Influence of Cooking Procedures on Properties of Cottage Cheese Curd

Influence of Cooking Procedures on Properties of Cottage Cheese Curd T.E.H. C H U A 1 and W. L. D U N K L E Y Department of Food Science and Technology University of California Davis 95616 ABSTRACT

Most cottage cheese technology has evolved from practical experience. Books (7, 10) and reviews (6, 8, 12) describe representative practices, but in comparison with hard cheese varieties, relatively little research has been reported on variables that influence the composition and properties of cottage cheese. Effects of selected variables during cooking of cottage cheese were studied to determine their influence on percent total solids and firmness of the curd. Although the experiments were restricted to laboratory-size batches made by the direct acid set method, the results should be helpful for pilot and commercial studies of cottage cheese made by either the direct acid or culture set method.

Cheese was made by the direct acid set method in laboratory-size batches with careful control of processing variables. Total solids in unwashed curd increased linearly with heating time (between 20 and 142 min) and temperature (between 42 and 64 C). Total solids in unwashed and washed curd at a given heating temperature increased with heating rate (between .18 and .50 C/min). Heating rate influenced the relation between percent total solids in unwashed and washed curd. In comparison with a low heating rate (.18 C/min), higher heating rates (.30 and .50 C / m i n ) y i e l d e d a firmer washed curd when the sample was taken at the same heating temperature and also with the same percent total solids, within ranges used commercially. Healing time (between 5 and 30 min) and time o f holding curd in whey (between 0 and 40 min) at cooking temperature did not influence total solids of unwashed curd. Increasing the rate of agitation increased total solids in the curd, breakage of curd particles, and amount of sedimentable solids in the whey. Cooking conditions are critical in determining total solids in cottage cheese curd, and they influence other curd properties such as firmness and size distribution.

MATERIALS AND METHODS Experimental Approach

Cottage cheese curd was made in a benchscale vat equipped with a variable speed agitator and automated control of heating rate. To minimize variables related to skim milk and cultures, skim milk was prepared by rehydrating nonfat dry milk (NDM), and the direct acid set method (1) was used for coagulation. Variables studied were heating temperature, rate of heating, healing time, holding time, agitation, and washing in relation to their effects on percent total solids and firmness of the cheese curd. In experiments in which agitation was varied, size distribution of curd particles and sedimentable solids in the whey were measured.

INTRODUCTION

Commercial manufacture o f cottage cheese is a relatively young enterprise compared with the well established industries for manufacture o f Cheddar, Swiss, and other popular varieties.

Received February 1, 1979. ~Carnation Company, 1310 I4th Street, Oakland, CA 94607. 1979 J Dairy Sci 62:1216-1226

Equipment

The vat, acrylic plastic (plexiglas) 1.5 cm thick, had inside dimensions 30 cm × 30 cm and 38 cm deep. A curd knife for the vertical cuts was placed in the b o t t o m of the vat before use. It consisted of a square frame made from 1.4 cm × .4 cm steel with the upper edges beveled out and fine wire (.2 mm diameter) soldered across the b o t t o m in each direction

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the vat was equipped with a high-speed propellertype stirrer and a submersible coil of tubing through which heated water was circulated. During setting the vat was enclosed in 2.5 cm thick foam rubber insulation to maintain uniform temperature. The curd-whey mixture was heated by steam injection. Temperature and rate of heating were controlled by a thermoregulator that actuated a solenoid valve in the steam line. The rate of heating was varied by changing the diameter o f a drive wheel on a synchronous clock m o t o r that adjusted the thermoregulator. To provide controlled agitation of the curd-whey mixture during cooking, two agitators, designated reciprocating and rolling, were used. The reciprocating agitator (in Figure 1A) consisted of a flat paddle pivoted 68.5 cm above the vat b o t t o m and moved back and forth by a variable speed motor. The rolling agitator (Figure 1B) provided motion similar to that o f the Verti-Stir agitator (Stoehing Bros. Co., Kiel, WI). A paddle suspended horizontally from a vertical shaft was moved with a horizontal circular motion which gave the liquid a rolling motion.

