Mobility and Stability Characterization of Model Food Systems Using NMR, DSC, and Conidia Germination Techniques

Abstract

One of the most important aspects of food stability to be monitored is the potential to support microbial growth and/or toxin production. Many researches have shown that water activity (a$\rm\sb {w}$) cannot adequately account for the observed stability behavior of many food systems. We hypothesize that water and solid mobility and glass transition temperature (Tg) of a food system may be useful tools for predicting and controlling its stability. Our objective was to study water and solid mobility and Tg of model food systems, and to evaluate the abilities of these parameters to serve as predictors of food stability by correlating them to mold conidia germination. Aspergillus niger conidia germination was used as a microbial probe of food stability in three systems; sucrose, starch and a 1:1 sucrose/starch mixture. A suite of NMR and DSC techniques were used to fully characterize water and solid mobility and Tg of the food systems, respectively. All experiments were done in duplicated at 20$\sp\circ$C. Water content and a$\rm\sb{w}$, were unable to predict mold germination time. Among the three deuterium NMR relaxation rates, R1, R2, and R2*, used to measure water mobility, R2* showed the best overall correlation with mold germination time. In the C-13 CP/MAS NMR study, the two starch samples which supported mold germination also showed the lowest T$\rm\sb{1p}$ values (i.e., high solid mobility) for the backbone carbons (C1 and C4) as compared to the other starch samples. Moreover, all samples supporting mold germination had DSC Tg midpoint values below the experimental mold germination temperature of 20$\sp\circ$C. These results suggest that it may be more accurate to predict or control food stability by using water and solids mobility and Tg of the food system, than using a$\rm\sb {w}$. These parameters could also provide the basis for developing new food systems or improving existing ones.