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EFFECTS OF HEAT-TREATING AND STORING MILK ON FARMS PRIOR TO CONVERTING INTO COTTAGE CHEESE AND QUARG

The following paper was presented by Dr. Robert R. Zall, Professor, Food Science Department, Stocking Hall, Cornell University, Ithaca, NY 14853, U.S.A., especially for the second biennial Marschall International Cheese Conference, held in the Coliseum of the Dane County Exposition Center, Madison, Wisconsin, on September 15, 16, 17, and 18, 1981.

Heating and storing milk on dairy farms before pasteurization in milk plants reduce psy-chrotrophs, stabilize acid degree values, and prolong delicate milk flavor. Neither cheese quality nor shelf life appears to be impaired when cottage and Quarg are made from such material. In fact, cottage cheese yield increases substantially when made from heated/stored milk. No suggestions appear necessary to suggest changes in current make procedures for these cheeses when heated/stored milk is used.

The use of farm bulk tanks has increased the problem of milk spoilage due to growth of psychrotrophic bacteria in refrigerated milk. Better methods are needed to minimize the number of psychrotrophs (8, 10, 11, 12). Pasteuriza-tion of milk in processing plants cannot eliminate the problem of psychrotro-phs in raw milk. Psychrotrophs frequently deteriorate milk quality by releas-ing undesirable metabolites and enzymes such as lipases and proteases into milk. Conventional milk pasteurization methods may eliminate most of the psychrotrophic bacteria, but they cannot remove or inactivate already formed heat-resistant metabolites and bacterial enzymes (1, 4, 5, 6).

To overcome this problem, Zall and Chen (13) suggested that milk could be heat-treated and stored on farms before pickup. The practice would be benefi-cial to fluid and cultured milk quality, and was deemed to be so after testing the idea in a study over a two-year period. By heating milk to 74¼ C (165¼ F) for ten seconds, psychrotrophic counts could be reduced to almost zero in raw milk. Raw milk could be stored up to seven days with no deleterious effects.

Milk so treated was found to be perfectly acceptable for buttermilk and yogurt manufacture.

The initial work did not study the effects on cheesemaking and cheese yields of heating and storing farm milk. To probe the effects of heating and storing milk on farms in cheesemaking, a second series of experiments were carried out over a summer and winter period in which raw milk was thermalized and stored before pasteurization, as in the earlier work, and then converted into cottage cheese and Quarg cheese. These varieties of soft cheese were selected because answers to some questions could be arrived at quickly.

Fresh milk samples were collected from the Cornell University dairy herd where approximately 400 cows are milked twice daily in a double ten Herring-bone De Laval milking parlor. A quantity of fresh, uncooled raw milk was heated at a subpasteurization level by passing it through a thin plate heat exchanger system called a Micro-therm, manufactured by Alfa-Laval, and stored in a refrigerated bulk tank at 3 + 10¼ C. (see Figure 1). Equal quantities of heated milk were added to the tank each day or every other day. After seven days, the stored milk was delivered to a milk processing plant where it was pasteurized. Chemo-biological analysis of the milk before and after pas-teurization indicated that farm-heated milk maintained satisfactory quality after seven days of storage at 3 + 10¼ C. Selected data on the stored milk were comparable to that of regular milk for bacterial counts, acid degree values, and amino acid composition. More than 800 people taste-tested the pasteurized, aged, farm-heated stored milk and a fresh milk control, as in a taste panel exercise. About 43 percent of the panelists expressed no sample preference; 31 percent of the panelists preferred fresh milk. Select data are presented in Figures 2, 3, and 4.

Psychrotrophic counts of the farm-heated/stored milk during storage with daily additions of equal quantities of freshly heated milk are shown in Figure 2. 

Data in Figure 3 show the psychrotrophic counts when equal portions of fresh, heat-treated milk were added to the refrigerated bulk tank on alternate days, three times a week. The bacterial counts and ADV data of alternate-day ex-periments were about the same as the data obtained in the experiments in which milk was added to the tank every day for six days. Approximately 20 percent of the initial total bacterial counts in the raw milk were psychrotrophic. 

The data on amino acid composition in Figure 4 show no difference between pasteurized farm-heated/stored milk and regular pasteurized milk. Forty individual samples of pasteurized milk from unheated and farm-heated sources were tested and compared for nutritive quality as measured by possible changes in amino acid content.

The activity of cultures grown in farm-heated milk and in fresh raw milk was examined; yogurt and buttermilk cultures at 2 percent and 5 percent inoculum were tested. The culture growth results, as shown in Figure 5, suggest that no discernible difference occurred between milk sources. The two kinds of milk supported culture activity about equally well, as monitored by titratable acidity and pH changes. Heated/stored milk was at least as good as the control milk samples in its ability to support typical milk fermentation for cultured products or cheesemaking.

Quantities of milk were collected from the Cornell University dairy herd. The milk was heat-treated in the Alfa Laval Micro-therm and stored in a refriger-ated vessel as in previous studies. After seven days, the milk was removed from storage and processed through an APV high-temperature short-time unit and used for cheesemaking. Milk was pasteurized at 71.70¼ C for 15 seconds, put into small laboratory-sized cheese vats, and set with culture and rennet in appropriate amounts according to a short-set procedure outlined by Emmons and Tuckey (3). A five-hour short-set method was used throughout the test period, in which more than 50 vats of cottage and 38 vats of Quarg cheese were produced.

