Abstract
Although the literature contains several references concerning to the browning reaction, that is to say, the amino-carbonyl reaction of heated milk, investigation on the main factors of this reaction has not been extended beyond the milk protein-lactose system, as source of the reaction. In view of the facts described herein, the most reasonable conclusion to be drawn from available data is that the urea contained in milk is the other main factor taking place in the amino-carbonyl reaction, besides the protein-lactose system. Urea, however, has been dealt with in previous reports as an inactive reactant in the amino-carbonyl reaction.
(1) The urea commonly contained in milk is completely decomposed by the addition of 1cc of 10 per cent urease solution, prepared from soya bean powder by the method of VAN SLYKE and CULLEN, when 10cc of milk has been incubated at 20° for 3hours. (Fig. 1)
(2) The browning of milk in which urea has been decomposed by the urease action before heating at 120° for 2 or 3hours is not more remarkable than the control milk to which (NH4)2CO3 was added, equivalent to the urea contained in milk, in order to cancel the reaction of (NH4)2 CO3 produced in the samples by urease action. (Table 1).
(3) The loss of urea in milk is 67 per cent during heating at 100° for 10hours. It appears that these changes obtained are maximum in amino compounds, the amino-carbonyl reaction taking place as shown by HODSON and KRUEGER. (Fig. 2).
(4) The browning of heated milk is increased by increasing the addition of urea to milk before heating. (Table 2)
