Evaluation of magnetic resonance for detection of bacterial contamination in low-acid, shelf-stable packaged soymilk

This study evaluated magnetic resonance (MR) as a nondestructive method for detection of bacterial contamination in shelf-stable soymilk and cheese sauce. To accomplish this, individual 355-ml polymeric trays filled with soymilk and inoculated with Bacillus stearothermophilus and Bacillus subtilis (10 ³ CFU) were incubated for up to 28 h at 55°C and 62 h at 37°C. respectively. MR relaxation times (T₂) of these samples were then correlated with the bacterial growth as well as viscosity and pH changes caused by the bacteria in the packaged soymilk. In addition, this study investigated the ability of MR to differentiate between regularly processed cheese sauce and cheese sauce that was modified with α-amylase as a spoilage simulation. Results showed increased MR T₂ relaxation times after the bacterial populations reached 10⁸ CFU/ml (after 18 h) and 10⁷ CFU/ml (after 44 h) for B. stearothermophilus and B. subtilis, respectively. B. subtilis had an undetectable influence on viscosity but a profound influence on pH. B. stearothermophilus, in comparison, significantly lowered the pH and increased the viscosity of the soymilk. MR was able to distinguish between regularly processed 85-g pouches of cheese sauce and other pouches with sauce that were modified with 0.5 ml of 1% α-amylase solution. These results showed that MR has the potential to be used for nondestructive detection of physical changes in soymilk and cheese sauce induced by bacterial growth and enzymatic activities, respectively.