Effects of Storage Conditions on the Stability of Isoflavone Isomers in Lactic Fermented Soymilk Powder

In this study, the powder of the Streptococcus thermophilus-fermented soymilk was stored under five conditions for 10 weeks: (1) at 25 °C with no added desiccant or deoxidant, (2) at 25 °C with added deoxidant, (3) at 25 °C with added desiccant, (4) at 25 °C with both deoxidant and desiccant added, and (5) at 4 °C with both deoxidant and desiccant added. It was found that the isoflavone content of the fermented soymilk powder declined during storage depending on storage condition and the kind of isoflavone isomer. The highest residual of isoflavone was found with the fermented soymilk powder stored at 4 °C with deoxidant and desiccant. Under this storage condition, total isoflavone, β-glucosides, acetylglucosides, malonylglucoside, and aglycone isoflavone in fermented soymilk powder showed residuals of 89.7%, 83.5%, 87.8%, 61.0%, and 93.1%, respectively, of its original content after 10 weeks of storage. Meanwhile, the smallest respective residuals of 71.2%, 29.3%, 47.9%, 51.8%, and 79.2% were noted with powder stored at 25 °C with neither deoxidant nor desiccant. However, the percentage of aglycone, the bioactive isoflavone, increased from an initial value of 77.95% to 86.76% in this sample.