LPT 311: Milk and Milk Products Technology (1+1)

PREPARATION OF CURD

In India curd is also called Dahi. As per PFA rules, dahi is a product obtained from pasteurised or boiled milk by souring natural or otherwise by using harmless lactic cultures.

Apparatus required

• Stainless steel wares, glass wares, water bath, incubator, refrigerator, etc.

Materials required

• Good quality milk, starter culture which includes Streptococcus diacetyl lactis, Streptococcus. cremoris, ith an aroma producing bacteria mainly Leuconostoc citrovorum or Leuconostoc dextranicum.

Procedure

• Pasteurise milk at 85^oC for 10 min and cool the milk immediately (20-25^oC) by keeping the container in running water, by simultaneous agitation. Add 0.5% of starter culture at this temperature and mix thoroughly. Distribute this cultured milk into a small sterile steel or glass container taking aseptic precautions, and fix a sterile Aluminium cap. Permit the cultured milk to remain undisturbed for 16-18 hours at 22-25^oC, or to reach the acidity of 0.8-0.98

Judging of dahi

• It is done to find whether the dahi offered for sale possess desirable characters for direct consumption.

Procedure

• If the container is taken from cold storage, it is allowed to warm slightly without opening the lid. Examine for the firmness of body, separation of whey at the bottom, middle or top, presence of gas bubbles and change in colour. Open the lid and immediately examine the contents for flavour, odour, etc.
• Desirable characters of market quality of dahi includes colour, appearance, flavour, body and texture and acidity.

While examining the body and texture following defects have to be noted..

1. Watery consistency : Due to low solids or due to poor quality of milk. Too high or insufficient heat treatment can also cause this defect.
2. Hard and lumpy curd: This is due to over ripening especially at high temperature and this defect is usually found along with same amount of whey formation.

Whey off: Wheying off in dahi may be due to low solid content or due to deliberate dilution with milk. Higher acidity also causes these defects. Disturbances or vibrations at the time of curd setting also result in whey formation.

3. Ropiness: Ropiness due to faulty fermentation resulting in low acid production and sweet curdling due to certain microbes.

Common flavour defects are:

• Bitter and cheasy: due to peptonising proteolytic organisms.
• Cooked defect: due to overheating
• Flat: due to low acid and diacetyl production
• Rancid: due to lipolytic organisms
• Fruity and alcoholic: due to yeast and molds
• Malty defect: due to S. lactis var. multigenes

Requirements for dahi

Characteristics	Requirement for sweet dahi	Requirement for sour dahi
Acidity, lactic (percentage weight)	0.70	1.0
Yeast and mould count/gm	100	100
Coliform count/gm	10	10
Phosphatase test	Negative	Negative