R. KARNAD                                         359

                                          SUMMARY

A new species of butter aroma bacteria Str. diacetyl aromaticus has been
isolated from Indian dahi by plating on lactose-litmus-agar. This charac-
teristic type of Streptococcus is a powerful producer of acetoin and diacetyl in
milk culture without the addition of citric acid or other acids and produces
less volatile acidity than the citrovorus group.

This organism is different from Sc. lactis in its biochemical characteristics.
It also differs from Sc. cremoris and Str. citrovorus and Str. paracitrovorus.

Aroma development in the culture is highest at a temperature of 25°-28°C
after 48 to 72 hours' incubation. The maximum amount of diacetyl is pro-
duced in the culture after 72 hours when the pH is about 4.8. Addition of
yeast extract to the milk improves the flavour and the yield of diacetyl. In
association with a lactic acid organism the aroma bacteria gives a higher yield
of diacetyl. Neutral sodium citrate increases the yield of diacetyl to an
appreciable degree. The volatile acid content of the culture is not increased
by adding citric acid and citrates (0.2 to 0.4 per cent) to the milk. Yeast
extract has a pronounced effect in increasing the volatile acidity. A mix-
ture of yeast extract and phosphate increases the volatile acid production.

The flavour of butter is improved on the addition of neutral sodium
phosphate to the milk before inoculating the culture. A mixture of phosphate
and yeast extract gives better results. Phosphates have a stimulating effect
on the production of gas by Str. diacetyl aromaticus.

This species of Streptococcus would appear to have great possibilities in
butter-making.

                                 ACKNOWLEDGEMENT

The author wishes to thank Dr V. Subrahmanyan and Mr B. N. Banerjee
for their kind guidance and help throughout this work.

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