Recent Developments

A number of cheese varieties have been manufactured from milk concentrated by ultrafiltration (3,4). Patents

Salt dispenser with oscillating boom spreader

Curd inlet

Levelling screw

Bulk salt receiver Heated salt hopper

Curd discharge trough with auger

Salt dispenser with oscillating boom spreader

Curd inlet

Levelling screw

Curd discharge trough with auger

Peg stirrer with parking position clear of curd on belt

Mellowing belt

Curd depth transmitter

Transfer plate

Salt whey outlet

Figure 3. Dual salt applicating and cheese curd mellowing system. Two stage continuous salting and mellowing of milled or granular curd. Source: Courtesy of Sherping Systems, Winsted, Minn.

Peg stirrer with parking position clear of curd on belt

Mellowing belt

Curd depth transmitter

Transfer plate

Salt whey outlet

Figure 3. Dual salt applicating and cheese curd mellowing system. Two stage continuous salting and mellowing of milled or granular curd. Source: Courtesy of Sherping Systems, Winsted, Minn.

have been issued (5) and some procedures have been reduced to practice, but after more than 20 years of research in this area only a minimal amount of the total world production of cheese is made with ultrafiltered milk.

Bactofugation used to remove microbes and spores from cheese milk has sparked some interest, especially in Europe (6). Microfiltration may find significant application in special milk treatment prior to, or during, product manufacture. Significant developments in recombinant biotechnology and fermentation technology may provide specificity previously not available for milk coagulation and cheese flavor development. New cultures, which are resistant to phage, may be on the horizon. New cheese-making technology may allow previously unattainable processes and product control to become reality.

Cheese Starter Cultures

Starter cultures are organisms that ferment lactose to lactic acid and other products. These include streptococci, leu-conostocs, lactobacilli, and streptococcus thermophilus. Starter cultures also include propionibacteria, brevibac-teria, and mold species of Penicillium. The latter organisms are used in conjunction with lactic acid bacteria for a particular characteristic of cheese, eg, the holes in Swiss cheese are due to propionibacteria, and the yellowish color and typical flavor of brick cheese is due to brevibacterium linens. Blue cheese and Brie derive their characteristics from the added blue and white molds, respectively. Lactic acid-producing bacteria have several functions:

1. Acid production and coagulation of milk.

2. Acid gives firmness to the coagulum, which influences cheese yield.

3. Developed acidity determines the residual amount of animal rennet influencing cheese ripening; more acid curd binds more rennet.

4. The rate of acid development influences dissociation of colloidal calcium phosphate, which then influences proteolysis during manufacture and the rheological properties of cheese.

5. Acid development and production of other antimicrobials control the growth of certain nonstarter bacteria and pathogens in cheese.

6. Acid development contributes to proteolysis and flavor production in cheese.

Types of Culture

There are some impending changes in the nomenclature of starter culture organisms; the nomenclature used here follows present standards (7). Mesophilic cultures are used for cheese types that do not exceed 40°C during cheese making (8). These starters are propagated at 21-23°C and include Streptococcus lactis ssp. lactis, S. lactis ssp. cre-moris, S. lactis ssp. diacetylactis and Leuconostoccremoris. These are used for Cheddar, Gouda, brick, Muenster, cream, cottage, and Quarg cheese types, leuconstoc and S. lactis ssp. diacetylactis ferment citrate to produce carbon dioxide and diacetyl to characterize Edam, Gouda, and cream cheese.

Thermophilic cultures are used in cheese types where curd is cooked to 45-56°C. These starters are propagated at 40-45°C and include S. thermophilus, sometimes fecal streptococci, Lactobacillus helveticus, L. delbrueckii ssp. lactis, and L. delbrueckii ssp. bulgaricus. These are used for Swiss, Emmentaler, Gruyère, Parmesan, Romano, mozzarella, and Gorgonzola cheeses (9).

