Determination of Water Separation from Cooked Starch and Flour Pastes after Refrigeration and Freeze-thaw

Transcription

1 ENGINEERING/PROCESSING Determination of Water Separation from Cooked Starch and Flour Pastes after Refrigeration and Freeze-thaw G. H. ZHENG and F. W. SOSULSKI ABSTRACT A procedure for determination of net syneresis in starch and flour materials was developed based on free water in freshly cooked pastes, syneresis water expelled during cold storage and absorbed water in the contracted gel. Free water values decreased from 50% to <5%, as starch concentrations increased from 4 to 10%, indicating the need for correction of expelled+absorbed water values from free water for calculating net syneresis. Normal cereal, root and dicotyledonous seed starches exhibited a broad range of expelled water values during refrigeration, but gave high absorbed water values during freeze-thaw cycles. Waxy starches had low net syneresis during refrigeration and, except waxy corn, during freeze-thaw. For comprehensive evaluation, two refrigeration and two freeze-thaw cycles are recommended. Key Words: free water, expelled water, absorbed water, net syneresis, starch INTRODUCTION STARCH IS ONE OF THE MOST ABUNDANT AND WIDELY DISTRIButed components in foodstuffs. After starch granules are gelatinized by cooking in the presence of water, the molecules reassociate during cooling into gel-like structures that affect the functional and sensory properties of foods. During cold storage, the reorganization of starch molecules may result in release of water (or syneresis) to adversely affect functional properties. Schoch (1968) compared tests for evaluating the cold storage stability and freeze resistance of several native waxy and modified food starches. The rate of water syneresis after freeze-thaw treatments correlated well with prolonged refrigeration tests and with quality evaluation of frozen starch-thickened foods. A technique for measuring syneresis of pasted starches using repeated freeze-thaw treatments was proposed for evaluation of cold storage stability of starchy materials. Although the technique was developed for cold-resistant starches, it has been used on a wide range of starches (Hood and Seifried, 1974; Hoover and Sosulski, 1985; Wu and Seib, 1990; Yanez et al., 1991; Kim et al., 1993). Published syneresis data has shown a broad range of values for a given material, reflecting the unreliability of data from this technique. Alternative methods of determining storage stability (White et al., 1989) or measuring syneresis (Dreher et al., 1983; Hoover et al., 1991; Ferrero et al., 1993) have been reported, but no direct comparisons have been made among such methods. A principal variable in syneresis tests has been the lack of consistency in initial water-to-starch/flour ratios used to prepare the cooked pastes. Widely different water-to-starch/flour ratios are used in various food applications. However, information is needed on the effects of such ratios or free water in cooked starch pastes on syneresis test values as well as on food characteristics. Schoch (1968) proposed that higher concentrations resulted in more rapid reassociation of macromolecules in cooked starch pastes. Kim et al. (1993) established that degree of syneresis was negatively related to starch concentration. Author Zheng is affiliated with the Dept. of Applied Microbiology & Food Science, and author Sosulski is affiliated with the Dept. of Crop Science & Plant Ecology, Univ. of Saskatchewan, 51 Campus Drive, Saskatoon, SK, S7N 5A8, Canada. The chemical definition of syneresis is the spontaneous separation of a liquid from a gel or colloidal suspension due to contraction of the gel (Lapedes, 1978). However, syneresis tests using centrifugation, as proposed by Schoch (1968), would measure the total of expelled water plus water absorbed in the gel. In food systems, however, separated or expelled water would have a different influence on functionality or quality assessment than would absorbed water (Pomeranz, 1985). Our objective was to develop methodology for measurement of net syneresis by quantitating the free water in freshly-cooked pastes, syneresis water that is expelled from the gel during cold storage and the absorbed water in the contracted gel. The three-stage method was compared to conventional centrifugation and suction filtration on samples of starches and flour pastes. MATERIALS & METHODS Starch and flour materials Normal and waxy corn starches were obtained from Cerestar USA (Hammond, IN), potato starch from Sigma Chemical Co. (St. Louis, MO), HRS wheat flour and durum wheat semolina from CSP Food