Characteristics of Commercial Nonfat Dry Milk for Cottage Cheese1

CHARACTERISTICS

OF C O M M E R C I A L N O N F A T D R Y M I L K F O R COTTAGE CHEESE 1

H. E. RANDOLPH .~D T. KRISTOEEERSEN Department of Dairy Technology, The Ohio State University AND

The Ohio Agricultural Experiment Station Columbus SUI~IMARY

Thirty samples of commercial low-heat nonfat dry milk were subjected to bacteriological, chemical, and physical tests to determine their uniformity. Cottage Cheese was made from 17 of the samples to determine the curd-making properties. Results are presented for standard plate, direct microscopic, psychrophilic, and coliform counts, titratable acidity, lactic acid, ash, calcium, nitrogen, whey protein nitrogen, dispersibility, and rennet curd tension. Powders obtained from different plants varied considerably, whereas those received from the same plant over a 4-too. period were relatively uniform. About 90% of the samples contained sweet-curdling organisms which were resistant to 80°C. for 20 rain. Considerable differences were observed in the Cottage Cheese-making properties of the reconstituted milks. There was no apparent relationship between curd quality and cooking time, or between any of the bacteriological, chemical, and physical properties and the curd-making characteristics of the powders. Cutting the curd at lower than normal titratable whey acidities resulted in a marked reduction in the time required for cooking the cheese. This was done without any impairment to curd quality of nonfat dry milks containing relatively low whey protein nitrogen, and low rennet curd tension values. A t frequent but i r r e g u l a r intervals, difficulties are encountered in the manuf a c t u r e of Cottage Cheese f r o m low-heat nonfat d r y milk which results in prolonged cooking periods and in curd of inferior quality (7, 17, 20, 21, 23, 24). The persistence of such difficulties during the years t h a t low-heat nonfat powders have been proposed as the sole source of solids for Cottage Cheese u n d o u b t e d l y has contributed to a decrease in the p o p u l a r i t y of such powders for this purpose (20). Various attempts (7, 8) have been made to develop a simple and r a p i d test to predetermine accurately the suitability of powder for Cottage Cheese. However, application of such tests to correlate processing t r e a t m e n t s with curdmaking characteristics has not always succeeded. V e r y little attention has been given to the bacteriological quality or the chemical composition of a powder as related to its curd-making properties. E i t h e r one or both of these characteristics could p l a y an i m p o r t a n t role. The purposes of the present investigation were: (a) to obtain detailed information concerning the bacteriological, chemical, and physical characteristics of commercial n o n f a t milk powders supplied for Cottage Cheese manuReceived for publication September 2~, 1960. 1 Technical article 16:60. The Department of Dairy Technology. Journal article 75-60, The Ohio Agricultural Experiment Station. Supported in part by Hatch Funds through the Ohio Agricultural Experiment Station and by the Ohio Dairy Products Research Fund. 833

834

H. E. RANDOLPI~

AND

T. KRISTOFFE~SEN

facture; (b) to relate these characteristics to the curd-making properties of the reconstituted milks; and (c) to determine the effect of cutting acidity on the curd-making properties of such milks. EXPERI:~[ENTAL PPROCEDURES

