Invited review: A commentary on predictive cheese yield formulas.

Predictive cheese yield formulas have evolved from one based only on casein and fat in 1895. Refinements have included moisture and salt in cheese and whey solids as separate factors, paracasein instead of casein, and exclusion of whey solids from moisture associated with cheese protein. The General, Barbano, and Van Slyke formulas were tested critically using yield and composition of milk, whey, and cheese from 22 vats of Cheddar cheese. The General formula is based on the sum of cheese components: fat, protein, moisture, salt, whey solids free of fat and protein, as well as milk salts associated with paracasein. The testing yielded unexpected revelations. It was startling that the sum of components in cheese was <100%; the mean was 99.51% (N × 6.31). The mean predicted yield was only 99.17% as a percentage of actual yields (PY%AY); PY%AY is a useful term for comparisons of yields among vats. The PY%AY correlated positively with the sum of components (SofC) in cheese. The apparent low estimation of SofC led to the idea of adjusting upwards, for each vat, the 5 measured components in the formula by the observed SofC, as a fraction. The mean of the adjusted predicted yields as percentages of actual yields was 99.99%. The adjusted forms of the General, Barbano, and Van Slyke formulas gave predicted yields equal to the actual yields. It was apparent that unadjusted yield formulas did not accurately predict yield; however, unadjusted PY%AY can be useful as a control tool for analyses of cheese and milk. It was unexpected that total milk protein in the adjusted General formula gave the same predicted yields as casein and paracasein, indicating that casein or paracasein may not always be necessary for successful yield prediction. The use of constants for recovery of fat and protein in the adjusted General formula gave adjusted predicted yields equal to actual yields, indicating that analyses of cheese for protein and fat may not always be necessary for yield prediction. Composition of cheese was estimated using a predictive formula; actual yield was needed for estimation of composition. Adjusted formulas are recommended for estimating target yields and cheese yield efficiency. Constants for solute exclusion, protein-associated milk salts, and whey solids could be used and reduced the complexity of the General formula. Normalization of fat recovery increased variability of predicted yields.

Transfer of protein from milk to cheese.

This report concerns measurement of paracasein in milk and transfer of protein from milk to cheese. In the main experiment, two vats of Cheddar cheese were made from each of 11 lots of milk from one large herd over a period of 7 mo. Exclusion of solutes from moisture in paracasein micelles in milk and cheese was central to estimation of paracasein and to the transfer of protein from milk to cheese and whey. Solute-exclusion by paracasein and its changes during cheesemaking could be visualized by considering paracasein micelles to be a very fine sponge. The sponge excludes solutes, especially the large solutes like whey proteins. The sponge shrinks during cheesemaking and expels solute-free liquid, thereby slightly diluting the whey surrounding the micelles inside the curd. Paracasein N in milk was calculated as the difference between total milk N and rennet whey N, the latter adjusted to its level in milk. Adjustment used appropriate solute-exclusion factors (h) of the protein fractions of whey and 1.08 for paracasein and associated salts. They were combined to give a factor Fpc, which adjusted the level of rennet whey N to its level in milk: 1.001 x (1 - 1.01 x FM/100 - Fpc x pc/100), where FM = fat in milk, pc = estimated paracasein, and 1.001 = dilution of milk by chymosin and CaCl2. The mean Fpc was 3.03. Differences in values were small among different procedures for calculating paracasein, but they are considered to be important because they represent biases, which, in turn, are important in analyses commercially. We conclude that solute exclusion by moisture in paracasein must have decreased during cheesemaking because the ratio of moisture to paracasein in the final cheese was 1.5, much less than the h of 2.6 for serum proteins by paracasein. Release of solute-excluding moisture from paracasein during cooking was likely the reason for lower total N in cheese whey than in the rennet whey in the paracasein analysis. Paracasein, estimated to be in cheese, curd fines, salted whey, and whey during cheddaring, was 98.21, 0.20, 0.25 and 0.19%, respectively, of the paracasein in milk for a total of 98.85% (SD of 22 vats = 0.46); the location of the missing paracasein is not known. On the other hand, recovery of milk N in cheese and wheys was 99.92% (SD = 0.37%). Nitrogen in paracasein and its hydrolysis products in cheese was estimated to be 98.51% of total cheese N. Proteose-peptone from milk appeared not to be included with the paracasein in appreciable amounts. Some was apparently included with denatured serum proteins during Rowland fractionation of whey, perhaps as a coprecipitate. Measured paracasein would include fat globule membrane proteins in milk containing fat, and denatured whey proteins in heated milks. It was concluded that the method of measurement and the associated calculations are integral parts of the definition and quantification of paracasein in milk.

Movement of moisture in refrigerated cheese samples transferred to room temperature.

