Rheological Characteristics and Microstructure of Probiotic Soy Yogurt Prepared from Germinated Soybeans

Mei Yang^1,2, Jing Fu^1,3 and Li Li¹*

¹Research and Development Center of Food Proteins, College of Light Industry and Food Science, South China University of Technology, Guangzhou 510641, PR China
²Research Group for Bioactive Products, Department of Biology and Chemistry, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kong SAR, PR China
³Bio-Resources Key Laboratory of Shaanxi Province, Shaanxi University of Technology, Hanzhong 723001, PR China

Article history:
Received February 24, 2011
Accepted June 28, 2011

Key words:
soybean germination, soy yogurt (sogurt), probiotics, rheological properties, microstructure, protein hydrolase

Summary:
Soy yogurts (sogurts) were prepared from germinated soybeans [Glycine max (L.) Merrill] with different hypocotyl lengths. This study is mainly focused on the effects of soybean germination on the rheological characteristics and the microstructure of sogurt. Results show that, after soybean germination, sogurt had lower storage module (G’) and loss module (G’’) in the linear viscoelastic region than the sogurt from ungerminated soybean. Shear sweep test demonstrated that the flow behaviour index (n) of sogurt obviously rose from 0.225 to 0.241 (p<0.05), while the yield stress (τ₀) and consistency coefficient (κ) decreased remarkably from 3.25 to 1.98 Pa and from 12.61 to 8.37 Pa·sⁿ, respectively. Furthermore, the three-dimensional network of sogurt became more ordered and open, with the diameter of interspaces increased from 1–2 to 5–6 μm. Protein hydrolase system assays indicated that endopeptidase, aminopeptidase, dipeptidyl aminopeptidase and carboxypeptidase in sprouted seeds showed different activities towards synthetic substrates. It can be concluded that, after the limited proteolysis of soybean storage proteins by endopeptidases and exopeptidases in germinated soybeans, the rheological properties and microstructure of sogurts can be improved.

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