The determination of drying end-point for asparagus by-products with the use of LF-NMR spectra

Drying is a simple but important method to utilize asparagus by-products for future use. We investigated the changes in LF-NMR spectra during the drying of various asparagus by-products including leaf, hard-stem and root. A shift of major peak in LF-NMR spectra toward left was noticed with the progress of drying. Also, a decrease in peak area for free water and an increase in peak area for strongly bound water were also observed until it reaches the safer level of moisture content. An end-point drying is the point when the material water activity is below 0.6, which was determined through the observation of a major peak at relaxation time below 1 ms. Correlation coefficient values suggested that both the moisture content and water activity were positively correlated with total peak area (0.892 to 0.991, and 0.860 to 0.975 respectively) and major peak area (0.876 to 0.987, and 0.900 to 0.963 respectively), while negatively correlated with the peak area for tightly bound water (-0.620 to -0.825, and -0.743 to -0.990, respectively). Since LF-NMR spectroscopy is quick and nondestructive method, in comparison to the conventional methods for moisture content or water activity, the observation of a major peak below 1 ms relaxation time indicated the drying end-point for these by-products.

Automated summary

The study revealed that as the drying process of asparagus by-products progresses, the major peak in LF-NMR spectra shifts towards the left, with a decrease in the peak area for free water and an increase in the peak area for tightly bound water until it reaches a safer moisture content level. The drying end-point is determined when the material's water activity is below 0.6.