glucan, [37] pen- 105 tosan, [37] protein, [37,39] and phenolic content [39] of dried samples or their antioxidant activity. [39] SS and HA drying techniques are characterized by various drying rates, subject to the difference in temperature between the surface of the sample and the drying medium, and the heat transfer 110 coefficient of the drying medium. [29] An inversion temperature is reached, above which the moisture evaporation rate under SS drying conditions exceeds the values noted in HA, and below which the reverse is observed. The inversion temperature of porous foods was determined in the 115 range of 125 C to 150 C. [28 30] Most research papers evaluate the effect of different drying techniques on the drying kinetics and quality of corn [19] and wheat distillers co-products, [21,24 26,37,38] as well as mixtures of corn=wheat-based distillers co-products. [39,40] 120 To date, there have been no published data on the effect of SS and HA drying on the color, crude protein, and amino acid content of corn=wheat distillers co-products, their drying kinetics and quality. Information concerning the moisture evaporation rate in SS and HA drying can 125 be used to optimize drying processes of distillers dried co-products. Optimized drying operations and, consequently, higher product quality will contribute to the economic viability of ethanol plants and the attractiveness and competitiveness of the livestock industry on the domestic 130 and global markets. Therefore, the objective of this study was to: (1) describe thin-layer drying characteristics of corn=wheat distillers co-products under SS and HA drying conditions; (2) evaluate the inversion point above which the drying rate in SS exceeds that in HA; (3) examine the 135 effect of SS and HA drying at 110, 130, and 160 Con the quality of corn=wheat DDG and DDS in terms of their color, crude protein, and amino acid content (in particular, lysine content); and (4) check for correlations between lysine content and color attributes of corn=wheat distillers co-products. MATERIALS AND METHODS Preparation of Experimental Material The experimental material was a mixture of corn and wheat (50% corn, 50% wheat) whole stillage obtained from a local distillery (Mohawk Canada Limited, a division of Husky Oil Limited, Minnedosa, MB). Solid and semi-solid fractions were separated from the remaining liquid by condensed matter centrifugation using a laboratory batch centrifuge (Sorvall General Purpose RC-3 Centrifuge, Thermo Scientific Co., Asheville, NC, USA). The centrifuge operated at relative centrifugal force of 790 g, with a 1000 ml sample container rotating at 2200 rpm for 10 min, with a rotation radius of m. Condensed matter centrifugation produced wet distillers grains (WDG) and centrifuged solubles (CS). Centrifuged solubles were not condensed in an evaporator. WDG and CS fractions were applied in drying experiments, whereas the remaining liquid was discarded. WDG and CS were bagged and deep frozen at 15 C until further experiments. Bags of frozen WDG and CS were thawed overnight and placed in the sample preparation area to achieve room temperature (23 C) before thin-layer drying runs. WDG and CS were dried separately to obtain distillers dried grains (DDG) and distillers dried solubles (DDS). A flow diagram of corn=wheat-based stillage processing is shown in Fig. 1. Superheated Steam and Hot Air Drying Drying Apparatus and Conditions Drying experiments were conducted at the Department of Biosystems Engineering, University of Manitoba, Canada. Two sets of drying experiments were performed: (1) drying with superheated steam (SS); and (2) drying with hot air (HA). The SS drying system consisted of a steam generator, superheater, drying chamber, water tank, condensation unit, steam conveying pipes and valves, data acquisition and control system. [38] Hot air drying experiments were performed in a laboratory oven (Precision Scientific Thelco 130D Laboratory Oven, Thermo Fisher Scientific Inc., ON, CA). SS drying was conducted at 110, 130, and 160 C under or near to the atmospheric pressure and steam velocity of 1m=s. The drying conditions and procedures were identical in HA and SS drying. HA and SS drying tests were conducted in triplicate. Experimental measurements of mass, lifting force, material temperature, medium temperature, steam flow, and pressure under different drying conditions are described below. Detailed information about the