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with 6.25 mm spacings. The knife for the horizontal cut was a frame made of 3.2 mm steel rod with fine wire soldered crosswise with 6.25 m m spacings. For the initial pH adjustment and heating,

Low heat NDM from one spray dryer run (Crystal Cream and Butter Co., Sacramento, CA) was used to prepare rehydrated skim milk, 9.0 ± .05% total solids. The skim milk was stored at 2 to 3 C for 1 to 4 days before use. Skim milk (23 liters) was measured into the cheese vat, and its pH was adjusted to 5.05 ± .03 by slow addition of 120 ml o f acid solution (Vitex 750, Diamond Shamrock Corp., St. Louis,.MO, diluted with an equal volume of water) from a burette with its discharge near the stirrer. The acidified skim milk was heated slowly to 32.5 C by water at 38 C circulated through the heating coil. As the temperature approached 32.5 C, the pH was checked and, if necessary, adjusted to 5.02 to 5.08 by adding acid solution. A mixture of 94.5 g deltagluconolactone (DGL, Vitex 850) dissolved in cold water and 2 ml cottage cheese coagulator (Vitex) was added, stirring was continued for 2 min, the stirrer and heating coil were removed, and the vat was enclosed in the foam rubber insulation. Cutting was at pH 4.70 ± .03, after setting for about 45 min. Healing time, the time Journal of Dairy Science Vol. 62, No. 8, 1979

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C H U A AND D U N K L E Y

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was used continuously to move curd gently from the corners of the vat. With the rolling agitator, three agitations designated R1, R2, and R3 were used. Times at the selected settings were R1, 5 rain each at 2, 8.5, 15, and 25 rpm, then 33 rpm for completion of cooking; R2, 10 min at 8.5 rpm, 5 rain each at 15 and 25 rpm, and at 33 rpm for completion; and R3, 15 rain at 15 rpm, 5 min at 25 rpm, and at 33 rpm for completion. With the rolling agitator, removal of settled curd from the corners consisted of one stroke with the paddle from each corner at 2-rain intervals until the fastest agitator setting was reached. For cooking, coo] rate of heating by steam injection was standardized at .30 C/min except wa rate of heating was an experimental when rate variable.

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Figure 2. Influence o f heating t e m p e r a t u r e (A) and time (B) on the percent total solids o f unwashed curd with two heating rates, .18 and .50 C/min. Reciprocating agitator• Each line represents data from one experiment,

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except when healing time was an intentional variable. After the healing period, the mechanical agitator and steam injector were mounted in the vat and agitation and heating were started. The pH of the curd-whey mixture was adjusted to 4.5 by slow addition of acid solution (Vitex

750). Agitation was varied by changing settings on the variable speed motor and was standardized at selected speeds. When the reciprocating agitator was used, it was operated for 5 min each at 2, 8.5, 15, and 25 rpm and then at 33 rpm for the remainder of the cooking period• With the reciprocating agitator, a hand paddle Journal of Dairy Science Vol. 62, No. 8, 1979

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HEATING TEMPERATURE(C) Figure 3. Influence o f reciprocating and rolling agitators on percent total solids o f unwashed curd with three rates o f heating.

COOKING COTTAGE CHEESE CURD TABLE 1. Correlation coefficient (r), slopes (a), intercepts (b), a and confidence limits (CL .05) of the intercepts for relations between total solids and heatingtime at different heating rates. Reciprocating agitator. Heating rate C/min .18 .23 .25 .30 .35

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and t h e n 5 h in a vacuum oven (70 C, 760 Torr). Firmness and distribution o f curd particle sizes were measured as described (4). Sediment-