Selection of powders. The study involved two phases: (a) the examination of powders from different plants and (b) several powders from individual plants. For the first phase, one 50- or 100-lb. sample of low-heat nonfat powder was obtained from each of 18 commercial plants selected arbitrarily from nine of the leading nonfat dr y milk producing states (22). These included five plants in Wisconsin, three in Ohio, three in Minnesota, two in Iowa, and one each in Idaho, Michigan, Missouri, New York, and Pennsylvania. For the second phase, three additional samples were obtained at monthly intervals from four of these plants located in different states (Wisconsin, Ohio, Minnesota, and Iowa). The powders were stored at 50 ° F. during the investigation (6 too.). Bacteriological quality tests. The powders were examined for total bacterial count, coliform type organisms, psychrophilic bacteria, and direct microscopic count according to standard methods (5). Levowitz and Weber's (14) singlesolution stain was employed for the direct microscopic count. Titratable acidity was determined on 10% reconstituted milk with N/10 NaOIt and phenolphthalein indicator. Lactic acid measurements were made according to the Ling method (15) as modified by Cope and Choi (6). The starter activity test of Horrall and Elliker was employed for detection of inhibitory substances (13). The rate of acid development of a fresh commercial starter in reconstituted milks of the commercial powders was compared with the activity of the starter in a sterile reconstituted milk known to be free of inhibitory substances. The presence of sweet-curdling microorganisms was detected by incubating 9 ml. of milk containing 10% solids (reconstituted in sterile water) for up to 24 hr. at 30 ° C. Observations for coagulation were made at 2-hr. intervals. Following coagulation, the milks were titrated with N/10 NaOH, with the titer serving as an arbitrary criterion for sweet curdling (coagulation at < 0.50% acid). The coagulated samples were examined microscopically to determine the morphological characteristics of the microorganisms. Chemical and physical methods. Ash was determined according to official procedures (4). Calcium was determined according to Reisfeld's modification of the Murexide method (19), and total nitrogen according to the Parnas method (18). The protein content was calculated from the nitrogen values, using the conversion factor of 6.38. Undenatured whey protein nitrogen was measured according to a modified Harland and Ashworth method (3). The relative dispersibility of the powders was determined by the centrifugal method of Gould and Bassett (11), which indicates both the wettability and the solubility of a milk powder. Rennet curd tension was determined by the method of Emmons et al. (8), exceDt that a submarine signal curd Tension Meter (Model 1935) was used. Acid-rennet curd tension values were obtained by a procedure simulating

835

NONFAT DRY MILK FOE COTTAGE CHEESE

Cottage Cheese-making conditions. The powders were reconstituted to 11.0% total solids in sterile water. Six hundred milliliters of reconstitute was prepared from each powder and divided equally between three 250-ml. beakers. The milk was adjusted to 90 ° F. and inoculated with 5% lactic starter. Following incubation at 90 ° F. for 1.5 hr., rennet was added at the rate of 1 ml. per 1,000 lb. of milk. Incubation was then continued until coagulation occurred. Curd tension measurements were made at 15, 45, and 75 min. following visible coagulation. Cottage Cheese-making. Cottage Cheese was made in six 55-1b. capacity stainless steel vats constructed to provide optimum control and uniformity. The powders were reconstituted to 11.0% solids and the conventional short-set procedure as outlined by Remaley (20) was followed. Cooking of the curd was controlled with the Lundstedt curd meter (16). A reading of from nine to • 11 was considered to indicate sufficiently firm curd. No cheese was cooked for more than 3 hr., as cooking beyond this time was considered commercially unacceptable. The finished curd was evaluated, according to A.D.S.A. standards (4), for body and texture and appearance by a panel consisting of three competent judges. RESULTS AND DISCUSSION

Bacteriological and general quality characteristics. S u m m a r y results for the 30 samples are presented in Table 1. Bacteriologically, the majority of the TABLE 1 Bacteriological properties of commercial low-heat n o n f a t powders

S t a n d a r d plate count a Direct microscopic count b Coliform count/milliliter Psychrophillc count/milliliter Titratable acidity ( % ) Lactic acid ( % dry basis) Starter activity ( % relative acidity) Sweet curdling e, d A. Hours for coagulation B. Acidity at coagxllation

Range

Average

15T.-75T. 28M.-56M.

39T. 41M.