Inc. (Saskatoon, SK). The semolina was reground to pass a 60-mesh sieve. Refined starches were extracted from seeds or flours of barley, wheat, oat, normal and waxy rice, canaryseed, buckwheat, field pea, tapioca, arrowroot, and waxy amaranth in the laboratory by steeping, grinding, washing with 0.2% NaOH, neutralization and drying at 40 C (Hoover and Sosulski, 1985). Oat, canaryseed and waxy amaranth starches were further defatted using hot propanol-water (3:1, v/v) at 80 C for 4h (Gibinski et al., 1993). Amylose contents were determined colorimetrically by the method of Williams et al.. (1970), although alternative methods have been reported ( Sievert and Holm, 1993; Gibson et al., 1997). Paste preparation Aqueous starch suspensions of commercial samples (normal and waxy corn, and potato starches) had a ph ranging from 5.5 to 6.3 while those isolated in the laboratory from 6.0 to 6.8, therefore, all starch slurries were adjusted to ph 6.5 with 0.01M NaOH or 0.01M HCl solutions prior to cooking. The ph of the flours was not adjusted. It was 5.7 for bread wheat and 5.5 for durum flours. The slurries (150 g), 8% d.b. (w/w) for starches or 10% d.b. (w/w) for flours, were contained in sealed 400-mL stainless steel chambers equipped with a mixing blade, and heated in a boiling water bath for 10 min with a mixing speed of 1,000 rpm using a Omni-Mixer (Ivan Sorvall Inc., Norwall, CT). Sodium benzoate (0.15g) was added to the 150g paste sample after cooking to prevent microbial activity during repeated refrigeration or freeze-thaw treatments. The pastes were cooled to room temperature ( 23 C) before free water in the fresh pastes was determined. Each paste was divided into two 60g samples contained in 70-mL bottles, which were sealed with screw lids, and stored at either 4 C (refrigeration) or -18 C (freezer). Terminology The terminology used throughout this paper needs to be defined at the outset to prevent confusion. These definitions were developed and apply to all results and data: Free water was the water separated by centrifugation from a 134 JOURNAL OF FOOD SCIENCE Volume 63, No. 1, 1998

2 freshly prepared paste. Expelled water was the amount released from a paste after refrigeration or freeze-thaw treatment, and it was removed by decantation and measured. Absorbed water was the water removed by centrifugation after removal of expelled water. Net syneresis was calculated from the formula: net syneresis = (expelled water + absorbed water) free water. Refrigeration The bottles containing starch or flour pastes were stored in a refrigerator at 4 C for 1 wk, then held at room temperature for 6h before measurement of expelled and absorbed water. This single refrigerated storage cycle was repeated four times for all samples. Freeze-thaw treatment Freeze-thaw treatments were performed by storing the pastes in the freezer for 16h, then thawing at 40 C for 2h. After determination of expelled and absorbed water, the gels were re-frozen and then thawed to repeat the cycle. After subjecting normal and waxy corn starches and wheat flour to zero, one and two freeze-thaw cycles in a preliminary trial, all starch and flour samples were treated with four freeze-thaw cycles. Conventional measurement of syneresis Conventional methods of centrifugation (Schoch, 1968) or suction filtration (Dreher et al., 1983) were applied to determine water separation in freshly cooked and cooled (zero cycle) or freeze-thawed (one or two cycles) pastes of normal and waxy corn starches, and wheat flour. For centrifugation, about 25g starch or flour paste, contained in a 35-mL polycarbonate conical centrifuge tube, was centrifuged at 3030 g (5000 rpm) for 10 min. The amount of water released from the pastes was decanted, weighed and calculated as a percentage of the original weight. For suction filtration, about 25g starch or flour pastes or gels contained in a 60-mL Pyrex Buchner funnel with a fritted disc were placed under -550 mm/hg at 23 C for 5 min. The surfaces of freezethawed corn starch and wheat flour gels were covered with a single layer of Whatman #4 filter paper (4 cm dia) prior to suction; the filter paper was removed after filtration. Weights of the empty funnel and funnel plus gel before and after filtration were recorded for calculating the percentage of water removal by the suction and absorbed by the filter paper. Determination of free water Fresh pastes of normal and waxy corn starches, and wheat flour, were prepared as described for 4, 6, 8 and 10 % d.b. (w/w) slurries. Free water contents of the pastes were determined in 5-cc B-D Plastipak Syringes (Becton Dickinson & Co., Rutherford, NJ), as shown (Fig. 1). The Luer-Lok