4 DRYING KINETOCS OF CORN=WHEAT DISTILLERS GRAINS 3 FIG. 1. A flow diagram of corn=wheat-based stillage processing. experimental set-up and procedure for SS drying tests can be found in the literature. [38] Moisture Content 190 WDG and CS were dried on a solid Teflon sphere (150 g, 5.0 cm in diameter) that served as inert material. Each time, the mass of approx g of wet material (equivalent to a 3 mm layer) was uniformly distributed on the surface of a Teflon sphere. Two hemispherical tea strainers whose 195 radius was approximately 3 mm larger than the radius of inert material were used to cover the sphere with WDG or CS. An external fiberglass mesh-cage prevented the dried material from dripping off the sphere. The Teflon sphere, covered with WDG or CS, was attached to a string 200 and suspended on a digital mass balance positioned at the top of the drying chamber. Changes in moisture content of a dried sample were calculated based on changes in mass measured with an electronic balance (Model TR-403, Denver Instrument Co., Arvada, CO) to the nearest g. Lifting Force The upward flow of steam through the drying chamber created a lifting force, which influenced actual mass readouts. A separate experiment was conducted after each 210 drying test to account for lifting force. Steam conditions were identical to those applied during WDG and CS drying. SS flow was manually controlled to ensure that it bypassed the drying chamber. Mass readouts with and without steam passing through the drying chamber were 215 recorded. Temperature of the Material and the Drying Medium The temperature of dried material was measured to the nearest 1 C by immersing a 30-gauge K-type thermocouple tip into the WDG layer or the CS layer to a depth of 1.5 mm. Four additional K-type thermocouples were used to measure steam temperature inside the drying chamber. Changes in the mass and temperature of WDG and CS dried in SS were measured in separate experiments that were conducted under identical conditions. Steam Velocity Steam velocity was calculated based on readouts from a computerized mass flow rate meter (Compart DXF 351 Flow Computer, Endress Hauser GmbH Co., Weil am Rein, Germany). Pressure Pressure in the drying chamber was controlled by three pressure gauges (one for boiler water and two for steam). Pressure was measured to the nearest 0.1 kpa. Chemical Composition Ten selected samples (i.e., WDG, CS, DDG dehydrated with SS and HA at 110 C and 160 C, and DDS dehydrated with SS and HA at 110 C and 160 C) were analyzed for their moisture content, crude protein, and amino acid content. Moisture content was determined by the AACC Method 44-19: Moisture-Air-Oven Method, frying at 135 C using a forced convection laboratory oven (Precision Scientific Thelco 130 D Laboratory Oven, Thermo Fisher Scientific Inc., ON, CA). [41] The crude protein (CP) content

5 4 ZIELINSKA on a dry weight basis was determined according to method : Crude Protein-Kjeldahl Method, Boric Acid Modification (AACC Standards, 2000). [42] Free amino acids were separated and quantitated by protein hydrolysis, oxidation of sulfur-containing amino acids, and ion-exchange liquid chromatography coupled with ninhydrin postcol- 250 umn derivatization before UV detection. [43 45] Free amino acids were separated and measured in a dedicated amino acid analyzer (Biochrom 20þ Amino Acid Analyzer, Biochrom Ltd., Cambridge, UK). Amino acid concentrations were expressed in percentage on a dry matter basis 255 (% db) (the only exception was lysine, which was also expressed as percentage of crude protein, % CP). Crude fat (CF) content on a dry matter basis was determined according to the AACC Method 30-25: Crude Fat in Wheat, Corn and Soy Flour, Feeds, and Mixed Feeds 260 (AACC Standards, 2000). [46] Total ash content on a dry matter basis was calculated according to the AACC Method 08-01: Ash-Basic Method (AACC Standards, 2000). [47] Crude protein, crude fat, and total ash were determined in a forage=feed analysis, and whatever was 265 left to make up 100% mass balance was regarded as total carbohydrates. Color Color was determined with a chroma meter (CR-410, Konica Minolta, Tokyo, Japan) equipped with a circular 270 sample holder (Attachment CR-A50, Konica Minolta, Tokyo, Japan). Dried samples were ground before color measurements. The color of samples was expressed in CIE- LAB space, where L represents lightness=darkness, þ( ) a represents redness=greenness, and þ( ) b represents 275 yellowness=blueness. Total color difference (DE ), total saturation difference (DC ), and total hue difference (DH ) were calculated using Eqs. (1) to (3) [48] : DE ¼ DC ¼ DH ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DL 2 þ Da 2 þ Db 2 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 þ b 2 qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi a 2 0 þ b 2 0 pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi DE 2 DL 2 DC 2 Changes in individual color parameters of samples during drying were calculated as follows [48] : L ¼ L L 0 ; Da ¼ a a 0 ; Db ¼ b b where L 0, a 0, and b 0 represent the color values of non-dried samples, and L, a, and b indicate the color values of dried samples. The criterion introduced by the International Commission on Illumination was applied, and the data were analyzed ð1þ ð2þ ð3þ ð4þ according to the method described in