Ultrafiltration of Skim Milk Prior to Cottage Cheese Making

Cottage cheese has the largest sales volume potential of all cultured dairy products in the United States today. Moreover, it can be a highly profitable product if proper control is exercised during its manufacturing (46). Commercial production of cottage cheese utilizes a significant portion of the total fluid milk produced in the U. S.A. After the 1955\u27s, yearly per capita consumption of cottage cheese increased from 1. 7 7 kg (3.9 lb) to a high of 2. 45 kg (5.4 lb) in 1972 (67) . Consumer demand for lower fat dairy products gradually increased, which put more pricing emphasis on the· nonfat milk solids. This caused a marked increase in the price of cottage cheese, which depressed the yearly per capita consumption to 1.9 5 kg (4.3 lb) by 1981 (67) . Poor yields are a problem of major concern in the cottage cheese industry today. In the 1950\u27s, yield factors of 1.8 to 1.85 kg of 20% solids curd per kg milk solids were commonplace and at that time skim milk usually contained between 9 and 10% total solids. During the 1960\u27s yields dropped to 1. 7 to 1. 7 5 kg of 20% solids curd per kg milk solids (5, 44) . Any increase in cheese yield and/or decrease in costs of production would improve profit for cheese producers and help maintain lower consumer prices. Most of the studies on cheese making have indicated the decreases in product yields during the past decades (4, 22, 66, 86, 91) were due to: 1) use of larger vats and mechanical agitators has become widespread, resulting in greater yield losses as curd fines; 2) a change in relative price supports of butter and nonfat dry milk (NFDM) increased the price of the latter, so many manufacturers of cottage cheese determined that fortification of skim milk with NFDM was no longer economically sound and the practice was discontinued. Hence, cottage cheese yields failed to reach projected levels, especially during hot, dry summer months when the total solids content of milk tended to be low (15, 66, 91); and 3) composition of the milk supply has changed as production per cow increased Lower solids-not-fat (SNF) in milk directly affects the recovery of milk solids in the cheese curd. Of particular interest to a cottage cheese manufacturer has been the decrease in casein content of milk, for casein comprises nearly 78% of the cottage cheese solids. Milk today (91) is more likely to contain 2.31% casein rather than the 2. 5% listed in earlier references (42, 100) . Currently, the methods of cottage cheese manufacture convert an average of only 74.9% of milk protein into cottage cheese, while the balance of the proteins remain in the whey (66, 86) . The proteins lost in whey are mainly a-lactalbumin and B-lactoglobulin which remain soluble under the conditions of cottage cheese manufacture and thus do.not become part of the curd (SO). They may be an added expense rather than a benefit to the dairy industry, since these proteins as whey solids must either be processed further, disposed of as waste, or both. Processing cottage cheese whey is expensive and difficult because it is both dilute (94% water) and highly acidic (pH 4. 6) (10, 50) . Whey proteins can be concentrated and removed from whey after cheese making (33, 50) or they may be recovered in the cheese by concentration in the milk with ultrafiltration (UF) (34, 43, 44, 47, 60) prior to cheesemaking. Ultrafiltration is a continuous method for separating high molecular weight solutes from fluid streams . Its ability to fractionate and concentrate complex fluids has led to several applications of ultrafiltration in the dairy industry (8, 23, 40, 62). Ultrafiltrating milk will raise the protein content; the· milk can then be used to make ripened types of cheeses. Milk concentrated by ultrafiltration requires less rennet and starter culture for cheese manufacture than does a conventional milk; moreover, processes based on ultrafiltrated milk produce less whey than do traditional processes (19, 20, 44, 47, 63). The use of ultrafiltrated milk in the production of cultured dairy products gives yields as much as 20% greater (44, 47, 56) due to the retention in the cultured products of proteins which normally would be lost in whey in traditional manufacturing processes. The present investigation was designed 1) to study the increase in total solids in starting skim milk after using UF to remove 25% (UF 25%) and 33% (UF 33%) of skim milk volume as permeate; 2) to explore the possibility that concentrating skim milk may increase cottage cheese yield; and 3) to determine if the acceptability of cottage cheeses which were obtained from UF concentrated skim milk were inferior, equal, or superior to those of cottage cheese manufactured without UF treatment of the skim milk