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a b o u t 4-min intervals after the curd was firm enough for sampling. For unwashed curd samples, curd-whey m i x t u r e (about 120 ml) was dipped f r o m the vat w i t h a ladle, poured through a screen strainer (1 m m holes), and the curd was allowed to drain 2 min. A b o u t 20 g were transferred to an a l u m i n u m dish for d e t e r m i n a t i o n of total solids. For washed curd, curd-whey m i x t u r e was ladled into 800 ml water (25 C, pH 4.5) in a l-liter beaker, the m i x t u r e was stirred gently for 15 s, allowed to settle 15 s, and the w a t e r was decanted off. The curd was washed a second t i m e with 800 ml cold w a t e r (2 C, p H 4.5), strained, drained 10 min at 2 C, and samples for total solids d e t e r m i n a t i o n were transferred to alumin u m dishes. Curd for size distribution m e a s u r e m e n t s was washed in the vat three times with volumes of wash water (4 C, pH 4.5) equal to t h a t o f t h e w h e y and with the rolling agitator. Whey for d e t e r m i n a t i o n of sedimentable solids was r e m o v e d f r o m the vat b y a 100-ml pipette w i t h o u t stopping the agitation. The pH was d e t e r m i n e d as described by E m m o n s and T u c k e y (7) with a digital pH m e t e r (Orion Model 701A) and c o m b i n a t i o n electrode (Markson P o l y m a r k gel JM-1808). Percent total solids in curd was d e t e r m i n e d essentially as described by E m m o n s , L a r m o n d , and Beckett (6). For drying, a p p r o x i m a t e l y 15-g samples were weighed in a l u m i n u m dishes, frozen at - 2 6 C with forced-air circulation, dried overnight in a c h a m b e r - t y p e freeze d r y e r

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TOTAL SOLIDS(°/o) Figure 4. Interrelations among rate of heating, heating temperature, percent total solids, and firmness of washed curd. Heating rates, .18,. 30, and .50 C/rain; rolling agitator. Journal of Dairy Science Vol. 62, No. 8, 1979

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able solids in whey were determined by the method of Raab, Liska, and Parmelee (11) except that the samples were centrifuged at 585 x g (Sorval Model GL-2). Results were recorded as sedimentable solids in ml/100 ml whey.

(Table 1). All coefficients of correlation were highly significant;the slopes of best-fit regression lines differed significantly, but the intercepts did not. Figure 4 reports interrelations among rate of heating, heating temperature, percent total solids, and firmness of washed curd with three heating rates. Figure 5 illustrates the relation between total solids of washed and unwashed curd and the influence of rate of heating on the relationship. Analysis of variance showed that effects of washing and rate of heating were significant. Figure 6 summarizes data that demonstrate the influence of rate of heating on rate of change of solids in both unwashed and washed curd. Figure 7 illustrates the influence of holding curd at three temperatures on percent total solids, with three different rates of heating to the holding temperatures. Regression lines were calculated for the first nine experiments and two repeated experiments. Only one slope, for holding at 54 C with .23 C/min heating rate (Figure 7A), differed significantly (P<.05) from zero. For the repeated experiment (plotted as open triangles), the slope did not differ from zero. The experiment with holding temperature 50 C and heating rate .50 C/min (Figure 7C) was repeated because the plot of the data for the first experiment suggested a curvilinear relation. For results of the repeated experiment (plotted as open circles), a straight line gave a

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Figure 2 summarizes results of representative experiments showing the influence of heating temperature and time on total solids of unwashed curd. Similar experiments used the rolling agitator with heating rates of .18, .30, and .50 C/min, and the results (Figure 3) showed similar trends. Additional data were obtained with heating rates between .18 and .50 C/min, and they were analyzed statistically Journal of Dairy Science Vol. 62, No. 8, 1979

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In developing the experimental procedures, care was taken to control variables as completely as possible, such as by using a single lot of NDM for the entire study, the direct acid set method to eliminate variables related to culture and pH control, and an automated thermoregulator to provide reproducible control of rate of heating. One important variable difficult to control and standardize was agitation during cooking. The early experiments were with the reciprocating agitator. When it was used, curd settled along the two sides of the vat parallel to the agitator paddle. Continuous gentle manual agitation was used to m o v e the settled curd

back into the bulk of the whey. During the study, we replaced the reciprocating agitator

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Figure 7. Influence of holding unwashed curd in whey at 50, 54, and 57 C on percent total solids. Rates of heating to the holding temperature were A, .23 C/rain; B, .30 C/rain;and C, .50 C/min. Reciprocating agitator. Open symbols represent data from repeated experiments.

satisfactory fit. Analysis of variance of the first nine experiments indicated that holding times did not influence percent total solids.