the literature. [48,49] The criterion relies on human perception of color and assumes that total color difference (DE ) of 0 to 2 is insignificant and unrecognizable by an experienced and qualified observer, whereas total color difference (DE ) greater than 5 is significant and recognizable even by an inexperienced observer. [49] For each sample, the results of color attribute measurements were averaged over 35 measurements. The relationship between total lysine content and the color of corn=wheat distillers co-products was assessed and described by a linear equation (5): y ¼ Ax þ B where y is lysine content, expressed by percentage content of feedstuff (% db); x represents lightness, L, or yellowness, b ;anda and B are coefficients in Eq. (5). Statistical Analysis The experimental data were processed by regression analysis and ANOVA with a completely randomized block design. The significance of differences between treatments was determined by one-way ANOVA and Duncan s multiple range test (p 0.05). Data were analyzed in Statistica 9.0 (StatSoft Inc., Krakow, Poland). RESULTS AND DISCUSSION Proximate Composition of Corn/Wheat WDG and CS The crude protein content of corn=wheat WDG and CS was analyzed because the suitability of distillers co-products for the production of feed for monogastric animals [10] is determined largely by their protein quality, in particular lysine content (Table 1). The moisture content, total carbohydrate content, crude fat, and total ash levels of the analyzed material are shown in Table 1. Moisture levels and crude protein, crude fat, and ash content of corn=wheat CS were significantly higher in comparison with corn=wheat WDG. Total carbohydrate levels in corn=wheat WDG were significantly higher than in corn= wheat CS. Mosqueda and Tabil [50] reported significantly higher crude protein levels in wheat CDS (approx. 46% db) than in wheat WDG (approx. 26% db), whereas Kingsly et al. [19] observed much higher crude protein concentrations in corn WDG (approx. 16% db) than in corn CDS (approx. 7% db). Amino Acid Profiles of Corn/Wheat WDG and CS The amino acid profiles of corn=wheat WDG and CS were evaluated in a protein analysis. The most abundant amino acid in corn=wheat WDG and CS was glutamic acid, followed by leucine and proline. Similar results were reported by Mustafa et al. [51] for corn and wheat wet distillers co-products. In their study, the most abundant amino ð5þ

6 DRYING KINETOCS OF CORN=WHEAT DISTILLERS GRAINS 5 TABLE 1 Quality attributes of corn=wheat wet distillers grains (WDG) and centrifuged solubles (CS) WDG CS Quality Attributes 1 2 Moisture, kg=kg db a b Crude Fat (CF), % db a b Ash, % db a b Carbohydrates, % db a b Crude Protein (CP), % db a b Lysine, % db a b Lysine, % CP a b Aspartic Acid, % db a b Threonine, % db a b Serine, % db a b Glutamic Acid, % db a b Proline, % db a b Glycine, % db a b Alanine, % db a b Cystine, % db a a Valine, % db a b Methionine, % db a b Isoleucine, % db a b Leucine, % db a b Tyrosine, % db a b Phenylalanine, % db a b Histidine, % db a b Arginine, % db a b L, a b a, a b b, a b Mean standard error of the mean of a set of observations are given in the table. Moisture content is expressed as kg of water per kg of dry mass (kg=kg db). Crude protein and amino acid contents are expressed as the percentage values of feedstuff (% db); additionally, lysine content is expressed as a percentage value of crude protein (% CP). Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between two fractions of stillage; namely, WDG and CS. acid in corn and wheat WDG and CS was glutamic acid, followed by proline in wheat and leucine in corn distillers 340 co-products. [51] The predominant limiting amino acids in corn=wheat WDG and CS were sulfur amino acids (methionine þcystine), followed by histidine, lysine, and tyrosine (Table 1). Our results are consistent with the findings of Wu [52] and Wu et al. [53] in studies of corn and wheat wet 345 distillers co-products. In our study, CS were significantly richer in amino acids than WDG (Table 1). Color of Corn/Wheat WDG and CS Color attributes (L and b ) are reliable predictors of lysine content of distillers co-products, [54] but there is a general scarcity of information concerning the color of wet co-products from ethanol plants. [55] As shown in Table 1, corn=wheat CS were significantly lighter (higher L values), less red (lower a values), and more yellow (higher b values) than corn=wheat WDG. Rosentrater and Lehman [55] determined the lightness, redness, and yellowness of corn WDG at , , and , respectively. Corn=wheat WDG analyzed in our study were less red and less yellow than the corn-based WDG examined by Rosentrater and Lehman. [55] No significant differences were observed between L values of corn=wheat WDG and corn-based WDG. [55] Drying Kinetics of Corn/Wheat WDG and CS The effect of different drying media (SS and HA) and