Fermented foods :

Fermented foods :

Prescott and Dunn defined fermentation as a process in which chemical changes are brought about in an organic substrate through the action of enzymes elaborate by microorganisms. Biochemically fermentation is the metabolic process in which carbohydrates are partially oxidized with the release of energy in the absence of any external electron acceptor.

Starter culture:

A starter culture is a single or mixed culture of microorganisms used as initial inoculum for fermentation of food. E.g., dairy starter cultures are LABs (Lactic acid bacterias) which are of 16 different genera. The main function of this starter culture is to produce lactic acid from lactose. LABs are of two types: (a) Homofermentative- those that produce lactic acid as the major or sole product of fermentation. E.g., Lactobacillus, Lactococcus, and Streptococcus. The homolactic are able to extract about twice as much energy from a given quantity of glucose compared to as heterolactics. (b) Heterofermentative- those lactics that produce equal molar amounts of lactate, CO2 and ethanol from hexoses. E.g., Leuconostoc, Enterococcus etc.

The other function of the starter culture are :

·     production of flavor, aroma, and alcohol.

·     Proteolytic and lipolytic activities

·     Inhibition of undesirable organisms

A good starter culture will

(i)            convert most of the sugar to lactic acid,

(ii)           increase the lactic acid concentration to 0.8% to 1.2% (known as titratic),

(iii)          drop of the pH to between 4.3 to 4.5.

Starter cultures can be produced in large quantities and preserved by freezing in liquid nitrogen or by freeze-drying (lyophilization). The main threat to the preservation of starter culture is by bacteriophages, stringent culture handling and plant sanitation programs are essential in preventing bacteriophage problems. Phage-resistant starter cultures are also be devised.

Yoghurt:

The starter culture for yogurt is a mixed culture of Streptococcus thermophilus and Lactobacillus bulgaricus in 1:1 ratio. S.thermophilus is responsible for acid production and grows faster than Lactobacillus, whereas L.bulgaricus adds the flavor and aroma by the production of the chief volatile flavor component i.e., acetaldehyde.

The process of preparation:

1.  Whole or skimmed milk (cow’s milk) is taken.

2.  Reduced the water content by at least 1/4th.

3.  Add 5% by weight of milk solids or condensed milk.

4.  Heat the concentrated milk to 82-93 C for 30-60 minutes.

5.  Cool to about 45C and add 2% yogurt starter culture (S.thermophilus and L.bulgaricus).

6.  Incubate at 45 C for 3-5 hours.

7.  Cool to 5 C.

The titratable acidity of yogurt is 0.85- 0.90 %. Yogurt is a thermophilic fermented milk product with high solid content.

Acidophilus milk:

Production procedure:

1.  Take sterile skimmed milk (pasteurized).

2.  Inoculate with Lactobacillus acidophilus (1-2% of inoculum).

3.  Cooled at 37 C for 8-12 hours.

Lactobacillus acidophilus is the inhabitants of the intestine and therefore has therapeutic value. Acidophilus milk has no typical flavor except a sour or lactic flavor due to the production of lactic acid (lactic acid production is 1-2%).

Differentiate between yogurt and acidophilus milk:

Yogurt:

1.    Starter culture is - S.thermophilus and L.bulgaricus. (mixed)

2.    Have characteristic flavor and aroma.

3.    High concentrate milk is used

4.    Reduced H2O content.

5.    Titratable acidity is 0.85-0.90%.

Acidophilus milk:

1.    Starter culture is - Lactobacillus acidophilus (only).

2.    No typical flavor.

3.    Skimmed milk is used not concentrated.

4.    Pasteurized milk was taken in which H2O content is relatively high.

5.    Titratable acidity is 1-2%.

Kefir:

·     Any type of milk is used.

·     Starter culture can be in any form of Kefir grains which contains Lactococcus lactis, Lactobacillus bulgaricus and a lactose fermenting yeast held together by layers of coagulated protein organisms get immobilized in the matrix synthesized by themselves.

