Continuous microwave heating to inactivate thermophilic spores in heating-sensitive skim milk concentrate

Abstract

Skim milk powders often contain high spore counts, originating from persistent thermophilic bacilli in the processing plants. Through germination and enzymatic spoilage, product defects in the reconstituted product formulation may result. Heating of skim milk concentrate prior to drying may inactivate these spores. Within this study, a new method to grow thermophilic spores isolated from dairy processing plants (Anoxybacillus flavithermus and Geobacillus stearothermophilus; N0 = 6 log10 cfu mL−1) was developed, followed by heat-inactivation experiments in reconstituted skim milk concentrate using a continuous pilot-scale microwave plant. Spore reductions were counted on tryptic-soy agar (55 °C) and formal reaction kinetics were calculated. Reductions of 1–3 log10 units while applying temperatures of 110–125 °C with heat holding times of 5–270 s were achieved, depending on the strain, whereby G. stearothermophilus 1978 spores were more heat resistant than the ones of A. flavithermus 1958.