temperatures (110, 130, and 160 C) on the drying characteristics of corn=wheat WDG and CS was evaluated. Changes in the drying characteristics of corn=wheat WDG and CS were noted when SS was used instead of HA (Figs. 2a and 2b). The drying kinetics of WDG was similar under exposure to SS and HA at a temperature of 160 C (Fig. 2a), but the drying kinetics of WDG at 110 C and 130 C differed significantly, subject to the applied drying medium. Significant differences in moisture loss were also reported in CS at the same drying temperature but under exposure to different drying methods (Fig. 2b). Steam condensation was observed on the surface of samples during the warm-up period of SS drying. The condensate was not removed from the surface of samples at 110 C; i.e., near the saturation temperature of SS. Additional water was removed from the surface of WDG and CS in 30 and 27 minutes, respectively. The higher the steam temperature, the faster the condensate disappeared from the sample surface. The moisture that accumulated on the surface of WDG and CS during SS drying at 160 C was removed in nine and six minutes, respectively. The initial warm-up period prolonged total drying time by approximately one-seventh (Fig. 2a and 2b). The drying times for reaching equilibrium moisture content in corn=wheat WDG and CS are given in Table 2. The drying times needed to reduce initial moisture levels in WDG and CS to equilibrium levels ranged between 40 and 240 minutes, depending on the stillage fraction, the drying medium, and temperature. Despite much higher initial moisture levels in CS than in WDG, SS drying was much shorter (8 33%) in CS than WDG. However, no sig- nificant differences were observed between the drying times of WDG and CS under the same HA drying conditions (Table 2). The higher the temperature of SS and HA drying, the shorter the drying times of WDG and CS. The

7 6 ZIELINSKA TABLE 2 Times of drying required to reduce the moisture contents of wet distillers grains (WDG) and centrifuged solubles (CS) from the initial values to the equilibrium moisture contents under different superheated steam (SS) and hot air (HA) drying conditions Sample Drying Medium Drying Temperature ( C) Equillibrium Moisture Content kg=kg db Drying Time (min) WDG SS a a a WDG SS b a d WDG SS c a 60 5 f WDG HA a a c WDG HA b a 90 5 e WDG HA c a 70 5 f CS SS a a b CS SS b a 90 5 e CS SS c a 40 5 g CS HA a a c CS HA b a 90 5 e CS HA c a 70 5 f Mean standard error of the mean of a set of observations are given in the table. Different superscripts a,b in the column indicate significant differences (p < 0.05) between different samples. FIG. 2. Drying curves of (a) corn=wheat wet distillers grains (WDG) and (b) corn=wheat centrifuged solubles (CS) dehydrated with superheated steam (SS) and hot air (HA) at temperatures of 110, 130, and 160 C. increase in SS temperature from 110 to 160 C shortened 400 drying times by 75% in WDG and by 81% in CS (Table 2). The increase in HA temperature from 110 to 160 C shortened drying times by 50% in WDG and by 52% in CS. At lower drying temperatures of 110 and 130 C, the drying times of WDG and CS were shorter in 405 HA than in SS (by up to 42%). The reverse was observed at 160 C when the drying times of WDG and CS were shorter in SS than in HA (by up to 43%) (Table 2). Initial drying rates of WDG and CS under different SS and HA drying conditions were analyzed to clarify these 410 observations. The initial drying rates of WDG and CS under different SS and HA drying conditions and their moisture levels are shown in Figs. 3a and 3b (SS drying rate curves started after the condensate evaporated from the surfaces and the samples regained their original moisture content). The higher the drying temperature, the higher the drying rates of WDG and CS under exposure to SS and HA. The initial rates of water evaporation from WDG and CS under exposure to HA at 110 and 130 C were significantly higher than in SS under the same drying temperatures. The initial drying rates of WDG under exposure to HA at a temperature of 110 and 130 C were and =min, respectively, and under exposure to SS and =min, respectively (Fig. 3a). The initial drying rates of CS under exposure to HA at 110 and 130 C were and =min, while under SS they were and =min, respectively (Fig. 3b). Similarly to drying times, the reverse was noted when drying temperature was increased to 160 C. At 160 C, the initial drying rate of WDG was =min under exposure to SS and =min under exposure to HA (Fig. 3a). The initial drying rates of CS under exposure to SS and HA at a temperature of 160 C were =min and =min, respectively (Fig. 3b). In this analysis, inversion temperatures were determined at 139 C for WDG and 132 C for CS (Figs. 4a and 4b). When SS and HA temperatures were below the inversion temperature, the drying rate under HA was higher than

