358                                          Aroma in Dahi

Effect of neutral sodium phosphate.—In another set of experiments neutral
sodium phosphate was added up to 0 02 per cent to the milk before inocula-
tion to find its effect on the flavour and aroma of butter. The aroma of the
resulting butter was superior compared to the blank. Yeast extract (1 to 2
per cent) in combination with phosphate, when added to the milk, gave a
much better sample of butter.

The stimulating effect of phosphate on the fermentation by the aroma
bacteria can be demonstrated by the increase in gas evolution in a culture
medium containing yeast extract, 2 per cent dextrose and 2 per cent calcium
carbonate. Addition of 0.2 per cent to 0.3 per cent phosphate re-starts the
fermentation in a culture when fermentation has stopped in the usual course
after inoculation of the culture of the aroma bacteria. The rate of the re-
action before and after the addition of phosphate is shown in Fig. 1.

[NLS note: a graphic appears here - see image of page]

FIG. 1. Effect of potassium phosphate on gas evolution by Str. diacetyl aromaticus
                                         in yeast extract.

These studies have shown that the new aroma bacteria gives a fine-
flavoured butter and is a powerful producer of acetyl methyl carbinol and
diacetyl. The fact that the aroma of butter can be considerably improved
by the addition of yeast extract and phosphates seems to open up new
possibilities in the field of cream ripening for butter-making.