<1-2


<1-6 0.13-0.15


0.019-0.098 73.9-95.2

0.040 87.1

12-20 0.125-0.68

15 0.26

Thousand/gram. b Million/gram. Incubation temperature 30 ° C. d One sample did not coagulate in 24 hr.

samples were of E x t r a Grade according to American D r y Milk Institute Standards (2). Also, the relatively low titratable acidity and lactic acid values would indicate that milk of good bacteriological quality was used in the manufacture of the powders. The starter activity test revealed the powders to be free from inhibitory substances. The observed variations in ability to support the growth of lactic starters may be accepted as normal due to differences in contents of growth-promoting substances of the milks. With respect to sweet curdling, the majority of the milks coagulated after 12 to 18 hr. of incubation and only one failed to coagulate after 24 hr. At the

/t. E. RANDOLPH AND T. KRISTOFFEI~SEN

836

time of coagulation, titratable acidity values ranged f r o m 0.12 to 0.68%. The m a j o r i t y of the samples contained less t h a n 0.50% acid, which would indicate the presence of high numbers of sweet-curdling microorganisms. Only two samples contained sufficient acid (0.58 and 0.68%) to have caused coagulation. Microscopic examinations revealed that large rod-shaped organisms were associated with milks coagulating at acidities below 0.20%, whereas short- and medium-chain cocci p r e d o m i n a t e d when coagulation was at acidities above 0.35%. Mixtures of large rods and short-chain cocci were present in milk coagulating at acidities between 0.20 and 0.35%. The microscopic examinations indicated t h a t the large rods were sporeforming varieties. This observation was s u p p o r t e d b y selecting powders containing a predominance of rod-shaped bacteria and heating the reconstituted milks at 80 ° C. for 10 rain., followed b y cooling and incubation at 30 ° C. All but one of these milks coagulated within 24 hr. with little or no acid development and contained large numbers of rod-shaped bacteria. Chemical and physical characteristics. S u m m a r y results for the 30 powders are presented in Table 2. Variations observed in the ash, calcium, and nitrogen TABLE 2 Chemical and physical propel'ties of commercial low-heat nonfat powders Characteristics

Range

Average

7.57- 8.25 1.21- 1.42 5.18- 5.82 33.05-37.13 2.52- 7.05

7.87 1.30 5.49 35.03 6.11

Dispersibility A. Solubility ¢ B. Wettability d

0.00-16.63 0.00- 8.90

1.78 1.73

Rennet curd tension (g.)

0.00-84.0

Ash (%) Calcium (%) Nitrogen (%) Protein (%)~ Whey protein nitrogen b

50.0

Nitrogen X 6.38. b Milligrams of ~vhey protein nitrogen per gram of nonfat solids. c Per cent increase in the total solids content of the bottom 25% of the centrifuge] sample over the control. Grams of nnstrainable material. contents of these samples m a y be explained by the normal compositional differences of milk. The values obtained are in agreement with previously reported results (2, 20). U n d e n a t u r e d whey protein, rennet curd tension, and dispersibility values, which to some degree reflect the intensity of the total heat treatment of the powders, indicated considerable variations in processing conditions. Nine powders r a n g i n g in rennet curd tension values f r o m 6 to 84 g. were selected for acid-rennet curd tension measurements. The results in Table 3, a r r a n g e d in increasing order of rennet curd tension values, indicate acid-rennet curd tension values r a n g i n g f r o m 0 to 22 g., 28 to 105 g., and 71 to 170 g. at the three periods of examination, respectively. The rate of increase in curd tension values with increasing incubation time varied for the different samples. Some milks showed the greatest increase between 15 and 45 rain. of incubation (Sam-

NON~AT DRY MILK FOR COTTAGE CHEESE

837

ples 3, 4, 6, 8) and others between 45 and 75 rain. The acid-rennet curd tension values were not related to titratable acidity of the whey and, except for Sample 1, did not relate directly to the rennet curd tension values. Uniformity of tow-heat qwn.fat dry milk from individual manufacturers. The results for the powders obtained over a 4-too. period from four individual plants are summarized in Table 4. These powders were quite uniform with respect to direct microscopic count, coliform count, and titratable acidity within each plant. However, differences in excess of 100% were noted in the standard plate counts of the four powders from a plant. This was of particular significance for Plants 3 and 4, since the number of bacteria per gram for two powders was sufficiently high (68,000 and 75,000) to cause degrading of the samples from Extra Grade to Standard Grade according to American Dry Milk Institute Standards (1). The lactic acid contents of the powders from Plants 1, 2, and 3 varied considerably, whereas those of the powders from Plant 4 were very uniform. TABLE

