Growth of milk fermentative bacteria in the presence of chitosan for potential use in cheese making

The effects of different molecular weight chitosans alone and their complexes with casein on the growth of three representative milk fermentative bacteria ( Lactobacillus delbrueckii ssp. bulgaricus, Streptococcus thermophilus and Propionibacterium freudenreichii) were compared. In nutrient broth, all chitosans showed a dose dependent inhibition of S. thermophilus and L. delbrueckii ssp. bulgaricus growth. Chitosan of high and low molecular weight, but not chitosan oligosaccharides, showed a dose dependent inhibition of P. freudenreichii that was dependent on the initial bacterial load. The inhibiting effects of chitosan were greatly reversed when the biopolymers were incubated with milk before interaction with bacteria. Under these conditions a cheese like product was made with high molecular weight chitosan as the coagulating agent. The product had lower firmness than the respective cheese control, probably because of a higher water retention. The effect of chitosan on milk fermentative processes appeared to be dependent not only on its molecular weight and concentration, but also on the presence of casein micelles or milk fat, that, acting in a competitive manner, could prevent the inhibitory activity of these biopolymers on bacterial growth.