8 DRYING KINETOCS OF CORN=WHEAT DISTILLERS GRAINS 7 FIG. 3. Drying rates of (a) corn=wheat wet distillers grains (WDG) and (b) corn=wheat centrifuged solubles (CS) dehydrated with superheated steam (SS) and hot air (HA) at temperatures of 110, 130, and 160 C. FIG. 4. The inversion temperature for (a) corn=wheat wet distillers grains (WDG) and (b) corn=wheat centrifuged solubles (CS) dehydrated with superheated steam (SS) and hot air (HA) at the temperature of 110, 130, and 160 C. under SS, as illustrated in Figs. 4a and 4b. The moisture 440 evaporation rate of samples dried under SS was higher than under HA at temperatures beyond the inversion point. Below the inversion point, faster moisture evaporation in HA can be attributed to a higher heat transfer coefficient and greater difference in temperature between the surface 445 of the sample and the drying medium. [29] When temperature exceeded the inversion point, heat flux and, consequently, moisture evaporation rate were higher in SS than in HA. Hot air drying at temperatures close to 160 C can produce a case-hardening effect, namely the formation of hard skin on the surface of a dried product. [35] The changes inside the sample and on its surface most probably slowed drying and prevented moisture from evaporating, which resulted in longer drying times. In several studies where porous foods were subjected to SS and HA drying, inversion temperature was determined in the range of 125 and 160 C. [30,36] SS and HA drying resulted in significantly higher initial drying rates of CS than WDG. The differences in the structure of the examined stillage fractions are shown in the images of corn=wheat-based WDG and CS (Fig. 5). WDG consisted of particulates from different structural parts of kernels, and their surface structures comprised

9 8 ZIELINSKA FIG. 5. The photographs of (a) corn=wheat wet distillers grains (WDG) and (b) corn=wheat centrifuged solubles (CS). flakes (mostly from tip caps and broken seed coats of kernels) and granules (from ground endosperm and germ) of varied size, shape, and composition. CS formed by a 465 diluted aqueous suspension of particulate solids were smoother and featureless. Effect of SS and HA Drying on Changes in Crude Protein and Amino Acid Content of Corn/Wheat WDG and CS The present study evaluated the influence of various dry- 470 ing media and drying temperatures on the crude protein and amino acid content of corn=wheat WDG and CS (Table 3). A minor decrease (up to 7%) in crude protein concentrations was observed during SS and HA drying of corn=wheat WDG and CS (Table 3). Similar results were 475 reported for SS and HA drying of wheat WDG. [41] In the present study, crude protein levels were significantly higher in corn=wheat DDS than corn=wheat DDG. The crude protein content of corn=wheat DDG varied from to % db, whereas crude protein 480 content of corn=wheat DDS ranged from to % db (Table 4). The content of selected amino acids decreased considerably during SS and HA drying due to prolonged exposure to high temperatures. The exposure of WDG and CS to 485 heat could produce the Maillard reaction, which reduces amino acid levels. The amino acids that were least susceptible to drying were leucine, tyrosine, phenylalanine, alanine, glutamic acid, and proline. Lysine was particularly susceptible to the Maillard reaction, which could take place 490 under rigorous drying conditions (Table 3). Glutamic acid, leucine, and proline were the most abundant amino acids, whereas methionine, cystine, lysine, and histidine were the limiting amino acids of corn=wheat DDG and DDS (Table 4). Subject to the dried material and the applied dry- 495 ing conditions, lysine loss during corn=wheat WDG and CS drying ranged between 8 and 50%. At drying temperature of 110 C, more lysine was lost when WDG and CS were exposed to SS than HA. The lysine content of WDG decreased by approximately 29% under exposure to SS and by approximately 15% under exposure to HA (Table 3). The above can be attributed to the fact that drying at temperatures close to the boiling point of water prolongs drying times and significantly decreases drying rates in SS in comparison with HA. When SS temperature exceeded the inversion temperature and when the drying rate under exposure to SS exceeded that reported under exposure to HA, the changes in the lysine content of WDG and CS were less profound under SS than under HA drying conditions. Lysine loss was determined at 35% when CS was dried at SS temperature of 160 C, and at 50% when CS was exposed to HA. Shorter time of exposure to higher drying temperature and the absence of hot air during drying could have minimized the Maillard reaction and increased the lysine content of SS dried samples. Similar results were reported in the literature. [24,56] The higher the temperature of SS and HA, the greater the decrease in the lysine content of WDG and CS. Most likely, the increase in SS