Cheesemaking was carried out by researchers who had both academic and industry experience before conducting the series of experiments. This fact seems important since cheesemaking remains both a science and an art.

Milk was analyzed for total nitrogen (TN), nonprotein nitrogen (NPN), and noncasein nitrogen (NCN). Cheese yields were determined using gravimetric methods which were adjusted to reflect the total solids and moisture in finished curd. Milk used in the different trials was examined for bacterial counts both by standard plate count and psychrotrophic count according to a rapid method described by Oliveria and Parmelee (9). All milk samples were also tested for inhibitory substances using the Delvotest method (2).

Cheese samples were analyzed for standard plate count (SPC), yeast and mold count, coliform count, and psychrotrophic count.

Whey was assayed for total solids, and cheese fines were determined using Whatman filters and drying residues at 100¼ C in a forced draft oven. In addi-tion, whey was analyzed for total nitrogen, noncasein nitrogen, and nonpro-tein nitrogen following RowlandÕs methodology using a Tecator Kjeltec sys-tem.

Five-pound samples of cheese were stored in food-grade plastic containers at 3 + 10¼ C for three weeks. At regular intervals, cheese was removed from refrig-erated storage and graded for appearance, texture, flavor, and aroma.

Table 1 shows cottage cheese yield results in different trials over the test pe-riod. The statistical significance of these data is that cottage cheese yield increased significantly (P 0.01) when made from heated/stored milk where t-test value is -2.839 with a significance level of 0.0087.

Microbial parameters are important because cheese so processed ought to be as good as, or better than, cheese made by more traditional methods. Summary data for the different cottage trials are shown in Tables 2 and 3.

Quarg was made according to methods outlined by Kroger (7) in the Proceed-ings of the first biennial Marschall International Cheese Conference. Basi-cally, it was an overnight set using 2 percent innoculums of culture incubated at a temperature of 22¼ C with rennet added. Summary data for different Quarg trials are presented in Tables 4 and 5.

Quarg yield results reflect product adjusted to 80 percent moisture. Table 5 shows yield results in different trials. The statistical significance of these data show that Quarg yield was not increased significantly when product was made with heated/stored milk (t-test value -0.681 at significance level of 0.5018).

The difference in mean values between yields in cottage cheese made from untreated and heat-treated/stored milk is about 5 percent. While the increase in yield of cheese in Quarg is less than cottage cheese, there was about a 2 per-cent increase in yield and this increase is still a considerable amount.

Increased yield of cheese made from milk heat-treated and stored appears to be due to inclusion of whey proteins with casein as a result of heating and storing treatments.

Work is currently going on in the laboratory by my co-researchers looking for the mechanism responsible for the increased cheese yields. Samples of milk and whey were saved from the different cheesemaking trials, and these are being analyzed for nitrogen movement from milk to whey into cheese, etc.

These tests are being run in concert with more samples of fresh product taken from cheesemaking trials duplicating original procedures. Some testing tech-niques being used include measuring the quantity of whey proteins in dif-ferent fractions, protein migration by gel phoresis, electron scanning micrographs, etc.

It is interesting to see, for example, that the total nitrogen in skim milk remains about the same when heat-treated in fresh samples and stored, but whey protein nitrogen is reduced in whey sampled from cheese made from heat-treated/stored milk. Table 6 shows total protein and whey protein in three successive trials where heat-treated/ stored milk was made into cottage cheese.

As to casein nitrogen, however, the data show that the casein levels moved upward from levels found in the control (unheated) milk in both the treated fresh or treated stored supply. To explain further where the whey proteins disappear, Table 7 shows that the increase in casein nitrogen increased with the decrease in whey protein nitrogen.

There are different ÒopinionsÓ as to why the heated/stored treatment of milk increases the cheese yields in cottage and Quarg cheeses. One hypothesis is that increased yields are due to the complexing of whey proteins with casein.

B-lactoglobulin combines with K-casein and precipitates with the casein. Our nitrogen balance studies show that there is a shift in some whey proteins to casein. It is reasonable to think that some form of complexing between casein and whey proteins occur.

Some researchers think that heating milk causes a disulfide linkage of K-casein and B-lactoglobulin, and this too seems logical. Two mechanisms of how the reaction occurs are being investigated. The hypothesis is that heating probably opens the protein molecule and increases numbers of polar ends that can react with the many functional groups of proteins. K-casein and B-lactoglobulin complexing is favored.

Another factor probably involves calcium. When milk is heated, calcium ions move into colloidal calcium phosphate, through which bridging may form between proteins. One of my students, David Dzurec, is studying this problem in great depth as part of his doctoral dissertation. We hope his work will con-tribute to the base of knowledge in this area.

There don't appear to be any special problems in making cheese farm heat-treated/stored milk, and no suggestions appear necessary to suggest changes in current make procedures. The heat treatment does not affect milk's taste nor its ability to be processed into cultured products. If, in fact, the heat-treated milk concept were adopted and we increased cottage cheese in the U.S. by 5 percent, this added yield, by itself, would be worth more than 40 million dollars.

The effects of heating and storing milk on farms prior to converting it into cottage cheese and Quarg are:

This research was sponsored in part by a grant from the Alfa Laval Company of Tumba, Sweden. Personnel from CornellÕs Teaching and Research Farm at Harford, N.Y., provided valuable day-to-day assistance to the milk handling study. Dr. Joseph Chen, David Dzurec, and Hung-Chang Chen contributed technical support to the project.

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