tips and plunger of the syringes were removed before use. A single piece of Whatman #114 filter paper (11 mm diam) was placed on the bottom of the syringes; g Celite (diatomaceous silica, acid-washed but not further calcined, Sigma Chemical Co., St. Louis, MO) was then added on top of the filter paper. Deionized water was added to the syringes under vacuum to moisten the Celite, and then the Celite layer was pounded with a glass rod to ensure a complete seal at the bottom of the syringe. The syringes were placed in mm polypropylene copolymer centrifuge tubes (Nalge Company, Rochester, NY) (Fig. 1), and then centrifuged at 3030 g (5000 rpm) for 10 min to remove surplus water from the Celite, after which the syringe was weighed (wt 0 ). Two to 2.5g samples were added to each syringe, and the syringe was weighed (wt 1 ) again. The syringe was then centrifuged at 3030 g (5000 rpm) for 10 min before final weighing (wt 2 ). Percent free water was calculated as: free water %=[(wt 1 -wt 2 )/(wt 1 -wt 0 )] 100 Subsequently, the slurry concentration of 8% d.b. (w/w) was adopted for all starch and 10% d.b. (w/w) for flour samples. Free water determinations were then made on fresh pastes for each sample. Determination of expelled and absorbed water Water expelled by the gels after the first refrigeration or freezethaw cycle was decanted from the bottles, weighed and calculated as percent of paste weight before determination of absorbed water by centrifugation. Expelled water after the second cycle of treatment was calculated as percent of remaining gel weight, and cumulative values of expelled water were recorded for each sample. After each cycle, the gels were also sampled for measurement of absorbed water. The g samples were placed in the 5-cc B-D Plastipak Syringes as described for measuring free water. However, the Celite layer was replaced by g of Padco Cotton (nonsterile, Acco Absorbent Cotton Co., Valley Park, MO) that was packed tightly onto the bottom of the syringe with a glass rod. After weighing and centrifuging the sample at 3030 g for 10 min, and reweighing the sample and syringe, the absorbed water in the gels was calculated by the same formula as for free water. Certain gels such as those from stored waxy starches tended to pass through the cotton packing. In those samples, Celite packing on filter paper, as described for free water determination, was effective in retaining the gel while removing absorbed water from the gel and syringe during centrifugation. Effect of centrifugal force Fresh normal corn starch paste, and normal corn starch gels after 1 wk refrigeration or two freeze-thaw cycles, and waxy amaranth starch gel after six freeze-thaw cycles were subjected to centrifugal forces of 480, 1090, 1940, 3030, 4300, 7740 and g (corresponding to 2000, 3000, 4000, 5000, 6000, 8000 and rpm) for 10 min. Previously, centrifugation times of 5, 10 and 15 min were compared at each centrifugal force and 10 min gave the same values as 15 min except for the two lowest centrifugal forces. Therefore, the centrifugation time of 10 min was adopted for all determinations. Calculations Net syneresis was calculated by subtracting free water from the sum of expelled and absorbed water after each refrigeration or freezethaw treatment. Each measurement was replicated four times for calculation of standard deviations (Fig. 2, 3; Table 1), or two times (Tables 2, 3) for LSD analysis using the SAS GLM procedure (SAS Institute, Inc., 1987). (Continued on next page) Fig. 1. Apparatus for centrifugation of cooked starch or flour samples. Volume 63, No. 1, 1998 JOURNAL OF FOOD SCIENCE 135

3 Water Separation from Starch and Flour Pastes... Table 1 Percent free water in freshly cooked pastes, or expelled and absorbed water after one and two freeze-thaw cycles of three starchy materials as determined by conventional centrifugation (CC), suction filtration (SF) or net syneresis (NS) procedures a Determination of Freeze- Waxy corn starch Wheat flour Normal corn starch water separation thaw cycles (8%, d. b., w/w) (10%, d. b., w/w) (8%, d. b., w/w) CC SF NS CC SF NS CC SF NS Free Water 0 0.1± ± ± ± ± ± ± ±1.0 Expelled +Absorbed Water b ± ± ± ± ± ± ± ± ±1.4 Net Syneresis c Expelled +Absorbed Water b ± ± ± ± ± ± ± ± ±1.5 Net Syneresis c a Mean±SD, n=4. b Expelled and absorbed water was determined simultaneously by CC and SF, but separately by NS. c Net syneresis=(expelled + absorbed water)-free water. RESULTS & DISCUSSION Slurry concentration When determined by centrifugation in syringes, free water in freshly cooked pastes of normal corn starch was very high, >20%, for slurry concentrations of 4 and 6 % but decreased to 