When cheese samples refrigerated at 4 degrees C in 120 mL plastic tubs were transferred to room temperature at 23 degrees C, moisture began to move from the warmer surface to the cooler interior; the difference after 1 h was 0.2-0.4%. Others had observed that moisture moved from the interior of warmer blocks of cheese to the cooler surface during cooling at the end of cheese manufacture. In loosely packed cheese prepared for analysis, part of the moisture movement may have been due to evaporation from the warmer surface and condensation on the cooler cheese. It is recommended that cheese be prepared for analysis immediately before weighing. Cheese samples that have been refrigerated, as in interlaboratory trials, should also be remixed or prepared again.

Variations of moisture measurements in cheese.

Data were accumulated during interlaboratory trials for cheese moisture determination from laboratories using officially recognized methods: AOAC; International Dairy Federation, and Standard Methods for the Examination of Dairy Products (SM). In one trial, ranges of means of 5 cheeses were 0.67, 0.56, and 0.19% for 5, 9, and 8 laboratories, respectively. The lower ranges for the SM method were typical of 3 other interlaboratory trials, with ranges of 0.27, 0.34, and 0.34% for 6, 7, and 5 laboratories, respectively. Within one laboratory, there were no significant differences among the 3 methods, but they all gave about 0.2% lower results than 2 other methods, one using freeze-drying, followed by drying in a vacuum, the other using cheese that was spread on sand and dried in a vacuum oven for 24 h. This finding indicated that none of the officially recognized methods removed all the moisture. Data showed that many laboratories tended to give either higher or lower results than the mean of all of them in a series of 7 interlaboratory trials. Constant results, free of biases or systematic errors, are important in application of formulas for prediction of yield of cheese for purposes of yield control, but are difficult to obtain. It is proposed that results by a laboratory in interlaboratory trials be compared with those obtained by one or more reference laboratories using a method that removes all the moisture from cheese. The difference would be applied as a constant in the predictive yield formula. That difference would likely be best as a running mean of differences in an ongoing series of trials. The reference laboratories would use frozen samples for quality control to ensure uniformity of results among trials. Mean moistures of 36.10 and 36.11% were obtained on subsamples before and after freezing for 7 months.

Factors Influencing Survival of Listeria monocytogenes in Milk in a High-Temperature Short-Time Pasteurizer.

Heat resistance experiments were carried out with Listeria monocytogenes which had been grown at three different temperatures (30, 39, and 43°C). Heated whole milk was inoculated with L. monocytogenes and then passed through a high-temperature short-time system at 72, 69, 66, and 63°C for a minimum holding time of 16.2 s. Heated cells were recovered both aerobically and anaerobically using four different methods: direct plating, most probable number, cold enrichment, and warm enrichment. Significant differences in recovery of L. monocytogenes were observed depending on the growth temperature. Cells grown at 43, 39, or 30°C, held 1 d at 4°C, and then heated at 69°C showed an overall decrease in numbers of approximately 2.1, 2.8, and 4.1 logs, respectively. Cells grown at 39°C and then held 3 d at 4°C appeared to be the most heat sensitive. Although cells grown at 43 and 39°C were capable of surviving at the minimum high-temperature short-time temperature (72°C), those grown at 30°C were not. In some instances, anaerobic incubation enhanced the recovery of L. monocytogenes , as compared to cells recovered aerobically, although these differences were not statistically significant. While L. monocytogenes can survive minimum pasteurization treatment (71.7°C/16 s) under certain conditions, common methods of handling, processing, and storing fluid milk will provide an adequate margin of safety.

Thermal inactivation of Campylobacter species, Yersinia enterocolitica, and hemorrhagic Escherichia coli O157:H7 in fluid milk.

Heat treatment of raw milk in an HTST pasteurizer operated at 60.0 to 72.0 degrees C for a minimum holding time of 16.2 s rapidly inactivated mixtures of hemorrhagic Escherichia coli O157:H7, Yersinia enterocolitica and Campylobacter spp. (C. fetus, C. coli, and C. jejuni). Each of the three genera in the mixture was inoculated at a level of approximately 1.0 x 10(5) cfu/ml. At 60.0 degrees C, hemorrhagic E. coli showed a maximum 2 log10 reduction in counts and no viability at greater than or equal to 64.5 degrees C. Yersinia enterocolitica and Campylobacter spp. showed greater heat sensitivity with a 4 log10 reduction in counts at 60.0 degrees C and absence of viable cells at greater than or equal to 63.0 degrees C. These findings reiterate the need for stringent control of thermal processes in the manufacture of dairy products from raw or heat-treated (non-pasteurized) milk.

Thermal resistance of Listeria monocytogenes in inoculated and naturally contaminated raw milk.

