Selection of Lactobacillus spp. for protective composition for in cheese production

Abstract

Background. Production of hard and semi-hard cheese depends significantly on the quality of raw milk, especially on the level of its microbiological contamination. Specific mode of milk pasteurization typical for cheesemaking does not guarantee complete inactivation of undesirable microflora, besides risk of secondary contamination is probable. There is also risk to obtain cheese with the defects of microbial origin. To prevent excessive development of undesirable microorganisms protective cultures are used. They have significant inhibitory effect on the growth of the above mentioned microorganisms.
The aim is to determine the most important criteria for the selection of lactic acid bacteria for protective compositions used in the production of hard and semi-hard cheese.
Material and methods. Basing on the complex of technological properties nine lactic bacteria were taken from 247 strains from non-commercial dairy products. They identified according to combination of physiological and biochemical properties according to [15]: 9 strains were assigned to the species L. casei, one strain to the species L. paracasei and one strain to the species L. acidophilus. The level of antagonistic activity of LAB concerning harmful microflora was determined by "diffusion holes" method [16], the intensity of antagonistic activity was evaluated by the diameter of no growth area of test culture around the holes with metabolites of LAB. The maximum acidity of milk clots was determined according to GOST 3634–92, synergistic affects according to [17]. Proteolytic activity of lactobacilli was determined using Eijkman agar; resistance to salt, sodium nitrite and temperature of 55 °C according to [18].
Results. The most important selection criterion is high antagonistic activity to the undesired microflora – сoliform bacteria, Staphylococcus aureus, spore-forming microorganisms. However, the selection of microorganisms for protecting cultures should be made on the basis of their low to none antagonistic activity to the starters. Besides, the moderate parameters of acidifying activity and maximum acidity in milk should be characteristic for such milk acid bacteria, together with their resistance to physical and chemical factors (temperature and salt content), and the milk curds produced therewith should exhibit good syneresis. Such important selection criteria of biotechnology for all microorganisms as resistance to bacteriophages should also be taken into account.
The strains of lactobacilli selected by us met these criteria: they showed antagonistic activity to test-culture of technically harmful microflora on the level of 18 ÷ 25 mm, the maximum acidity of milk at the level (200 ÷ 250) °Т, sufficient level of thermal stability and low sensitivity to nitrate and salt, and formed milk clots of proper (48 ÷ 51 %) synergistic parameters.
Conclusion. Selection criteria of microorganisms for use as protective cultures in the production of natural cheese are determined based on the analysis of world scientific data together with the results of own experimental studies. Characteristics of lactobacilli strains we have selected prove their perspective use in biotechnological developments as protective cultures for cheese.