Technological properties of lactic acid bacteria isolated from Silter

Silter is a traditional Italian cheese made from raw cow's milk in the Sebino Bresciano and Valcamonica areas. It is frequently the product of marginal farmers who adopt traditional technology in their small-size milk production and processing. These farmers have long been endeavouring to maintain an old tradition by handing it down to the present time, generation by generation. Raw milk cheeses are characterised by the presence of indigenous microflora that is associated with a more intense, and qualitative, difference in flavour. The aim of this study was to characterize, and evaluate, important technological properties of the autoctonous lactic acid bacteria (LAB) isolated during the manufacture of Silter produced on the mountain and on the plain. The microbiological analyses show high levels (107cfu mL-1 in the milk and 108 cfu g-1 in the curd) of LAB species in the samples from both mountain and plain. 484 strains of LAB were isolated: 67 strains, selected after sensory analyses of cheeses, were evaluated for phenotypic characteristics, and for technologically relevant biochemical activities such as acid production and redox activity in milk, proteolytic activities. The strains were identified by Api 20 Strep, Api 50 CHL (Api System BioMérieux France), Biolog AN and GP Microplate (Biolog System USA) and were subjected to Specie-specific PCR in order to confirm phenotypic identification. A high level of biodiversity in the indigenous microflora: Lactococcus lactis ss lactis predominated (31/67) (Fig.1). The strains showed an Eh value between -217.0 mV and +151.4 mV: in 39 strains the reduction activity was high (Eh < -102.0 mV). The titratable acidity after 24 h ranged from 5 to 10 g lactic acid L-1 for 38 strains. Regarding acidifying activity, that of Streptococcus thermophilus was the fastest, reducing the pH of skim milk to 4.30 after 6 hours and 3.84 after 24 hours of incubation. The results suggest that the strains isolated from the raw milk and curd are a potential source of new starters with particular properties.