5 6% in the 8 and 10% slurries (Fig. 2a). Wheat flour pastes exhibited higher level of free water for 6 and 8% slurries than normal corn starch. Free water separated spontaneously from 4% wheat flour when the paste was cooled to room temperature. The waxy corn starch pastes showed much lower levels of free water, <10%, in the 4 and 6% slurries. However, the free water percentages decreased to the same level as the normal corn starch and wheat flour in the 8 and 10% slurries. Further studies are needed to explain the similarities in free water levels between the 8 and 10% slurries of these different samples. From these results, it appeared that use of 8% slurries of starch and 10% of flour samples would minimize the levels of free water in gels for determination of net syneresis after storage stability tests. In addition, free water analysis was conducted on all samples to remove any bias to syneresis measurements. It also showed that this measurement would enable use of more dilute slurries in functional tests for certain food applications. In contrast to the small increases in free water contents (Fig. 2a), the total water separation of expelled + absorbed water from waxy starch gel after one freeze-thaw cycle or 1 wk refrigeration increased with decreased slurry concentrations (Fig. 2b). At a slurry concentration of 8%, the total water separation of expelled + absorbed water for waxy corn starch after cold storage was 5 6% of paste weight (Fig. 2b). Since the free water constituted 4.6% of paste weight (Table 1), the net syneresis was 1 2% (Fig. 2c). However, the more dilute slurry at 6% gave a total water separation of 10 15% (Fig. 2b). At the 4% slurry concentration, the total water separation from waxy corn starch gel after cold storage rose markedly to 38 50% and net syneresis to 33 41%, suggesting a breakdown in water-holding capacity in very dilute slurries. Total water separation from normal corn starch and wheat flour gels after either 1 freeze-thaw cycle or 1 wk refrigeration correlated negatively (r 2 =0.87 to 0.97) with slurry concentration (Fig. 2b). For 4 and 6% normal corn starch gels, similar amounts of total water separation were obtained after one freeze-thaw cycle and 1 wk refrigeration. At 8 and 10%, however, the freeze-thaw treatment resulted in more total water separation than did refrigeration. Net syneresis of normal corn starch gels after one freeze-thaw cycle showed an increase with the increase of slurry concentration to 8%, then the net syneresis decreased at 10% concentration (Fig. 2c). Normal corn starch gels after 1 wk refrigeration exhibited a similar pattern of net syneresis in relation to slurry concentration as that after 1 freezethaw cycle but to a lesser extent (Fig. 2c). Wheat flour gels after one freeze-thaw cycle exhibited 40 to 58% water separation but after 1 wk refrigeration they gave 4 to 33% total water separation for 10 to 6% slurries (Fig. 2b). Net syneresis of wheat flour gels increased from 18% of gel weight for 6% slurry to 36% of gel weight for 8 and 10% slurries after 1 freeze-thaw cycle. However, net syneresis after 1 wk refrigeration decreased from 4 % of gel weight for 6% slurry to 1% of gel weight for 8 and 10% slurries (Fig. 2c). These results demonstrated that free water in the freshly cooked pastes, and slurry concentration had considerable influence on the estimation of syneresis after cold storage. Also, some material (e.g. wheat flour) may give different results after freeze-thaw treatment compared to refrigeration. Therefore, the storage stabilities of a broad range of starchy materials were assessed in both repeated freeze-thaw and refrigeration tests. Centrifugal force Increasing centrifugal force from 480 to g for freshly cooked corn starch paste or waxy amaranth starch gel after six freezethaw cycles resulted in small but progressive increases in total water separation (Fig. 3). The increase from <5 to 15% indicated the need Fig. 2 Effect of slurry concentration on free water in the freshly cooked pastes (a), total expelled+absorbed water separation (b), and net syneresis (c) from the gels after one freeze-thaw cycle (open symbols) or 1 wk refrigeration (solid symbols) for wheat flour (, ), normal (, ), and waxy (, ) corn starches. Vertical bars (a, b) indicate standard deviations (n=4). 136 JOURNAL OF FOOD SCIENCE Volume 63, No. 1, 1998