and HA temperature enhanced the rate of the Maillard reaction. [56] When CS was dried in HA, lysine loss was only 8% at 110 C, but it reached 50% when temperature was increased to 160 C. Higher drying rates and=or shorter drying times during CS dehydration resulted in lower lysine loss than during WDG drying. The exposure to SS at a temperature of 160 C led to 45% lysine loss in WDG and 36% loss in CS (Table 3). Lysine content, expressed as the percentage of feedstuff (% db), ranged from to % db in corn=wheat DDG and from to % db in corn=wheat DDS. The percentage content of crude protein (% CP) is an indicator of lysine quality in dried distillers co-products. [17] CP values above 2.80% are indicative of high quality, whereas lower values testify to low quality

10 DRYING KINETOCS OF CORN=WHEAT DISTILLERS GRAINS 9 TABLE 3 The percentage change in the amino acid contents of corn=wheat wet distillers grains (WDG) and centrifuged solubles (CS) during different superheated steam (SS) and hot air (HA) drying WDG WDG CS CS Attributes SS 110 C HA 110 C SS 160 C HA 160 C SS 110 C HA 110 C SS 160 C HA 160 C Crude Protein 2 1 aa 1 0 aa 1 1 ab 2 0 ab 2 0 aa 0 1 ba 6 0 ab 7 0 bb Lysine 29 0 aa 15 3 ba 45 2 ab 48 1 ab 14 4 aa 8 4 aa 36 1 ab 50 2 bb Aspartic Acid 10 8 aa 4 15 aa 12 9 aa 7 5 aa 6 8 aa 9 5 aa 4 8 aa 5 5 ab Threonine 7 1 aa 5 5 aa 11 3 aa 7 0 aa 7 4 aa 9 6 aa 4 0 ab 8 5 ab Serine 5 0 aa 2 7 aa 10 3 ab 7 0 aa 4 3 aa 6 5 aa 7 1 ab 9 0 ab Glutamic Acid 2 4 aa 3 4 aa 8 0 ab 9 5 aa 4 4 aa 2 5 aa 13 1 ab 14 2 aa Proline 3 3 aa 6 2 aa 7 2 aa 4 2 aa 0 4 aa 3 4 aa 12 2 ab 12 2 ab Glycine 8 2 aa 7 7 aa 4 5 aa 4 1 aa 1 2 aa 5 5 aa 4 1 ab 8 7 ab Alanine 2 1 aa 1 5 aa 4 4 aa 3 2 ba 14 4 aa 18 6 aa 10 0 aa 7 1 bb Cystine 0 29 aa 5 31 aa 19 1 aa 5 15 ba 1 6 aa 6 3 aa 28 2 ab 32 3 ab Valine 1 3 aa 1 1 aa 1 1 aa 3 5 aa 5 4 aa 8 6 aa 3 2 ab 5 3 ab Methionine 4 27 aa 7 32 aa aa 1 10 aa 3 4 aa 8 4 aa 14 2 ab ab Isoleucine 1 4 aa 1 2 aa 3 0 aa 5 6 aa 7 8 aa 6 6 aa 6 1 ab 10 3 ab Leucine 2 4 aa 4 0 aa 0 0 aa 6 4 ba 12 3 aa 15 6 aa 5 1 ab 7 3 ab Tyrosine 3 4 aa 9 3 aa 8 1 aa 8 7 aa 7 4 aa 11 5 aa 0 2 ab 3 2 ab Phenyloalanine 1 4 aa 4 1 aa 4 1 aa 6 6 aa 4 5 aa 7 5 aa 3 1 ab 6 2 ab Histidine 5 3 aa 3 2 aa 9 2 aa 6 1 ba 4 1 aa 7 4 aa 3 1 ab 11 1 bb Arginine 8 2 aa 1 1 ba 10 2 ab 16 1 bb 2 6 aa 4 4 aa 8 1 aa 18 4 bb Mean standard error of the mean of a set of observations are given in the table. Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between successive pairs of samples dried by different drying media: (1,2) WDG dried by SS and HA at 110 C; (3,4) WDG dried by SS and HA at 160 C; (5,6) CS dried by SS and HA at 110 C; (7,8) CS dried by SS and HA at 160 C. Different superscripts A,B indicate significant differences (p < 0.05) between successive pairs of samples dried at different drying temperatures: (1,3) WDG dried by SS at 110 and 160 C; (2,4) WDG dried by HA at 110 and 160 C; (5,7) CS dried by SS at 110 and 160 C; (6,8) CS dried by HA at 110 and 160 C. of lysine in corn-based DDGS. [51] The lysine content of wheat-based DDGS has been estimated in very few studies, 535 many of which were conducted in old-generation plants that may not be representative of the quality of contemporary products. [57] In the present study, the lysine content of corn=wheat DDG and DDS ranged from to % CP, and from to % 540 CP, respectively. Effect of SS and HA Drying on Changes in the Color of Corn/Wheat WDG and CS The effect of different drying media and drying temperatures on total changes in color (DE ), saturation (DC ), and 545 hue (DH ) of corn=wheat WDG and CS during SS and HA drying at 110, 130, and 160 C is shown in Table 5. Total changes in the color (DE ) of corn=wheat WDG and CS during SS and HA drying ranged from to and from to , respectively. According to the criterion introduced by the International Commission on Illumination (CIE), those numbers indicate significant (noticeable to even inexperienced observers) changes in the color of corn=wheat WDG and CS under different SS and HA drying conditions. Significant changes in the saturation (DC ) of corn=wheat WDG and CS were also observed during SS and HA drying, ranging from to and from to , respectively. The changes in the hue (DH )of corn=wheat WDG and CS during SS and HA drying ranged from to and from to , respectively. The changes in color, hue, and saturation can be attributed mostly to changes in lightness (L ) and yellowness (b ) of samples, rather than to changes in redness (a ). Maillard reactions between reducing sugars and amino acids could occur during rigorous SS and HA