3

R e l a t i o n s h i p of r e n n e t c u r d t e n s i o n v a l u e s to a c i d - r e n n e t c u r d t e n s i o n v a l u e s i n selected s a m p l e s of c o m m e r c i a l n o n f a t p o w d e r s Acid-rennet curd tension Sample No.

Rennet curd tension

15 rain. ~

45 min. a

g.

T.A. b

0 2 12 6 22 11 9 11 10

.46 .45 .49 .49 .43 .46 .45 .50 .46

g.

75 mi n. ~ T.A. b

g.

T.A. b

.49 .48 .51 .52 .49 .48 .50 .51 .49

71 140 125 116 165 132 170 132 154

.54 .55 .54 .54 .54 .54 .55 .53 .56

(g.) 1 2 3 4 5 6 7 8 9

6 44 47 55 57 66 69 78 84

28 60 105 66 77 95 66 100 60

a M i n u t e s a f t e r t h e first v i s i b l e c o a g u l a t i o n . b P e r cent t i t r a t ~ b l e w h e y a c i d i t y .

All powders contained sweet-curdling microorganisms. On a plant basis, small variations were apparent in the coagulation times and the titratable acidities of the reconstituted milks. The acidities of 0.12 to 0.13% observed in the coagulated milks of powders from Plant 2 represented a decrease of 0.01 to 0.02% from the original values. Powders from individual plants did not vary greatly in ash, calcium, nitrogen, and undenatured whey protein contents. The differences were not greater than what may be expeeted due to natural variations in the milk supply. Similarly, dispersibility values were fairly uniform from month to month. Significant variations were observed in rennet curd tension values of the powders from three of the plants (Plants 1, 2, and 4).

Cottage Cheese-making properties of commercial low-heat nonfat dry milk. Seventeen powders representing a wide range of whey protein nitrogen and

TABLE Variations

in

bacteriological,

chemical,

and

physical

4

p r o p e r t i e s of low-heat n o n f a t dry milk f r o m i n d i v i d u a l p l a n t s Plant Number

1 Characteristics Standard plate count " Direct microscopic c o u n t b Coliform count Psychrophilic count Sweet c u r d l i n g ° A. H o u r s f o r c o a g u l a t i o n B. A c i d i t y a t c o a g u l a t i o n

2

3

4

Range

Av.

Range

Av.

Range

Av.

Range

Av.

18 T . - 4 9 T. 41M.-53M. <1 <1

30 T. 46M. <1 <1

15 T . - ¢ I T. 30M.-43M. <1 <1

26 T. 34M. <1 <1

30 T . - 6 8 T. 33M.-54M. <1 <1

44 T. 45M. <1 <1

35 T . - 7 5 T. 25M.-46M. <1 <1

52 T. 38M. <1 <1

15.5 .18

12-12 .14-.32

12 .195

.14-.14 0.029-0.03.2

.14 0.031

14-18 .14-.23

16 .19

12-16 .12-.13

i4 .13

14-16 .14-.24

Titratable acidity (%) Lactic acid ( % dry basis)

