Effect of Spoilage Bacteria on Biacetyl Content and Flavor of Cottage Cheese

Observations on growth of certain cottage cheese spoilage bacteria indicated that their activity in the cheese usually was accompanied by loss of desirable butter culture aroma in the product.The bacterial species involved and the nature of the gelatinous or slimy spoilage defect have been described in an earlier paper (4).Elliker and Horrall (3) observed that samples of butter contaminated withPseudomonas putrefaciens displayed a marked loss of aroma during keeping quality tests.The reduction of aroma preceded the typical cheesy or putrid defect commonly associated with this organism.Chemical analyses correlated the loss of typical butter aroma with destruction of part of the biacetyl present.Later, Elliker (2) was able to demonstrate the ability of a number of types of bacteria to destroy biacetyl.The importance of the Pseudomonas group was emphasized in studies on commercial and experimental butters.It was pointed out that the loss of biacetyl due to microorganisms may be more important in some commercial foods than is generally realized.In the present study, as in those on butter, the loss of proper aroma often was observed to precede the pronounced physical or flavor manifestations of the spoilage.Thus it seemed obvious that at least some strains of the spoilage organisms were destroying a major aroma component (probably biacetyl) ; consequently, further studies were suggested.Purposes of the investigation were to study the relationship of hiacetyl to the desirable aromatic flavor of cottage cheese and to determine the effect of certain spoilage organisms on biacetyl content of cottage cheese, as well as on biacetyl added to milk.EXPERIMENTAL Analytical method.Analyses for biacetyl were conducted with a modified Pien, Baisse, and Martin method (1, 5) more recently described by Elliker (2).In general, the analytical method represented a colorimetric analysis of a steamdistilled fraction of the product.A further modification included in the present study involved generation of steam for the distillation in a 2-1.balloon flask holding a 20 ohm nichrome wire coil immersed in water.Steam flow was regulated by adjusting the voltage applied to the coil by means of a Variac transformer.Fifty-g.portions of cottage cheese were acidified with 2 ml. of lactic acid which had been steamed previously to remove any traces of biacetyl.This mixture was carefully steam distilled and two 9-ml.portions of distillate collected