MVSc

An experiment was conducted to find out the effect of preservatives such as nisin and potassium sorbate on the keeping quality of dahi. An attempt was also made to study the quality of dahi produced under household conditions with a special emphasis on the maintenance of starter.

Dahi was prepared under laboratory conditions and under household conditions. Both were divided into four parts and applied four treatments namely (1) Treatment A – with 1000 IU nisin /g curd (2) Treatment B – with 10 ml of nisin producing organism capable of producing 1000 IU of nisin/100 g curd (3) Treatment C – with 0.2 per cent potassium sorbate and (4) Treatment D – Dahi stored as such without any treatments. All treatments were compared with dahi at 0 hour (control).

The dahi after treatments A, B, C and D from both the methods were mixed well and transferred to 100 ml cups and stored at room temperature. Samples from each treatment were subjected to chemical, microbiological and organoleptic evaluation on the 2nd, 3rd, 4th, 7th, 10th and 21st day of storage for acidity, pH, lipolysis, proteolysis, diacetyl, total lactic count, coliform count and yeast and mould count.

There was a decrease in acidity for all treatments from the initial level under laboratory method whereas acidity was maintained at the initial level under the house hold method. There was only a slight change in pH during different periods of storage for all the treatments under both methods.

Proteolysis of dahi remained to the initial level upto 24 hours then increased gradually under both methods. Tyrosine value was higher under household conditions which could be due to the presence of proteolytic bacteria other than starter organisms that have gained entry into dahi through starter culture.

An abrupt reduction in diacetyl contents was noticed in dahi during storage. This could be due to the reduction of diacetyl by the enzyme diacetyl reductase produced by certain streptococci. With regard to the free fatty acid content treatments A, B and C were effective in controlling the breakdown of fat during storage.

Addition of nisin did not alter the total lactic count and it was maintained to initial level even after storage for 4 days. Nisin producing organisms multiplied during storage. In potassium sorbate added dahi a reduction in lactic count was noticed probably due to depletion of nutrients rather than the direct effect

Growth and multiplication of yeast, mould and coliforms were controlled in both methods by nisin or nisin producing organisms in dahi. Potassium sorbate completely eliminated yeast and mould and coliform growth in dahi but it reduced flavour score during sensory evaluation.

From the above findings it was concluded that the low acidity produced on storage might be due to the fact that the lactic acid bacteria added were low acid producers. There was a reduction in flavour score for potassium sorbate added dahi but at the same time it was found to suppress the yeast and mould growth completely. Hence it is recommended that potassium sorbate preferably at a reduced level of 0.1 per cent along nisin producing organisms can keep up the quality of dahi under room temperature. Also it is advisable to change the culture at least once a week to get good quality dahi with characteristic diacetyl flavour under house hold conditions.