Raw whole milk inoculated with 10(5) CFU/ml of Listeria monocytogenes was thermally processed at 60-72 degrees C for a minimum holding time of 16.2 s with survival being observed at temperatures up to 67.5 degrees C. In addition, milk naturally contaminated with L. monocytogenes serotype 1 (around 10(4) CFU/ml) was pooled for 2 to 2.5 days and then run through an HTST pasteurizer at temperatures ranging from 60-78 degrees C. Viable L. monocytogenes were detected in the temperature range of 60-66 degrees C. No viable Listeria were detected after treatment at temperatures of 69 degrees C and above in any of five trials. Efficacy of pasteurization and widespread use of processing conditions well above the minimum HTST guidelines ensure the absence of Listeria in pasteurized milk products. However, survival of Listeria at sub-pasteurization temperatures (60-67.5 degrees C) is of concern with regard to heat-treated or raw-milk cheeses.

Thermal Inactivation of Salmonella Species in Fluid Milk 1.

The thermal resistance of Salmonella senftenberg 775W, Salmonella muenster previously isolated from raw fluid milk, and two mixtures each consisting of ten Salmonella strains commonly isolated from human or non-human sources was tested. Cells were suspended in whole milk at a final concentration of 105 cells/ml. The inoculated milk was thermally processed at temperatures ranging from 60°C to 74°C using a pilot-scale plate pasteurizer unit. The mean and minimum residence time of milk in the holding tube of the pasteurizer was 17.6 and 16.2 s, respectively. The maximum temperature at which viable salmonellae were detected in the human (61.5°C) and non-human (64.5°C) mixtures was considerably lower than that obtained with S. senftenberg 775 W (67.5°C). S. muenster failed to show any milk-adapted response and could not be recovered at temperatures greater than 63.0°C. Treatment at 63°C produced a 4 log10 or greater reduction in the number of viable Salmonella including the heat resistant S. senftenberg 775 W, and a minimum 2 log10 decrease at 60°C. These findings warrant caution in the use of subpasteurizing temperatures for thermal processing of fluid milk.

Effect of lipids in milk replacers on calf performance and lipids in blood plasma, liver, and perirenal fat.

Various fats (20% of dry matter) were fed in milk replacer to calves, from 3 to 31 d of age, to compare their effect on calf performance, feed efficiency, and lipids in blood plasma, liver, and perirenal fat. Dietary fats tested were tallow (control), canola oil, canola soapstocks, corn oil, reclaimed restaurant cooking fat, and a high phospholipid waste product. Corn oil plus tallow (1:1) diet promoted scours and poor calf gains, but canola oil diet, despite a high content of unsaturated fatty acids, gave excellent calf performance and feed utilization and no scours. Canola soapstocks plus tallow (1:1) and restaurant waste cooking fat lowered gains by 25 and 15% and reduced diet intake. Calves effectively utilized high phospholipid (23%) in dietary lipids. Main lipid classes in blood plasma were cholesterol esters and phosphatidylcholine, and in liver phosphatidylethanolamine and phosphatidylcholine. Fatty acid composition of the major blood plasma and liver lipids, and of perirenal fat, tended to reflect dietary fatty acid concentrations.

Influence of triglycerides and free fatty acids in milk replacers on calf performance, blood plasma, and adipose lipids.

In a 4-wk study of 48 3-day-old calves we compared effects of feeding various fats or their free fatty acids in skim milk-powder based milk replacer, on calf performance, feed utilization, and blood plasma and adipose lipids. When fat was fed, calf performance and feed utilization were equivalent for tallow and coconut oil diets but markedly poorer for corn oil. Complete replacement (tallow) or one-half replacement (coconut and corn oils) of the fats with their free fatty acids reduced calf gains and feed efficiency. Tallow free fatty acids gave lower digestibilities of palmitic and stearic acid and reduced calcium absorption. Free fatty acids from both coconut and corn oils reduced diet palatability and intake; those from tallow and coconut oil markedly interfered (in vitro) with rennet clotting of milk replacers. The main lipid classes in blood plasma for all treatments were cholesteryl esters and phosphatidylcholine. High concentrations (56 to 87%) of linoleic acid occurred in cholesteryl esters for all diets despite low concentrations of linoleic acid in the tallow and coconut oil diets.

Digestion of fat in calves fed milk replacers prepared by homogenization of low-pressure dispersion.

High-fat (40% of dry matter) milk replacers were fed to calves from 3 to 14 days of age to determine the effect of fat dispersion method on abomasal retention of dietary fat and crude protein and on digestion of fat in upper parts of the gastrointestinal tract. Relative to homogenization, low pressure dispersion of fat into milk replacer resulted in a) much larger rennet clots in the abomasum, b) increased abomasal retention of dietary triglycerides and crude protein, and c) increased triglyceride hydrolysis in the abomasum, duodenum, and upper jejunum at 4 h after feeding. When high-fat diet was fed to postnatal calves, formation of large firm clots was beneficial for promoting a slower release of fat and protein into the duodenum, and digestion of fat was improved in the gastrointestinal tract. Pregastric esterase hydrolysis of dietary triglycerides (tallow, coconut oil) in the abomasum resulted in preferential release of capric, lauric, and myristic fatty acids and a proportionately low release of palmitic and stearic acids.