.14-.15 0.036-0.098

.145 0.057

.14-.145 0.024-0.045

.14 0.031

.14-.15 0.019-0.049

Ash (%) Calcium ( % ) Nitrogen (%) Protein (%)d Whey protein nitrogen ~

7.71-7.80 1.26-1.38 5.53-5.60 35.26-35.72 6.51-7.0~6

7.77 1.34 5.55 35.41 6.75

7.87-8.25 1.26-1.37 5.26-5.82 33.56-37.13 5.66-6.32

8.03 1.33 5.51 3~5.15 6.02

8.01-8.05 1.21-1.28 5.18-5.58 33.30-35.60 5.85-6.32

8.03 1.25 5.33 34.01 5.98

7.72-8.06 1.26-1.33 5.48-5.59 34.96-36.55 6.07-6.56

7.89 1.30 5.56 35.4.7 6.26

0.38-1.76 0.57-1.94

1.19 1.17

0.18-1.75 0.65-2.33

0.86 1.41

0.38-4.10 1.98-3.40

2.03 2.39

0.18-3.2.6 0.19-1.68

1.03 0.75

Dispersibility A. Solubility f ]3. W e t t a b i l i t y g R e n n e t c u r d t e n s i o n (g.)

56-78

63

44-74

53

44-47

.15 0.031

46

a Thousand/gram. b Million/gram. c I n c u b a t i o n t e m p e r a t u r e 30 ° C. d N i t r o g e n × 6.38. e M i l l i g r a m s of w h e y p r o t e i n n i t r o g e n p e r g r a m of n o n f a t solids. P e r cent i n c r e a s e in the total solids c o n t e n t of t h e b o t t o m 2 5 % o f the c e n t r i f u g e d s a m p l e over the control. g G r a m s of u n s t r a i n a b l e m a t e r i a l .

47-55

52

NON:FAT DRY MILK

FOR

COTTAGE

CHEESE

839

rennet curd tension values were selected for Cottage Cheese-making. The results in Table 5 are arranged in decreasing order of whey protein nitrogen values. The cooking times required for the various curds, which included come-up time to 120 ° F. (75 min.) and subsequent holding at this temperature, ranged from 80 min. to 3 hr., with 14 of the samples requiring 2 hr. or longer. The body and texture scores of the curds ranged from 34.5 to 31.0 (Excellent to Poor). Twelve of the samples received scores of 32.5 or better (Excellent to Good). The most common defects were grainy and mealy. Scores for appearance ranged from 14.5 to 12.5 (Excellent to Fair), with the majority of the samples ranging from 14 to 13 (Good). Most of the curds were criticized for lacking uniformity. There was no apparent relationship between cooking time and curd quality. Comparisons between the curd-making properties of the powders and the various bacteriological, chemical, and physical characteristics revealed a lack of definite relationships. General relationships were observed between whey protein and rennet curd tension values, and cooking time and curd quality. Powders with relatively high values for whey protein nitrogen and rennet curd tension usually required a shorter cooking time and resulted in better quality curd than powders with low whey protein nitrogen and rennet curd tension values. However, marked exceptions to this general trend were noted. For example, Powder 7 (Table 5) contained relatively high whey protein nitrogen and rennet curd tension values, but yielded a curd which required an excessively long cooking time (3 hr.). On the other hand, Powder 15 contained relatively low whey protein nitrogen and rennet curd tension values and resulted in a curd which firmed comparatively rapidly (2 hr.).

Effect of cutting acidity on c~rd-maki~g properties of lo.w-heat qwnfat powders. With regular skimmilk, cutting the coagulum at a low whey acidity (0.40 to 0.45%) results in curd which firms quickly during cooking but which is of a tough, firm character. Since a major criticism of Cottage Cheese made from much commercial low-heat powder is the excessively long period required for firming the curd, an experiment was designed to study the effect of whey acidity at cutting on the resulting curd quality. Powders with relatively low and high whey protein nitrogen and rennet curd tension values were selected for this purpose. The cheese was manufactured according to the standard procedure, except that the curd was cut at titratable whey acidities ranging from 0.40 to 0.65%. Typical results in Table 6 for selected powders reveal a direct relationship between the acidity of the whey at cutting and the time required for cooking the curd. With powders of low whey protein nitrogen and rennet curd tension values a decrease in cutting acidity from 0.56 to 0.40% resulted in a reduction of the cooking period from approximately 3 hr. to 1.5 hr. The attainment of a close to normal cooking period by cutting at lower whey acidities was accomplished without a decrease in curd quality. Curd from powder of high whey protein and rennet curd tension values should be cut at recommended acidities. Although cutting at lower acidity will