the reciprocating agitator were repeated with the roiling agitator. Although results with the different agitators should not be compared directly, when experiments on effects of other variables were repeated except that the roiling agitator was used instead of the reciprocating agitator, the results of the t w o groups of experiments showed consistent relationships. In experiments in which percent total solids was determined in both washed and unwashed curd, the analyses of the unwashed curd were more reproducible. Because of the greater reproducibility, and also to save the extra time required to prepare samples, in many experiJournal of Dairy Science Vol. 62, No. 8, 1979

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CHUA AND DUNKLEY 24

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the reciprocating than with the rolling agitator,

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the influence of heating temperature on total solids was similar. Mthough all of the coefficients of correlation of total solids and heating temperature in Table 1 were highly significant, the coefficient for the lowest heating rate was lower than the others. The slopes, a, which indicate the change of total solids during heating, show that the rate of change increased with rate of heating. Because the data fit a straight line with a high correlation over the range studied, the intercepts, b, are indicative of the percent total solids soon after the start of cooking (end of healing). The intercepts in Table 1 did not differ significantly. However, in comparing results of replicated experiments, the intercepts were not as reproducible as the slopes. The gentle agitation associated with cutting the curd and the first stirring of the curd appeared to have an important influence on the initial percent total solids (i.e., the intercepts). Even with care to standardize the cutting and the first gentle manual agitation, variability of shrinkage of the curd during healing and the first few minutes of cooking appeared to be a source of variability of the data. Determinations of total solids in unwashed curd are indicative of the extent of shrinkage of

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Rate of Heating

The results in Figure 2 are typical of many that show changes in total solids of unwashed curd during cooking. Total solids increased directly with both time and temperature, but the rate of increase was greater at the higher heating rate. Results in Figure 3 with the reciprocating and rolling agitators show that although percent total solids was higher with Journal of Dairy Science Vol. 62, No. 8, 1979

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COOKING COTTAGE CHEESE CURD the curd during cooking. Cooking conditions, however, influence properties of the curd after it is washed. This is illustrated by data in Figure 4. Figure 4A shows that for washed curd, rate of increase of total solids with increase of heating temperature is related inversely to heating rate rather than directly related as for unwashed curd (Figure 2A). Differences in slopes of the lines in Figure 4B show that firmness of washed curd increases more rapidly during cooking with the two higher heating rates than with the lowest rate. Also, at total solids typical of commercial cottage cheese curd, the higher heating rates produced firmer curd than the lowest rate (Figure 4C). The comparisons in Figure 5 of percent total solids in unwashed and washed curd made with different heating rates provide additional evidence of the influence of cooking treatments on changes in curd during washing. For the lowest heating rate, the percent total solids in the unwashed curd was higher than in the washed curd. At the highest heating rate, percent solids in the unwashed curd was lower than in washed curd at the lower total solids concentrations, but higher at the higher total solids. The influence of rate of heating on total solids in washed and unwashed curd is shown also in Figure 6, in which rate of change of total solids during cooking is plotted against heating rate. The figure includes a comparison of effects of the reciprocating and rolling agitators. The difference related to washing was much greater than that related to type of agitator. We do not have an explanation for the influence of rate of heating on changes in total solids during washing. Analyses of the solids for soluble constituents (e.g., lactose and whey proteins), if done, would have indicated whether the effect was at least partly related to differences in composition of the solids. If so, rate of heating would influence curd yields. Selection of the optimum heating rate for the commercial manufacture of cottage cheese necessitates compromises among a number of variables. Heating rapidly reduces the time required to achieve the desired total solids. With higher rates of heating, however, the curd is firmer than with a low heating rate (Figure

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4). Heating rate influences behavior of cottage cheese curd during washing (Figures 5 and 6). Cooking too rapidly is reported to cause mealiness (7). Some cheesemakers believe that heating rapidly causes formation of a skin on cottage cheese particles which interferes with expulsion of whey during cooking (8, 13), but this hypothesis has been questioned (9). Use of cooking temperatures below 55 C may result in spoilage by psychrophilic bacteria (2) and has been associated with development of a bitter flavor during storage. To avoid such problems, it may be necessary to select a heating rate at which the desired total solids will be reached only at cooking temperatures above 55 C. In a survey of cottage cheese factories (4), one plant used a heating rate of .58 C/min, higher than in our study, without evidence of poor texture or keeping quality of the cheese. Holding Curd at Cooking Temperature