4 Table 2 Composition of free water in freshly cooked pastes, and syneresis of expelled and absorbed water from starch and flour gels during 4 weekly cycles of refrigeration Materials by class Amylose Free water Expelled water a Absorbed water Net syneresis b % d.b. % gel wt % gel wt % gel wt % gel wt Cycles Cycles Cycles Fresh pastes Waxy starches Amaranth Rice Corn Cereal Starches Barley Wheat Oat Normal Rice Canaryseed Normal Corn Root and Tuber Starches Tapioca Potato Arrowroot Dicotyledon and Legume Starches Buckwheat Field Pea Wheat Flours Durum Bread LSD (p<0.05) acumulative values. bnet syneresis=(expelled water+absorbed water)-free water. to standardize the applied centrifugal force. Eliasson and Kim (1992) had reported that centrifugal force affected the syneresis values for a hydroxypropylated potato starch, suggesting the necessity for standardizing this procedure. For samples that exhibited high levels of water separation such as freeze-thawed normal corn starch, 50% water separation was detected at the relatively low centrifugal force of 480 g, and the maximum total water separation was recorded at 3030 g. Similarly, normal corn starch gel after one refrigeration cycle exhibited the near maximum absorbed water separation at 3030 g. Therefore the centrifugal force of 3030 g for 10 min was adopted for absorbed water determinations. Fig. 3 Effect of centrifugal force on the separation of free water from fresh pastes ( ), or absorbed water from refrigerated ( ) and freeze-thawed ( ) gels of normal corn starch (,, ) and waxy amaranth starch ( ). Vertical bars indicate standard deviations (n=4). The dashed vertical line identifies the centrifugal force of 3030 g. Conventional vs net syneresis analysis The proposed net syneresis (NS) procedure of measuring free, expelled and absorbed syneresis water in cooked starch pastes and stored gels was evaluated by comparing results with conventional centrifugation (CC) and suction filtration (SF) methods in three diverse materials, normal and waxy corn starches and wheat flour (Table 1). The samples showed an identical order of net syneresis after each treatment for the 3 methods. However, the CC and SF procedures did not show the presence of free water in the freshly cooked pastes (zero freeze-thaw cycles) whereas % free water was removed by the centrifuged syringe procedure. The measurement of expelled and absorbed syneresis water after freeze-thaw treatment was determined as a combined value by the CC and SF procedures. For comparison, the two values from the NS procedure were combined (Table 1). After 1 freeze-thaw cycle, the expelled + absorbed syneresis values were much lower for waxy starch than for normal corn starch while wheat flour gave intermediate values. For wheat flour and normal corn starch, the SF values exceeded those obtained by CC, but the NS technique gave values that were 79% and 42%, respectively, higher than SF values. Hoover and Sosulski (1985) had reported 26% syneresis for normal corn starch after 1 freeze-thaw cycle and analysis by the CC method. Hoover et al. (1991), using SF, obtained 50% syneresis for a similar corn starch sample after 1 freeze-thaw cycle. For waxy corn starch gel, the second freeze-thaw cycle caused a fivefold increase in CC and SF values for expelled+absorbed syneresis water while that of NS increased tenfold (Table 1). This suggested that repeated freeze-thaw cycles were needed to fully characterize starches such as waxy corn starch. After freeze-thaw, the increases in syneresis for wheat flour and normal corn starch were substantial but of a lesser order. In each case, the CC values were lowest and NS values highest. Additional water could be pressed manually from the spongy gels after the determination of water separations by CC and SF techniques. Also, the NS procedure was considered more reliable than the CC and SF because the standard deviations were smaller. However, the major benefit of the NS technique was the more accurate and comprehensive description of the stability of starches during storage, irrespective of starch-to-water ratio (Fig. 2). Characteristics of starches The net syneresis properties of 14 starch sources and two flours Volume 63, No. 1, 1998 JOURNAL OF FOOD SCIENCE 137

5 Water Separation from Starch and Flour Pastes... Table 3 Composition of free water in freshly cooked pastes, and syneresis of expelled and absorbed water from starch and flour gels during 4 freeze-thaw cycles Materials by class Amylose Free water Expelled water a Absorbed water Net syneresis b % d.b. % gel wt % gel wt % gel wt % gel wt Cycles Cycles Cycles Fresh pastes Waxy starches Amaranth Rice Corn Cereal Starches Barley Wheat Oat Normal Rice Canaryseed Normal Corn Root and Tuber Starches Tapioca Potato Arrowroot Dicotyledon and Legume Starches Buckwheat Field Pea Wheat Flours Durum Bread LSD (p<0.05) acumulative values. bnet syneresis=(expelled water+absorbed water)-free water. were evaluated