Effect of mixing during fermentation in yogurt manufacturing.

Discipline: yogurt; Keywords: lactic acid bacteria, stirring, low-speed agitation, process time, acidity profile, microbiological dynamics.

Is agitation during fermentation beneficial to yogurt characteristics? This question was addressed by E.J. Aguirre-Ezkauriatza and coworkers in a study entitled: Effect of mixing during fermentation in yogurt manufacturing, which was published in the Journal of Dairy Science (2008), Volume 91, pages 4454-4465.

Commercially, yogurt is manufactured through fermentation by lactic acid bacteria at temperatures in the range of 27 to 40oC. Normally the yogurt is not stirred or agitated during the fermentation phase but after fermentation in the case of liquid formulations or formulations mixed with fruit. Stirring during fermentation, however, might be beneficial, particularly in improving heat and mass transport across the fermentation tank. The authors investigated these possibilities by studying the effect of low-speed agitation during fermentation on process time, acidity profile and microbiological dynamics. For that purpose they used two laboratory-scale fermentors (3 and 5 liter) with different heat-transfer characteristics and Lactobacillus bulgaricus and Streptococcus thermophilus as fermenting bacterial species.

It was found that agitation during fermentation diminished system heterogeneity by significantly improving heat transfer. In the fermentation system with the greater potential for severe temperature heterogeneity, the overall specific growth rate of Lactobacillus bulgaricus was more than twofold greater in the agitated than in the non-agitated fermentations. The final population count of the species was also consistently greater. The results differed, depending on experimental conditions applied: In bioreactors with smaller temperature gradients under non-agitated conditions, agitation during fermentation did not directly affect intrinsic growth rates or overall lactic acid production rates. In fermentation investigations at low to moderate inoculum concentrations, conducted under agitated conditions, an overall economy in processing time was observed because of a reduction in the lag phase (as defined by pH changes). An unexpected result was the greater concentration of non-dissociated lactic acid in agitated versus non-agitated fermentations. This is probably related to the creation of a slight micro-aerobic environment through the incorporation of small amounts of air via agitation, and its relevance is that it may moderately inhibit bacterial growth rate. The inhibition, however, was not noticeable in fermentors with severe heat-transfer limitations, because temperature heterogeneity hindered the effect.

In terms of sensory quality, yogurt samples agitated during fermentation had lower viscosities and poorer gel properties, which might be improved by adding a gelation agent. With regard to flavour, the evaluating panel could not distinguish between the agitated and non-agitated samples.

Bottom line: The study suggests that agitation during fermentation to produce liquid yogurt might have practical benefits, particularly in fermentation vessels with poor heat-transfer characteristics. Nowadays, traditional yogurt manufacturing is restricted to small-scale fermentation vessels, precisely because of the heat-transfer limitations. Agitation might allow fermentations at controlled temperature on a larger scale without sacrificing product quality characteristics.