In the experiments comparing different rates of heating, the heating rate was constant during each experiment. Therefore, time and temperature were related directly, and the data did not indicate the relative importance of each. To study the influence of holding curd in whey at constant temperature on percent total solids, curd was cooked at constant rate to a selected temperature, and then it was held at constant temperature (Figure 7). Time of holding at constant temperature did not influence total solids. In the commercial manufacture of cottage cheese, it is sometimes necessary to delay washing the curd after completion of cooking (e.g., when a washer-cooler still contains curd from a previous vat). The results of these experiments, and of others in which the rolling agitator was used (results not reported), suggest that delay in washing may not further change total solids of the curd. However, as these experiments were based on analyses of unwashed curd and were not accompanied by measurements of other curd properties (e.g., curd firmness), their applicability in commercial operations should be verified. Healing Time

The data in Figure 8 were obtained in experiments designed to study the influence of Journal of Dairy Science Vol. 62, No. 8, 1979

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CHUA AND DUNKLEY

healing time on total solids in the curd. The significant difference for unwashed curd (Figure 8A) is attributed primarily to the difference in position o f the line for 15-min healing time. As indicated b y the confidence limits o f the intercepts in Table 1, positions of lines from individual experiments were not as reproducible as their slopes. In view of the similarity of data for 5- and 30-min healing times, we conclude that healing time did not have an important influence on total solids of the unwashed curd. The greater reproducibility of total solids in unwashed than in washed curd is illustrated by differences between how closely the data fit the lines in Figure 8A and B. Differences related to healing time in Figure 8B were within the confidence limits of the regression lines. Experiments were conducted also to study the influence of healing time on the relation between total solids and firmness of the curd. The influence of healing time was significant, and the slopes of the lines indicated that the shortest healing time resulted in the greatest increase in firmness with increase in total solids. The data are not reported, however, because the experiments were not repeated, and, for the 30-min healing time experiment, the coefficient of correlation of total solids and firmness was significant at only P< .05.

whey suggests that the first gentle agitation promoted rapid shrinkage of the curd until its total solids approximated that o f the intercept o f the line at zero time. In experiments on effects o f agitation on total solids in the curd, differences in total solids related to the t y p e o f agitator (Figure 3) were greater than those related to intensity of agitation with the rolling agitator, within the range of settings studied (Figure 9). On visual observation, the reciprocating agitator appeared to give more variable agitation of individual particles than the rolling agitator. As the paddle moved back and forth, the velocity of flow and acceleration and deceleration of the whey-curd mixture varied appreciably at different locations in the vat. The paddle moved more slowly at the end of a stroke (where it reversed directions) than at mid-stroke. Hence, at mid-stroke the velocity of flow around the paddle and through its holes was greater than at the end. Also, the curd in front of the paddle appeared to be pressed against the walls of the vat at the ends of the strokes. At the other extreme of agitation, some curd accumulated at the b o t t o m of the vat at the front and back in relatively quiescent pockets. The rolling agitator appeared to give more uniform agitation throughout the vat, and it was more effective in preventing settling of curd into quiescent pockets at the bottom. Increasing the RPM of the rolling

Agitation

On the basis of observations of curd motion viewed through the transparent walls of the vat, the first gentle agitation of the curd after healing was much more important than the healing period in promoting the initial shrinkage of the curd. Even during the 30-min healing period (Figure 8), there was little separation of whey from the curd. But as soon as the curd was disturbed by inserting the agitator, the volume of whey appeared to increase rapidly. The results in Figure 8 emphasize that the percent total solids in the curd changed rapidly immediately after healing. At cutting, the soft gel had 9% total solids, the same as the skim milk. The observation that little whey separated during healing indicated that the curd must have had about 9% total solids until the end of healing. The close fit to a straight line of the total solids in the unwashed curd in the first samples removed from the Journal of Dairy Science Vol. 62, No. 8, 1979

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need for gentle agitation early in the cooking step. A l t h o u g h some of the results m a y be directly applicable to the m a n u f a c t u r e of cottage cheese using cultures, special consideration should be given to steps in which the culture process m a y result in appreciable differences f r o m t h e direct acid set m e t h o d with regard to effects of variables. For example, during healing, the p H continues to d r o p f r o m bacterial action when cultures are used, but not in the direct acid set m e t h o d . Therefore, differences related to healing t i m e m a y be m o r e i m p o r t a n t for cottage cheese made by cultures than by the direct acid set m e t h o d .