TABLE 5 R e l a t i o n s h i p of whey p r o t e i n n i t r o g e n a n d c u r d t e n s i o n to body a n d t e x t u r e a n d a p p e a r a n c e scores a n d c r i t i c i s m s of C o t t a g e Cheese m a d e f r o m c o m m e r c i a l low-heat n o n f a t d r y milk Sample No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

Whey protein nitrogen 7.05 6.99 6.92 6.75 6.70 6.68 6.68 6.56 6.32 6.26 6.13 6.02 5.97 5.85 5.85 5.84 5.36

Rennet curd tension 61 57 37 78 69 66 58 55 47 45 54 84 6 74 45 30 41

Body and texture Cooking time

(hr.)

(rain.)

2 2 3 2 2 2 3 1 2 1 3 1 3 3 2 3 3

O0 30 00 00 30 20 00 30 40 35 00 20 00 00 00 O0 O0

a Excellent Good Fair Poor 35-34.5 34-32.5 32-30 29.5-0 Excellent Good l~air Poor 15-14.5 14-13 12.5-11 10.5-0 SL--slight; D.--distinet; S.--strong.

Score "

Criticism ~

34.0 33.0 34.5 33.0 32.0 34.0 33.0 31.0 32.0 32.5 33.5 32.5 32.5 33.5 32.0 33.5 32.0

S1. r u b b e r y SI. g r a i n y , sl. weak N o criticism S1. g r a i n y D. g r a i n y S1. firm D. firm S. g r a i n y , sl. firm S. weak S1. g r a i n y , sl. firm S1. g r a i n y SI. g r a i n y , sl. firm D. weak, sl. g r a i n y S1. g r a i n y , sl. weak D. g r a i n y S1. w e a k D. g r a i n y , sl. firm

Appearance Score b 14.5 14.0 13.5 14.0 13.0 14.0 14.0 12.0 14.0 13.0 13.5 14.0 13.0 13.0 13.5 12.5 14.0

Criticism ¢ N o criticism S1. lacks u n i f o r m i t y D. lacks u n i f o r m i t y S1. lacks u n i f o r m i t y S1. s h a t t e r e d curd S1. lacks u n i f o r m i t y S1. s h a t t e r e d curd D. s h a t t e r e d c u r d S1. lacks u n i f o r m i t y D. lacks u n i f o r m i t y D. lacks u n i f o r m i t y S1. s h a t t e r e d c u r d Shattered curd S1. s h a t t e r e d c u r d D. lacks u n i f o r m i t y D. s h a t t e r e d c u r d S1. lacks u n i f o r m i t y

TABLE 6 ReJationship of whey protein nitrogen, curd tension, and c u t t i n g acidity to the quality of curd and cooking time of Cottage Cheese made f r o m commercial low-heat n o n f a t dry milk Sample No.

1

Whey protein nitrogen

Rennet curd tension

5.36

Score a

Criticism c

Score b

Criticism °

(hr.)

(rain.)

41

3 3 2 ~ 1

00 00 35 45 35

33.5 34.0 33.5 33.5 34.0

S1. weak S1. g r a i n y S1. grainy, sl. weak S1. grainy, sl. weak S1. grainy, sl. firm

13.5 14.0 13.5 13.5 13.5

S1. shattered curd S1. lacks u n i f o r m i t y D. lacks u n i f o r m i t y D. lacks u n i f o r m i t y D. lacks u n i f o r m i t y

.60 .56 .50 .47 .40

3 3 2 l 1

00 00 45 55 30

33.5 33.0 32.0 32.5 33.5

S1. weak S1. g r a i n y D. weak, sl. grainy I). weak, s]. grainy S1. grainy, sl. weak

