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 .
Milk protein concentrate (MPC) and micellar casein (MC) powders are commonly used to increase the protein concentration of cheese milk. However, highly-concentrated milk protein powders are challenging in terms of solubility. The research question was whether and how incompletely dissolved agglomerates affect the protein functionality in terms of rennet gelation behavior. For the experiments, skim milk was enriched with either MC88 or MPC85 to a casein concentration of 4.5% (w/w) and sheared on a laboratory and pilot scale in rotor/stator systems (colloid mill and shear pump, respectively) and high-pressure homogenizers. The assessment criteria were on the one hand particle sizes as a function of shear rate, and on the other hand, the rennet gelation properties meaning gelling time, gel strength, structure loss upon deformation, and serum loss. Furthermore, the casein, whey protein, and casein macropeptide (CMP) recovery in the sweet whey was determined to evaluate the shear-, and hence, the particle size-dependent protein accessibility. We showed that insufficient powder rehydration prolongs the rennet gelation time, leading to softer, weaker gels, and to lower amounts of CMP and whey protein in the sweet whey.
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