·     Bacteria may be responsible for lactic acid production and yeast for alcohol production.

·     Acid and alcohol content of kefir can be 1% and effervescence is 2%

Kumiss:

·         Mare’s milk is used.

·         The starter culture is the same organism but not in the form of grains but present as a mixed liquid culture.

·         Alcohol content is 2%

·         Acid content is 1-1.2%.

·         Effervescence is 2%.

Dahi:

·     Any kind of milk can be used.

·     No standard solid content is required.

·     Pasteurize the milk,

·     then add starter culture (Lactococcus lactis, Lactobacillus, and yeast).

·     Incubate at 25-37 C for 6-8 hours.

·     Acidity is 0.7-0.8 %.

·     Flavor is because of production of diacetyl.

Differentiate between yogurt and dahi:

Yogurt:

1.  Starter culture is - S.thermophilus and L.bulgaricus.

2.  Incubation is required at 45 C for 3-5 hours.

3.  Whole or skimmed milk is taken.

4.  Titratable acidity is 0.85-0.90%.

5.  Flavor is because of acetaldehyde.

Dahi:

1.  Starter culture is - Lactococcus lactis, lactobacillus, and yeast.

2.  Incubation is at 25-37 C for 6-8 hours.

3.  Any kind of milk can be used.

4.  Acidity is 0.7-0.8%

5.  Flavor is because of production of diacetyl.

Cheese:

Cheese results from lactic fermentation of milk. Cheese is the curd of milk separated from the whey and pressed into a solid mass. Basic steps involved in cheese production are:

1.  Coagulum formation

2.  Separation of curd from whey

3.  Ripening of the cheese

1.  The milk coagulation occurs due to the following:

·     Inoculation with bacterial culture

·     Incubation with rennin

The starter culture for cheese production may differ depending on the amount of heat applied to the curd. Streptococcus thermophilus is employed for acid production in cooked curds because it is more heat tolerant. Combination of Streptococcus thermophilus and Lactococcus lactis is employed for curd's that receive an intermediate temperature. These cultures result in lactose degradation to produce lactic acid which lowers the pH to about 4.6.  The incubation with rennin cleaves gamma-casein to paracasein and smaller peptides and leads to coagulation of alpha and beta-casein.

2. Separation of curd: whey is the watery part of milk that separates after the milk sours and thickens. The coagulum is first heated to 37 C and then cooled. This eliminates rennin activity and separates the whey. The curd is separated from the whey and salted. The bricks are pressed to remove excess moisture to enable proper ripening.

2.    Ripening the cheese bricks are inoculated with fungi like Penicillium roqueforti or P.camemberti. for the development of appropriate flavors through protease and lipases activities, Cheddar cheese uses proteases from Bacillus amyloliquefaciens to enhance flavor. In the case of Swiss cheese, a mixed culture of Lactobacillus bulgaricus, Streptococcus thermophilus and Propionibacterium Shermanii is used.

The blue cheese is known as 'Roquefort cheese’, is inoculated with spores of Penicillium roqueforti which affects the ripening and imparts the blue-veined appearance characteristic of this type of cheese. Types of cheese based on the moisture content:

·     Very hard cheese (maximum 34% moisture)

·     Hard cheese ( maximum 34% moisture)

·     Semi Hard semi-soft cheese (39-50% moisture)

·     Soft cheese (50-80% moisture)

On the basis of ripening cheese divides into:

1.  Unripened

2.  Ripened by mold

Cheese production in brief:

1.  Raw cow’s milk - pasteurization

2.  Starter culture is added and incubated at 37 C for 30 mins.

3.  Acid production takes place

4.  Rennin is added (440 units/ 30 C for 40 mins.) which leads to coagulation

5.  Cutting, cooking (38 C for 40 mins.) this inactivate rennin.

6.  Whey drainage

7.  Brine, salting (15% NaCl at 20 C for 1 hour)

8.  Cheese formation - this could be stored at 4 C for 28 days or could be ripened by bacterial or fungal species.