13.0 13.0 13.5 13.5 13.5

S1. shattered curd S1. shattered curd D. lacks u n i f o r m i t y D. lacks u n i f o r m i t y D. lacks u n i f o r m i t y

.64 .57 .49 .60 .56 .52

2 1 1 2 1 1

20 25 15 40 45 20

33.5 33.0 32.5 33.5 32.5 32.0

Sl. weak SI. grainy S]. rubbery S1. g r a i n y S1. rubbery D. rubbery

14.0 14.0 13.0 14.0 14.0 14.0

S1. lacks u n i f o r m i t y S1. lacks u n i f o r m i t y Sh shattered curd SI. lacks u n i f o r m i t y S1. lacks u n i f o r m i t y S1. lacks u n i f o r m i t y

5.97

6

3

6.02

84

4

6.75

78

b Excellent 15-14.5

Cooking time

.65 .56 .51 .45 .40

2

Excellent 35-34.5

Body a n d texture Cutting acidity

Good 34-32.5

Fair 32-30

Poor 29.5-0

Good 14-13

Fair 12.5-11

Poor 10.5-0

Sl.--slight; D.--distinct; S.--strong.

842

i . E. I%ANDOLPtt AND T. KRISTOFFERSEN

result in a reduced cooking period, the finished curd will be too firm or tough to be acceptable. Actually, the results with such powders indicated that cutting at slightly higher than normal acidity resulted in finished curds of somewhat better quality than did cutting at the generally recommended acidity. These results indicate it would be advisable to supplement, or perhaps even supplant, whey protein nitrogen and rennet curd tension measurements with pilot manufacture of Cottage Cheese to assess the suitability of a powder for Cottage Cheese, Stone et al. (21) first recommended this approach. However, their objective was to establish if a powder was suitable according to recommended Cottage Cheese manufacturing procedures. These recommendations establish conditions under which a powder has optimum Cottage Cheese curd-making properties. The results obtained in this study would indicate that most, and perhaps all, commercial low-heat nonfat powders are suitable for Cottage Cheese, provided the curd is cut at the proper acidity. Emmons et al. (9, 10) found a relationship between the heat treatment given plain or condensed skimmilk and the whey acidity at cutting of Cottage Cheese curd manufactured from these products. The A-C test was developed as a result of these observations. It is doubtful that irregularities in commercial low-heat nonfat powders for Cottage Cheese are due entirely to their processing heat treatments. However, there is a similarity between the findings of Emmons et al. (8) and those of the present study on the relationship of the whey acidity at cutting and the length of the cooking time and the curd quality. This suggests that the A-C test may be suitable for the rapid and simple assessment of the optimum cutting acidity of Cottage Cheese curd manufactured from low-heat nonfat powders. REFERENCES (1) AMI~RICAN DAII~Y SCIENC~ ASSOCIATIOI~. Score Card and Guide for Cottage Cheese. J. Dairy Sci., 41: 214. 1958. (2) AI~IEI~ICAN DRY MILK INSTITUTE, INC. The Grading of Nonfat Dry Milk Solids and Sanitary and Quality Standards. Bull. 911 (rev.). 1954. (3) A~IYAZICANDRY MILK INSTITU~, INC. The Turbidimetrie Estimation of Undenatured Serum Proteins in Nonfat Dry Milk. Unpublished release. 1957. (4) AMF~IOAN PUBLIC HF~ALTH ASSOCIATION. Standard Methods for the Examination of Dairy Products. 10th ed. 1953. (5) ASSO~IATIOBI OF OI~I~ICIAL AGI~ICULTURAL CHEMISTS. Official Methods of Ana]ysis. 8th ed., p. 267. Washington, D.C. 1955. (6) COPE, B., AND CHOI, R. P. Adaptation of the Ling Method for the Determination of Lactic Acid in Nonfat Dry Milk. American Dry Milk Institute, Inc. Unpublished release. 1957. (7) COULTF~, S. T., J~.NNESS, R., AND H.~LZ~ND, H. The Manufacture of Low-Heat Nonfat Milk Solids. J. Dairy Sci., 37: 476. 1954. (8) EM~ONS, D. B., MF~RRILL~ D. W., SWANSON, A. M., AND PKICE, W. V. Tests Currently Being Investigated in Research on the Evaluation of N o n f a t Dry Milk Solids for Cottage Cheese Manufacture. University of Wisconsin. Unpublished. 1954. (9) EMMONS, D. B., PI~IC]~ W. V., AND SWANSON, A. M. The A-C Test. Circ. 541, University of Wisconsin Ext. Service. 7 pp. 1957. (10) EM~ONS, D. B., SWANSON, A. M., AND PRIC~, W. V. Effects of Skimmilk Heat Treatments on Cottage Cheese Manufacture. J. Dairy Sci., 42: 1020. 1959.