under refrigeration and freeze-thaw conditions. Waxy amaranth and waxy corn starches contained only 1% amylose compared to 3% in waxy rice (Table 2). The normal rice starch had 17% amylose as compared to 20 25% in the other cereal starches and flours. Tapioca also contained 16% amylose as compared to 22% in potato and 24% in arrowroot starch. Dicotyledonous seeds are not important commercial sources of starch; buckwheat starch contained 21% amylose whereas field pea starch had 34% amylose. Free water separation from the freshly cooked starch pastes differed little among the starch and flour materials, ranging from 3.2 to 5.2% (Table 2). This was expected, since all pastes were prepared from 8% slurries for starches and 10% for wheat flours that exhibited little excess water (Fig. 2a). Syneresis during refrigeration The waxy starch gels from amaranth, rice and corn gave very little expelled water and centrifugation released little additional absorbed water (Table 2). It appeared that waxy starches had exceptional storage stability during refrigeration, which confirmed previous findings (Schoch, 1968). When the free water values were subtracted from expelled and absorbed water, total net syneresis ranged from 1.2 to 2.6% after the fourth refrigeration cycle. Among the normal cereal starches, barley gel was low in expelled and absorbed water released during each refrigeration cycle, and net syneresis was 6% after the fourth cycle (Table 2). Similarly, tapioca was very stable to refrigerated storage among starches in the root and tuber group. Wheat and oat starches were intermediate in resistance to syneresis, as was potato starch among the root and tuber starches. Wheat starch gel tended to release more expelled water than potato starch gel where centrifugation was necessary to separate the absorbed water. On the basis of expelled water, potato starch showed less syneresis during refrigeration. Thus the three-stage analysis of syneresis was valuable for characterizing starch properties and potential performance in food systems. In contrast, normal corn and arrowroot starch gels exhibited high expelled and absorbed water beginning with the first cycle and gave final net syneresis values of 65 and 60%, respectively. They were highest in amylose within their groups. On average, net syneresis increased by 46% between the first and second cycle but subsequent cycle means increased by 28 and 21%, respectively. Also, correlations between the first and the third or fourth cycles were lower than those between the second and the third or fourth cycles, particularly for normal starch gels (Table 4). Therefore two weekly cycles are recommended for reliable repetitive determination of refrigeration stability in normal starches. Syneresis during freeze-thaw cycles Compared to firm gel structures that developed during refrigeration, the starch pastes formed sponge-like gels during freeze-thaw. These spongy gels expelled little free water during the 16-hr cycles at 18 C (Table 3). Only oat and field pea starches gave decantable water after the first freeze-thaw cycle. Canaryseed, normal corn and tapioca starches yielded expelled water by the fourth cycle. This pattern of free water release was quite different than that found in the refrigeration experiment except that the three waxy starches and normal barley starch showed little or no expelled water under either storage conditions. Centrifugation, however, released high but variable levels of absorbed water in the spongy gels, even after the first freeze-thaw cycle (Table 3). Waxy amaranth was highest in freeze-thaw stability over the four cycles, followed by waxy rice. Waxy corn starch gel was stable for the 1st cycle but resistance diminished during the 2nd cycle when net syneresis values exceeded 60%. White et al. (1989) reported that waxy corn starch had a similar freeze-thaw stability to that of normal corn starch. Only durum flour, and to a lesser extent bread flour, had intermediate freeze-thaw stability throughout the four cycles (Table 3). Barley, buckwheat and wheat starches were intermediate in absorbed and net syneresis values after the 1st cycle but showed high values in later cycles. Starches with the least freeze-thaw stability were field pea<normal corn<arrowroot, which was similar to results for refrigeration stability. Again, correlations between first and later cycles were very low, while those between the second and later cycles were quite high for normal starches (Table 4). Refrigeration vs freeze-thaw stability Net syneresis values of wheat flours showed a very high correlation (r 2 =1.00) between refrigeration and freeze-thaw treatments across all cycles. Similar results were found among the waxy starch values (Table 4). Schoch (1968) had reported that the rate of water syneresis after repeated freeze-thaw treatments agreed well with prolonged refrigeration tests for waxy and modified starches. During the 4 wk refrigeration tests, small differences were found among the three waxy 138 JOURNAL OF FOOD SCIENCE Volume 63, No. 1, 1998

