Notes

Abstract

some time,l but mostly involve preparation of basal media containing twenty or more amino acids; such preparation is timeconsuming and relatively expensive. Plate assays are often needed at regular intervals for samples available only towards the end of a working day, when microbiological plate methods can usually be more readily carried out than chemical determinations.Lewis and Olcotta used a casein hydrolysate free from glutamic acid for tube assays, and it therefore seemed possible that hydrolysed casein might be freed from both glutamic and aspartic acids by an ion-exchange process and be suitable for use in plate assays. PREPARATION OF EXPERIMENTAL CASEIN HYDROLYSATE FOR BASAL MEDIUM-Light, white, soluble casein was hydrolysed by 25 per cent. sulphuric acid (500ml of acid to 120g of casein), neutralised with barium hydroxide and filtered. The filtrate was treated with successive amounts of animal charcoal (20 g per litre) until it was a pale straw colour and was then evaporated to give a 20 per cent. w/v solution.This solution was then allowed to run through a column containing the strongly basic anionexchange resin De-Acidite FF (obtainable from the Permutit Co. Ltd., London) that had been conditioned in the acetate form with 6 per cent. acetic acid. After passage of the hydrolysate solution, the resin was washed with distilled water until the effluent gave a negative ninhydrin test. The hydrolysate solution and the water washings were combined and evaporated under reduced pressure to give an approximately 15 per cent. w/v solution, the pH of which was adjusted to 6.4. The casein hydrolysate for use in the basal media was obtained by spray-drying this treated solution. The De-Acidite FF resin removed glutamic and aspartic acids and also sufficient of the tyrosine for the hydrolysate to be used in media for plate assays of these amino acids; tryptophan was already removed during the acid hydrolysis. TABLE I COMPOSITION OF BASAL MEDIUM