Application of mild pulsed electric fields on starter culture accelerates yogurt fermentation

The goal of this work was to investigate the influence of pulsed electric fields (PEF) operational parameters on a mixture of Streptococcus thermophilus DIL 5218 and Lactobacillus delbrueckii subsp. bulgaricus DSMZ 20081 T with regards to the culture’s acidification capability in reconstituted skim milk medium. We investigated the effects of field strength, pulse frequency and total number of pulses by use of design of experiments and a two-level full factorial design. The responses were the cell counts of the two microorganisms after PEF application, the pH lag phase λ pH , the maximum pH change rate µ max , the maximum pH change and the oxidation/reduction potential (ORP). The application of PEF on the mixed culture accelerated the acidification of milk by an average of 12 min in an approx. 160 min lasting control λ pH . In contrast the maximum pH change rate µ max and the maximum pH change decreased slightly in fermentations with PEF-treated cultures. Furthermore, a significantly faster decrease of the oxidation/reduction potential (ORP) already within the first 30 min and a lower final ORP was observed in milk fermented with PEF-treated culture. The total number of pulses applied was the most influencing factor in most of the responses measured. We hypothesized that the reason for the enhanced performance of the PEF-treated culture was a combination of an oxidative stress response of S. thermophilus DIL 5218 and an enhanced proteolytic phenotype in L. delbrueckii subsp. bulgaricus DSMZ 20081 T .