Heat-resistant bacteria contamination investigation in Chinese soybean curd industrial processing using high-throughput gene sequencing and MALDI-TOF-MS

Abstract

Heat-resistant microbial contamination during soybean curd (dried tofu, SC) processing requires sterilization for food safety and shelf-life extension, which causes significant texture damage and adverse taste. In this work, SC texture stiffening was confirmed to be temperature dependent. 121 °C and 110 °C autoclaving caused over 2.0- and 1.5-fold increase of SC hardness, respectively, while 85 °C thermal treatment had no obvious impact on SC texture. Heat-resistant bacteria caused thermal treated SC spoilage, and high-throughput 16S rDNA gene sequencing analysis suggested that Bacillus, Clostridium, Lysinibacillus and Brevibacillus were the main contaminations. Furthermore, we made a comparative study on bacterial contamination of two different industrial SC processing lines, using viable colony counting, high-throughput 16S rDNA gene sequencing and MALDI-TOF-MS. Staphylococcus, Macrococcus, Psychrobacter, Bacillus and Acinetobacter were the predominant genera in each semi-finished product. These two processing lines showed very low planktonic and sedimented colony values. The air-exposed food contact surface introduced most non-spore forming contaminating bacteria, with the exception of Bacillus from coagulation. Although raw soybean and processing aids including coagulant and marinade also had low colony values, they introduced heat-resistant bacteria strains, which attached on pipelines and formed biofilms on tofu-coagulation cloth causing sustained and serious contamination in coagulation process.