11 TABLE 4 Crude protein (CP) and amino acid (AA) contents of corn=wheat distillers dried grains (DDG) and distillers dried solubles (DDS) dehydrated with superheated steam (SS) and hot air (HA) Attributes DDG SS 110 C DDS SS 110 C DDG HA 110 C DDS HA 110 C DDG SS 160 C DDS SS 160 C DDG HA 160 C DDS HA 160 C Crude Protein, % db a b a b a b a b Lysine, % db a b a b a b a b Lysine, % CP a a a a a a a a a Aspartic Acid, % db a b a b a b a b Threonine, % db a b a b a b a b Serine, % db a b a b a b a b Glutamic Acid, % db a b a b a b a b Proline, % db a b a b a b a b Glycine, % db a b a b a b a b Alanine, % db a b a b a b a b Cystine, % db a b a b a a a a Valine, % db a b a b a b a b Methionine, % db a b a b a a a a Isoleucine, % db a b a b a b a b Leucine, % db a b a b a b a b Tyrosine, % db a b a b a b a b Phenylalanine, % db a b a b a b a b Histidine, % db a b a b a b a a Arginine, % db a b a b a b a b Mean standard error of the mean of a set of observations are given in the table. Crude protein and amino acid contents are expressed as the percentage values of feedstuff (% db); additionally, lysine content is expressed as a percentage value of crude protein (% CP). Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between successive pairs of samples: (1,2) DDG and DDS dried by SS at 110 C; (3,4) DDG and DDS dried by HA at 110 C; (5,6) DDG and DDS dried by SS at 160 C; (7,8) DDG and DDS dried by HA at 160 C. 10

12 TABLE 5 The total changes in color (DE ), saturation (DC ), and hue (DH ) of corn=wheat wet distillers grains (WDG) and centrifuged solubles (CS) during superheated steam (SS) and hot air (HA) drying at the temperatures of 110, 130, and 160 C WDG WDG WDG CS CS CS Color Indices SS 110 C HA 110 C SS 130 C HA 130 C SS 160 C HA 160 C SS 110 C HA 110 C SS 130 C HA 130 C SS 160 C HA 160 C DE, DC, aa aa ac bb ab ab aa ba ab bb ab bc aa aa ab ab ab ab aa ba ab bb ab bc DH, aa aa ab ba aa aa aa ba ab bb ab bb Mean standard error of the mean of a set of observations are given in the table. Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between successive pairs of samples dried by different drying media: (1,2) WDG dried by SS and HA at 110 C; (3,4) WDG dried by SS and HA at 130 C; (5,6) WDG dried by SS and HA at 160 C; (7,8) CS dried by SS and HA at 110 C; (9,10) CS dried by SS and HA at 130 C; (11,12) CS dried by SS and HA at 160 C. Different superscripts A,B,C indicate significant differences (p < 0.05) between three samples dried at different drying temperatures: (1,3,5) WDG dried by SS at 110, 130, and 160 C; (2,4,6) WDG dried by HA at 110, 130, and 160 C (7,9,11) CS dried by SS at 110, 130, and 160 C; (8,10,12) CS dried by HA at 110, 130, and 160 C. 11

13 TABLE 6 Color attributes (L white=black, a red=green, b blue=yellow) of corn=wheat distillers dried grains (DDG) and distillers dried solubles (DDS) dehydrated with superheated steam (SS) and hot air (HA) at the temperatures of 110, 130, and 160 C Color Attributes DDG DDS DDG DDS DDG DDS DDG DDS DDG DDS DDG DDS SS 110 C HA 110 C SS 130 C HA 130 C SS 160 C HA 160 C L, a b a a a a a b a b a a a, a a a b a a a b a b a b b, a b a a a a a b a b a a Mean standard error of the mean of a set of observations are given in the table. Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between successive pairs of samples: (1,2) DDG and DDS dried by SS at 110 C; (3,4) DDG and DDS dried by HA at 110 C; (5,6) DDG and DDS dried by SS at 130 C; (7,8) DDG and DDS dried by HA at 130 C; (9,10) DDG and DDS dried by SS at 160 C; (11,12) DDG and DDS dried by HA at 160 C. 12

14 TABLE 7 Coefficients A, B, R, and R 2 of Eq. (5) showing the relationships between the total lysine content and color attributes of corn=wheat distillers co-products Coefficients of Eq. (5) WDG, SS DDG CS, SS DDS WDG, HA DDG CS, HA DDS Lys - L Coefficient A aa aa aa aa Coefficient B aa aa aa bb Correlation Coefficient R Determination Coefficient R Lys - b Coefficient A aa aa aa aa Coefficient B aa ba aa bb Correlation Coefficient R Determination Coefficient R Mean standard error of the mean of a set of observations are given in the table. Explanation of the column headings: The Lys-L and Lys-b relationships were derived from the lysine content and color parameters (L and b ) of corn=wheat wet distillers grains (WDG), centrifuged solubles (CS), distillers dried grains (DDG), and distillers dried solubles (DDS) dehydrated with superheated steam (SS) and hot air (HA) under 110, 130, 160 C. Different superscripts a,b in two adjacent columns indicate significant differences (p < 0.05) between successive pairs of samples: (1,2) distillers grains and solubles dried by SS at 110, 130, and 160 C; (3,4) distillers grains and solubles dried by HA at 110, 130, and 160 C. Different superscripts A,B indicate significant differences (p < 0.05) between the samples dried by different drying medium: (1,3) distillers grains dried by SS and HA at 110, 130, and 160 C; (2,4) distillers solubles dried by SS and HA at 110, 130, and 160 C. 13