NONFAT DRY MILK ]~0R COTTAGE CHEESE

843

(11) GOULD, I. A., AND BASS~TT, H. J. Effect of Variations in the Composition of Milk with Special Reference to F a t on the Dispersibility of Dehydrated Milk. Rept. No. 12 for the Quartermaster Corps. 1953. (12) HAt%LAND,H. A., AND ASKW0~TH, U. S. A Rapid Method for Estimation of Whey Proteins as a a Indication of Baking Quality of N o n f a t Dry Milk Solids. Food Research, 12: 2~7. 1947. (13) H0~RAI~, B. E., AND ELLIKEE, 1~. P. An Activity Test for Cheddar and Cottage Cheese Starters. J. Dairy Sei., 33: 245. 1950. (14) L~vowlTz, D., AN]) WBBE% M. An Effective ~ ' S i n g l e Solution S t a i n . " J. Milk and Food Teehuol., 19(5) : 121. 1956. (15) LING, E. R. The Determination of Lactic Acid in Milk. J. Sci. Agr., 2: 279. 1961. (16) LUNDSTI!~DT, E. New Curd Meter Takes Guess Work Out of Cheesemaking. Food Eng., 2 7 ( 1 1 ) : 9 7 . 1955. (17) Mom~is, I{. A., COUL~E~, S. T., COMBS, W. B., A n HEINZI~L, L. ~. Estimation of the Serum Protein Content as a Method for Evaluating N o n f a t Dry Milk Solids for Use in Cottage Cheese Manufacture. J. Dairy Sci., 34: 487. 1951. (18) PA~N)~S, J. K. The Kjeldahl Method for Determining Nitrogen by the Method of P a r n a s and Wagner. Anal. Chem., 114: 261. 1938. (19) RmS~Er.D, R. A. A Study of the Calcium-Binding Properties of Whole Casein, a-Casein, and fl-Casein. Ph.D. thesis, Ohio State University. 1957. (20) REMALI~Y, R. J. The Use of N o n f a t Dry Milk in Making Cottage Cheese. Milk Ind. Foundation Cony. Proc. P l a n t Sect. p. 19. 1957. (21) S T o ~ , W. K., LA~aE, P. M., AN]) GI%A~, G. C. Determination of the Curd Making Quality of N o n f a t Dry Milk. J. Dairy Sci., 43:~8. 1960. (22) U. S. D]~eT. OF AGm Rm)O~T. N o n f a t Dry Milk Solids Production. 1956. Dairy Products Data Sheet. Olsen Publishing Co., Milwaukee, Wisconsin. (23) WHI~A~:~, 1~. The Selection and Use of N o n f a t Dry Milk Solids in the Manufacture of Cottage Cheese. J. Dairy Sci., 39: 231. 1956. (24) WILKOWSK]~, H. H. Relationship Between Titratable Acidity and pH During Lactic Acid Fermentation in Recenstituted N o n f a t Milk. J. Dairy Sci., 37: 22. 1954.