HEATING TIME (min) Figure 11. Influence of rate of agitation on the amount of sedimentable solids in whey at three rates of agitation. Rates of agitation 1, 2, and 3 as described in text. Rolling agitator; heating rate; .3 C/rain.

ACKNOWLEDGMENT

The research was supported in part by a grant f r o m Dairy Council o f California.

REFERENCES

agitator increased the speed o f the rolling m o t i o n within the vat but did not appear to increase the intensity o f agitation as m u c h as with the reciprocating agitator. The intensity o f agitation with the rolling agitator influenced the size distribution o f the curd particles (Figure 10) and sedimentable solids in the w h e y (Figure 11). Most of the increase in sedimentable solids t o o k place during the first 10 to 20 rain of t h e cooking period. These results support the r e c o m m e n d a tion o f E m m o n s and T u c k e y (7) that during the initial stages o f cooking, agitation should be as gentle as possible because the fragile curd shatters easily. The results should help cheesemakers who use the direct acid set m e t h o d to i d e n t i f y conditions that influence composition, and perhaps o t h e r properties, o f cottage cheese curd. For example, the key role of cooking t e m p e r a t u r e in determining percent total solids is confirmed. Rate o f cooking also appears to have an i m p o r t a n t influence on c o m p o s i t i o n and curd firmness. In contrast, holding t i m e at cooking t e m p e r a t u r e appears to have l i t t l e influence on percent t o t a l solids although it m a y influence o t h e r properties such as firmness of the curd. The experiments with different types and intensities of agitation illustrate the i m p o r t a n c e of this variable and especially the

1 Anon. 1975. Creamed cottage cheese processing instructions. Vitex 750, 850 and coagulator system. Vitex/American, Diamond Shamrock Corp., St. Louis, MO. 2 Collins, E. B. 1961. Resistance of certain bacteria to cottage cheese cooking procedure. J. Dairy Sci. 44:1989. 3 de Man, J. M. 1968. Cottage cheese texture. Can. Inst. Food Technol. J. 1:76. 4 Dunkley, W. L., and D. R. Patterson. 1977. Relations among manufacturing procedures and properties of cottage cheese. J. Dairy Sci. 60:1824. 5 Emmons, D. B. 1963. Recent research in the manufacture of cottage cheese. Dairy Sci. Abstr. 25:129, 175. 6 Emmons, D. B., E. Larmond, and D. C. Beckett. 1970. Food composition: Determination of total solids in heterogeneous heat-sensitive foods. JAOAC 54:1403. 7 Emmons, D. B., and S. L. Tuckey. 1967. Cottage cheese and other cultured milk products. Chas. Pfizer and Co., Inc., New York, NY. 8 Ernstrom, C. A., and N. P. Wong. 1974. Pages 662 to 718 in Fundamentals of dairy chemistry. 2nd ed. The AVI Publishing Company, Inc. Westport, CT. 9 Glaser, J., P. A. Carroad, and W. L. Dunkley. 1979. Surface structure of cottage cheese by electron microscopy. J. Dairy Sci. 62:1058. 10 Kosikowski, F. V. 1978. Cheese and fermented milk foods. 2nd ed. F. V. Kosikowski and Assoc. Brooktondale, NY. 11 Raab, J. A., Jr., B. J. Liska, and C. E. Parmelee. 1964. Temperature-programmed cooking of cottage cheese. J. Dairy Sci. 47:612. 12 Reidy, G., and T. Hedrick. 1968.1. Cottage cheese Journal of Dairy Science Vol. 62, No. 8, 1979

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33(8):531; 33(9):617. 13 Tuckey, S. L. 1964. Properties of casein important in making cottage cheese. J. Dairy Sci. 47:324.