15 14 ZIELINSKA drying of corn=wheat WDG and CS, which causes darkening of the dried samples. [58,59] The free amino group required for the Maillard reaction came from lysine residue in protein, whereas reducing sugars were provided by 570 starch that was degraded during corn and wheat processing. Lower concentrations of heat-damaged xanthophyll pigments could reduce product yellowness. [60,61] When dried under identical conditions, more profound changes in color, saturation, and hue were observed in SC than in 575 WDG. A significantly greater reduction in yellowness (b ) of samples was observed during SS and HA drying of CS than WDG. The reason for this could be more intense degradation of xanthophyll pigments (zeaxanthin and lutein) present in the form of pigment-protein complexes 580 or as direct components of crude fat. [61] The changes in the color, saturation, and hue of WDG and CS were more profound under SS than HA drying conditions. The observed differences were determined mostly by temperature and product residence time in the dryer. At lower tem- 585 peratures (110 and 130 C), drying times were longer under exposure to SS than HA, which probably resulted in more intensive Maillard reactions and greater pigment degradation. When WDG and CS were dried at 160 C, color degradation was more severe under SS than HA drying 590 conditions, probably due to sample burning under exposure to the high temperature environment of SS. The changes in the color, saturation, and hue of WDG and CS were significantly more pronounced under exposure to higher drying temperatures of 160 C and 130 C than C. As the drying temperature increased, the color of the dried samples shifted toward less yellow and darker regions. Significant differences in the color attributes of corn= wheat DDG and DDS exposed to SS and HA at 110, , and 160 C are shown in Table 6. The L values of DDG and DDS ranged from to and from to , respectively. The a values were positive, and the redness of DDG and DDS ranged from to and from to , respectively. The b values were all positive, with the exception of samples dried under SS at 130 and 160 C. The yellowness (b ) of DDG and DDS ranged from to and from to , respectively. 610 Correlation Between Lysine Content and Color of Corn/ Wheat Distillers Co-Products The present study was conducted to check for correlations between lysine content and selected color attributes of corn=wheat distillers co-products dried with SS and 615 HA. Total lysine content and lightness (L )ofcorn=wheat distillers co-products were positively correlated with R values of The values of R 2 between and indicated a good fit of Eq. (5) to empirical data. Total lysine content and yellowness (b ) of corn=wheat distillers co-products were also positively correlated with R values of and High values of R 2 between and also pointed to a good fit of Eq. (5) to empirical data. The stillage fraction and the drying medium did not exert a significant influence on the value of coefficient A in Eq. (5), but they had a significant impact on the values of coefficient B in Eq. (5) (Table 7). Lightness (L ) and yellowness (b ) emerged as reliable predictors of the lysine content of corn=wheat distillers co-products, where dark-colored samples contained less and light-colored samples contained more lysine. Similar results were reported for corn [54] and wheat [25] distillers co-products by other authors. CONCLUSIONS The present study demonstrated the effect of SS and HA drying on drying kinetics and qualitative changes in corn= wheat WDG and CS. The drying times of WDG and CS under exposure to SS and HA ranged from 40 to 240 minutes, and the initial drying rates of WDG and CS ranged from to =min. An inversion temperature was reached at 139 C for WDG and 132 C for CS. SS and HA drying led to a significant decrease in the lysine content of WDG and CS, which ranged from 8% to 50%. The lysine content of DDG and DDS ranged from to and from to % CP, respectively. The total change in the color (DE ) of corn=wheat WDG and CS during SS and HA drying ranged from to and from to , respectively, and it was induced mainly by changes in L and b. The L values of DDG and DDS ranged from to and from to , respectively. The a values of DDG and DDS ranged from to and from to , respectively. Yellowness (b )of DDG and DDS ranged from to and from to , respectively. The lightness (L ) and yellowness (b ) of the samples were positively correlated with the lysine content of dried samples. This study explores the potential use of SS dryers in industrial dehydration of distillers co-products, and its results constitute valuable input for manufacturers of drying systems, ethanol plants, and animal feed suppliers. ACKNOWLEDGMENTS The author would like to thank Stefan Cenkowski, of the University of Manitoba, for making his laboratory available for the needs of the project; Malgorzata Stasiewicz, of the University of Warmia and Mazury in Olsztyn, for providing technical support and performing chemical analyses of amino acids; and the local distillery of Mohawk Canada Limited, Minnedosa, MB, Canada, for donating stillage