Development of a medium simulating Emmental juice for propionibacteria growth

Abstract

Our aim was to develop an Emmental juice-like medium (EJLM) for the growth of propionibacteria. Cheese juices were extracted by pressing three different Emmental cheeses before entering the warm room (i.e. before propionic fermentation) and their composition was determined. They contained (g/kg) lactate 34·6–36·9, protein 23·8–29·6, NaCl 15–23, Ca 5·9–7·3 and no residual sugar, and had a pH of 5·4. In order to simulate this composition, EJLM was constituted from the microfiltrate of a milk, enriched in native casein and fermented by a Lactobacillus helveticus strain until lactose exhaustion. PO₄³⁻, K⁺, Mg²⁺, Ca²⁺ and Cl⁻ were added to reach a mineral balance and an ionic strength close to those of juice. The main difference was the nitrogen content: that of juice was ∼3·5 times that of EJLM. Four Propionibacterium freudenreichii strains grew with similar growth rates in EJLM and in juice at 24°C in anaerobiosis and reached >3×10⁹ cfu/ml. Using EJLM, Prop. freudenreichii TL 173 grew at pH 5·4 or 6·0, at 24 or 30°C, in semi-anaerobiosis (static culture) or anaerobiosis (CO₂ atmosphere), showing a synergic inhibition of growth at low pH and temperature and in semi-anaerobiosis. EJLM provided a way to study propionibacteria in an environment similar to that in Emmental cheese.