Effect of different drying methods on drying characteristics and qualities of lemon slices

Abstract:Lemon tea has become one of indispensable health drinks in the market which are popular for many consumers (especially women consumers). Dried lemon slices are generally processed by air drying (AD) or freeze-drying (FD). FD has been considered as one of the best methods for obtaining dehydrated foods with high quality. However, conventional freeze-drying (CFD) process technology is characterized by many disadvantages including complicated process steps, large space occupation, huge equipment investment, frequent materials transferring, long drying time, and high production cost. In order to simplify the food CFD processes and shorten the drying time, a novel integrated freeze-drying processing technology (so-called vacuum-freezing-drying, VFD) was proposed based on the principle of vacuum cooling and vacuum freeze dryer. In order to investigate the effects of different drying methods on the drying characteristics and quality of lemon slices, comparison experiments and multi-index testing on the drying of lemon slices were conducted with the methods of VFD, CFD and AD. Two different freezing methods were adopted in VFD and CFD, which resulted in 25.62% and 4.55% mass loss in the lemon slice samples during freezing stage, respectively. The drying process time of VFD and CFD was 12.5 and 17.5 h, respectively, which was more than twice of that of AD. Compared with the CFD process, and the VFD process reduced by 2.5 h in either freezing stage or drying stage and saved 14.27% of power consumption, which was a considerable economic advantage. The retention ratio of vitamin C in dried lemon slices with VFD, CFD and AD was 66.03%, 45.45% and 19.14%, respectively, and the rehydration ratio was 4.60, 3.97 and 2.24, respectively, both showed the significant difference (P<0.05). Color of lemon slice samples was measured with an automatic colorimeter. The comprehensive color difference index in air-dried lemon slice was significantly higher than that in the 2 kinds of freeze-dried lemon slices which showed no significant difference in the comprehensive color difference index (P>0.05). Volatile components of lemon slices were analyzed by using solid-phase microextraction - gas chromatography - mass spectrometry (SPME-GC-MS). The top 5 volatile compounds were separated and identified from the dried lemon slices, which were ranked in descending order as d-limonene, terpinene, Levate-β-pinene, β-pinene and pinene. The highest retention ratio of the top 5 volatile compounds was achieved in the dried lemon slices by the VFD method, then the CFD method and the AD method. Transverse relaxation time (T2) of dried lemon slices was measured with low field nuclear magnetic resonance (LF-NMR). Free water, non-flowing water and bond water were observed in the atlas of T2 of the 3 groups of dried lemon slices. Bond water constituted the highest proportion of the residual water, and non-flowing water and free water constituted the much smaller proportion. And the T2 peak of the hot-air dried lemon slices showed a slight right-shift trend compared with the other 2 groups. The sensory characteristics of the 2 groups of freeze-dried lemon slices showed little difference. The overall sensory characteristics of air-dried lemon slices were relatively weak. The sensory characteristic intensities in color, texture, and lemon flavor of air-dried lemon slices were much less than the 2 freeze-dried groups. The freeze-drying method is suitable for the processing of dried lemon slices with high quality and value. Compared with the CFD, the VFD method can shorten the process time, reduce energy consumption and obtain better or equivalent quality of lemon slices, deserving further application in the industry. All the above research conclusions can provide some theoretical and technical basis for the processing of lemon slices drying.