6 Table 4 Regression coefficients (r 2 ) of net syneresis determinations for waxy and normal starches after each cycle of refrigeration and freeze-thaw treatments Cycles Cycles of refrigeration Cycles of freeze-thaw (ft) treatment 1 wk 2 wk 3 wk 4 wk 1 ft 2 ft 3 ft 4 ft Waxy starches 1 wk wk wk wk ft ft ft ft Normal starches starches that were tested (Table 2). In contrast, freeze-thaw treatments established significant differences among the waxy starches after three cycles (Table 3). Therefore repeated freeze-thaw tests are recommended for evaluating cold storage stabilities of waxy and modified starches. For normal starches, however, correlations between refrigeration and freeze-thaw tests were very low (r 2 = 0.25 to 0.69) irrespective of cycles (Table 4). Although net syneresis values between the second and later freeze-thaw cycles correlated highly (r 2 = 0.75 to 0.92, Table 4), few differences were found among the 12 freeze-thawed samples after two cycles (Table 3). In contrast, the refrigeration tests gave significant differences among samples (Table 2). Refrigeration tests appeared to be more discriminating than freeze-thaw treatments and, while some results were similar in both tests, other features were unique. Therefore, both types of stability tests can provide useful data for routine evaluation and comparisons of non-waxy starches. REFERENCE Dreher, M.L., Tinsley, A.M., Scheerens, J.C., and Berry, J.W Buffalo gourd root starch. Starch/Starke 35: Eliasson, A-C. and Kim, H. R Changes in rheological properties of hydroxypropyl potato starch pastes during freeze-thaw treatments. I. A rheological approach for evaluation of freeze-thaw stability. J. Text. Stud. 23: Ferrero, C., Martino, M.N., and Zaritzky, N.E Effect of freezing rate and xanthan gum on the properties of corn starch and wheat flour pastes. Int. J. Food Sci. Tech. 28: Gibinski, M., Palasinski, M., and Tomaski, P Physicochemical properties of defatted oat starch. Starch/Starke 45: Gibson, T.S., Solah, V.A., and McCleary B.V A procedure to measure amylose in cereal starches and flours with concanavalin A. J. Cereal Sci. 25: Hood, L.F. and Seifried, A.S Effect of frozen storage on the microstructure and syneresis of modified tapioca starch-milk gels. J. Food Sci. 39: Hoover, R. and Sosulski, F Studies on the functional characteristics and digestibility of starches from Phaseolus vulgaris biotypes. Starch/Starke 37: Hoover, R., Rorke, S.C., and Martin, A.M Isolation and characterization of lima bean (Phaseolus lunatus) starch. J. Food Biochem. 15: Kim, H.R., Muhrbeck, P., and Eliasson, A-C Changes in rheological properties of hydroxypropyl potato starch pastes during freeze-thaw treatments. III: Effect of cooking conditions and concentration of the starch paste. J. Sci. Food Agric. 61: Lapedes, D.N. (Ed.) Dictionary of Scientific and Technical Terms, 2nd ed. McGraw-Hill Book Co., New York. Pomeranz, Y Water. in Functional Properties of Food Components. Y. Pomeranz (Ed.), p Academic Press, Inc., New York. SAS Institute, Inc SAS: Statistics guide for personal computer. 6th ed. SAS Inst., Cary, NC. Schoch, T.J Effects of freezing and cold storage on pasted starches. in The Freezing Preservation of Foods, Vol 4. D.K. Tressler, W.B. Van Arsdel, and M.J. Copley (Ed.), p The AVI Publishing Co., Westport, CT. Sievert, D. and Holm, J Determination of amylose by differential scanning calorimetry. Starch/Starke 45: White, P.J., Abbas, I.R., and Johnson, L.A Freeze-thaw stability and refrigerated-storage retrogradation of starches. Starch/Starke 41: Williams, P.C., Kuzina, F.D., and Hlynka, I A rapid colorimetric procedure for estimating the amylose content of starches and flours. Cereal Chem. 47: Wu, Y. and Seib, P.A Acetylated and hydroxypropylated distarch phosphates from waxy barley: paste properties and freeze-thaw stability. Cereal Chem. 67: Yanez, G.A., Walker, C.E., and Nelson, L.A Some chemical and physical properties of proso millet (Panicum milliaceum) starch. J. Cereal Sci. 13: Ms received 4/14/97; revised 7/17/97; accepted 7/28/97. Financial support from the ADF Value-Added Processing (Crops) Strategic Fund by Saskatchewan Agriculture and Agri-Food, Regina was greatly appreciated. Volume 63, No. 1, 1998 JOURNAL OF FOOD SCIENCE 139