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In: Biology

describe the metabolic process of the bacteria that is used during cheese making, indicating the type...

describe the metabolic process of the bacteria that is used during cheese making, indicating the type of respiration that is happening during the process and the chemical reactions necessary for the formation of lactic acid in great detail. Please use your own words, thanks

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Expert Solution

Cheese preparation :

Chesse is prepared from Milk. (Milk is first standardised to optimise protein fat ratio to produce good quality of cheese)

The metabolic process starts with inocculation of milk with starter and non starter bacteria and ripening process.

More commonly starter bacteria are employed.The bacteria convert milk sugar (Lactose) into lactic acid. The common starter bacteria are Lactococcus, Lactobacillus or Streptococcus families. (Lactic acid bacteria)

The steps involved in cheese preparation.

1. Standardization of Milk : Milk is often standardized before cheese making to optimize the protein to fat ratio to make a good quality cheese with a high yield

2. Pasteurize/Heat Treat Milk : The milk will be pasteurized or mildly heat-treated to reduce the number of spoilage organisms and improve the environment for the starter cultures to grow.

3. Cool Milk : Milk is cooled after pasteurization or heat treatment to 90°F (32°C) to bring it to the temperature needed for the starter bacteria to grow. If raw milk is used the milk must be heated to 90°F (32°C).

4. Inoculate with Starter & Non-Starter Bacteria and Ripen : The starter cultures and any non-starter adjunct bacteria are added to the milk and held at 90°F (32°C) for 30 minutes to ripen. The ripening step allows the bacteria to grow and begin fermentation which lowers the pH and develops the flavor of the cheese.

5. Add Rennet and Form Curd : The rennet is the enzyme that acts on the milk proteins to form the curd. After the rennet is added, the curd is not disturbed for approximately 30 minutes so a firm coagulum forms.

6. Cut Curd and Heat : The curd is allowed to ferment until it reaches pH 6.4. The curd is then cut with cheese knives into small pieces and heated to 100°F (38°C). The heating step helps to separate the whey from the curd.

7. Drain whey : The whey is drained from the vat and the curd forms a mat.

8. Texture curd : The curd mats are cut into sections and piled on top of each other and flipped periodically. This step is called cheddaring. Cheddaring helps to expel more whey, allows the fermentation to continue until a pH of 5.1 to 5.5 is reached, and allows the mats to "knit" together and form a tighter matted structure. The curd mats are then milled into smaller pieces.

9. Dry Salt or Brine : For cheddar cheese, the smaller, milled curd pieces are put back in the vat and salted by sprinkling dry salt on the curd and mixing in the salt.

10. Form Cheese into Blocks : The salted curd pieces are placed in cheese hoops and pressed into blocks to form the cheese.

11. Store and Age : The cheese is stored in coolers until the desired age is reached. Depending on the variety, cheese can be aged from several months to several years.

12. Package : Cheese may be cut and packaged into blocks or it may be waxed

METABOLIC PROCESSESS AND CHEMICAL REACTIONS IN CHEESE PREPARATION :

Fermentation of lactose to lactic acid by lactic acid bacteria is an essential primary reaction in the manufacture of all cheese varieties.

The reduced pH of cheese curd which reaches 4.5 to 5.2 depending on the variety, affects at least the following characteristics of curd and cheese :

syneresis (collection of whey on surface of yogurt and hence cheese composition),

retention of calcium (which affects cheese texture),

retention and activity of coagulant (which influences the extent and type of proteolysis during ripening),

the growth of contaminating bacteria.

Most (98%) of the lactose in milk is removed in the whey during cheese making, either as lactose or lactic acid. The residual lactose in cheese curd is metabolized during the early stages of ripening.

During ripening lactic acid is also altered, mainly through the action of nonstarter bacteria. The principal changes are

(1) conversion of L-lactate to D-lactate such that a racemic mixture exists in most cheeses at the end of ripening.

(2) in Swiss-type cheeses, L-lactate is metabolized to propionate, acetate, and CO2, which are responsible for eye formation and contribute to typical flavor.

(3) in surface mold, and probably in surface bacterially ripened cheese, lactate is metabolized to CO2 and H2O, which contributes to the increase in pH characteristic of such cheeses and that is responsible for textural changes.

(4) in Cheddar and Dutch-type cheeses, some lactate may be oxidized to acetate by Pediococci.

Cheese contains a low level of citrate, metabolism of which by Streptococcus diacetylactis leads to the production of diacetyl, which contributes to the flavor and is responsible for the limited eye formation characteristic of such cheeses.


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