Propagation

Cultures are propagated in milk or a medium containing milk components and other nutrients and are heat treated (85°-90°C for 45 min) to render milk free of contaminants. The medium is then cooled to incubation temperature before inoculation. This processing is accomplished in a jacketed stainless-steel vessel provided with agitation and supplied with sterile air pressure. Culture inoculum is available from suppliers in frozen or freeze-dried form. It consists of appropriate mixture of culture strains and is free of contamination. After inoculation, culture is allowed to grow in quiescent state for 12-18 h. At the end of growth the medium is acidic with a pH of 4.5 or lower, depending on the culture. Ripened culture cell population is about 109 cfu/g. The ripened culture is cooled to at least 5°C. This form of propagation is called bulk-starter propagation. The culture may be used immediately or held for several days. The growth medium may contain buffering salts or the acid produced may be neutralized and controlled by the addition of ammonia, potassium hydroxide, or sodium hydroxide under controlled conditions. The latter is called pH-controlled propagation.

Starter cultures are susceptible to bacteriophages (viruses), which destroy the cultures and hamper cheese production. Cultures are also sensitive to antibiotics and bac-teriocins (proteinaceus bacterial inhibitors). Propagated cultures are used at 1-5% of cheese milk. Highly concentrated (1010 cfu/g) frozen cultures are available for direct addition to cheese milk (350 g/5,000 lb). These are called direct vat set (DVS). This form is convenient, but expensive.

Coagulation

Coagulation of milk is essential to cheese making. The coagulation entraps fat and other components of milk. Most proteolytic enzymes can cause milk to coagulate. Rennet (chymosin, EC 3.4.23.4) is widely used for milk coagulation in cheese making (10). It is extracted from the fourth stomach of young calves. Commercial rennets may include blends of chymosin and pepsin (bovine or other animals). Microbial rennets with similar functionality prepared from M. miehei are known as Marzyme, Hannilase, Rennilase, and Fromase. Preparations from M. pusillus and Endothia parasiticus are called Emporase and Sure Curd, respectively. Proteases from plants are known to coagulate milk but are not used in commercial cheese making. Coagulation of milk is also effected by lowering pH to about 4.6 in quiescent state either by fermentation of lactose to lactic acid or by hydrolysis of gamma—delta lactones to produce cottage, Quarg, ricotta, and cream cheese.

Animal and microbial rennets are aspartic proteases with optimum activity under acidic conditions. Their molecular weights range from 30,000 to 38,000. The primary stage of rennet coagulation involves partial hydrolysis of if-casein, the principal stabilizing factor of milk protein, at Phe-105 to Met-106 bond. Destabilization of the residual protein (para-casein) occurs in the presence of Ca2+ at temperatures >18°C in the secondary, nonenzymatic stage of rennet coagulation. About 30% of calf rennet is retained in the curd before pressing and only 5-8% after pressing (11). The amount of rennet retained is governed by the pH of the curd. Microbial rennets are retained to a lesser extent (3-5%).

During coagulation, linkages between casein micelles are formed and many micelles are joined by bridges, but later these appear to contract, bringing the micelles into contact and causing partial fusion. The firmness of curd is due to an increase in both the number and strength of linkages between micelles. It is suggested that phosphoryl side chains of casein specially ^-casein are involved. These may be linked by Ca2+ bridges. In cheese, as-1-casein plays a structural role (12).

Casein aggregation and fusion continues after coagulum cutting and throughout cheese making and early stages of cheese ripening. This process of rennet coagulation of milk is fundamental to the conversion of milk to cheese curd and its manipulation is essential for the control of cheese manufacture (Table 1). Casein aggregation leads to curd formation and syneresis. Increase in acidity and temperature and the addition of calcium chloride accelerates the rate of curd formation and its syneresis. On the other hand, cold storage of milk, homogenization, higher pasteurization temperature, and increased fat content impede the two processes. Acid coagulation of milk, as in cottage-cheese making results in less aggregation of casein than is seen with rennet. During cottage-cheese making the pH of milk casein drops to its isoelectric point resulting in a soft, fragile gel, firm enough to be cut. For large-curd cottage cheese, a very small amount of rennet is added. Essentially all added rennet is inactivated at the cooking temperature of curd (57-60°C).

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Homemade Pet Food Secrets

Homemade Pet Food Secrets

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