Effect of Thermal Treatment and the Addition of Texture Modifiers on the Rheological Properties and the Microflora of Reconstituted Kefir Powder

The present study examines the effect of low-temperature thermal treatment before drying, through storage at −10 °C and 4 °C for 72 h, respectively, on the physicochemical and microbiological properties of spray-dried kefir powder. Furthermore, with the intention of improving the rheological behavior of the reconstituted product, texture modifiers were employed including milk proteins (milk proteins, sodium caseinates, and whey protein concentrates) and carrageenan. According to the results, the low-temperature thermal treatment of kefir, prior to drying, resulted to an increased moisture content and yellowness of the kefir powder samples, with a simultaneous drop to the whiteness index and an increase of the particle size in both the powder and the reconstituted samples. The sample with prior treatment at 4 °C for 72 h, exhibited decreased pH values and increased acidity for both kefir and reconstituted product, while it also improved post drying population survival of lactobacilli and yeasts. The reconstituted sample with prior treatment at −10 °C for 72 h, exhibited evident pseudoplastic behavior, which, at low shear rates, yielded viscosity values very close to those of the fresh control kefir. Addition of sodium caseinates, in the absence and/or simultaneous presence of carrageenan, resulted to the highest viscosity increase of the reconstituted products. Milk proteins with the combined presence of carrageenan exhibited similar apparent viscosity values to the control.