Effect of vacuum freeze-drying methods on retention of volatile flavor compounds of lemon
(2.School of Food Science, Nanjing Xiaozhuang University, Nanjing, China 211171)
【Abstract】Volatile flavor compounds are the main factors that constitute the flavor characteristics of food. Vacuum freeze-drying (FD) is internationally recognized as the best method of producing high-quality dehydrated food. However, the retention of volatile flavor compounds by FD cannot meet our expectation. It had been verified by many researchers that volatile flavor compounds in food materials would be lost in FD process, depending on the material type and the processing parameters. Lemon is a citrus fruit with extremely high nutritional and medicinal value. It is rich in the volatile flavor compounds such as terpenes, alcohols, aldehydes, esters, and ketones. The lemon slice or powder products made by FD are popular with lots of consumers. In order to explore the effect of FD methods on the retention characteristics of volatile flavor compounds in lemon, two different processing methods including the integrated FD method and the conventional FD method were employed to dry lemon slice for comparing the distribution of volatile flavor compounds. The previous research results showed that nearly one third of the initial moisture in lemon slices was removed during the vacuum freezing process in the method of the integrated FD method, which contributed 2.5 h to saving time of pre-freezing process and 2 h to saving time of sublimation drying process in contrast to the conventional FD method, respectively. Smaller and denser porous network structure was formed in the dried lemon tissue after the integrated FD, and their cell wall chambers were more complete without broken or cracked piece. The monoterpene compounds predominated among all the volatile flavor compounds in the raw lemon materials, mainly including D-limonene, terpinenes, L-β-pinene, and β-pinene. The main sesquiterpene flavor compounds in raw lemon materials were 1-caryophyllene, valencian eneene, and β-windproofene. The main aldehyde compounds were citral, dimethyl-octendialdehyde, nonanal, decanal, and hexanal. The main alcohol compounds were α-terpineol, nerol, 4-nonenol, linalool, and geraniol. The main ester compounds were nerol acetate and 5-methyl-2-4-hexen-1-ol acetate. The retention rates of all major volatile flavor compounds in the two FD lemons were very low, and most of them migrated out from the FD lemon and were captured by the cold trap or discharged outside by the vacuum pump. The contents of all above volatile flavor compounds in the lemon slices dried with integrated FD method were significantly higher than those in the lemon slices dried with the conventional FD method (P < 0.05), which showed that the integrated FD method had significant advantages in the retention of volatile flavor compounds. The content of the main monoterpene flavor compounds in cold trap ice of the conventional FD method was significantly higher than that of the integrated FD method (P < 0.05), while the opposite result was tested for the main sesquiterpene compounds, aldehyde compounds, alcohol compounds and lipid compounds in the cold trap ice of two FD methods (P < 0.05). The difference in the retention characteristics of the main volatile flavor compounds in two kinds of FD lemon may be related to their FD process time and micro-structure morphology. In general, the retention rates of the above-mentioned main volatile flavor compounds in the freeze-dried lemon slices with the two FD methods were both relatively low, most of them were lost in the cold trap ice and discharged into the environment. However, the integrated FD method had significant advantages for the retention of the major flavor compounds in lemon slices in contrast to the conventional FD method. The results of this study can provide some references for the improvement of fragrant quality in fruits and vegetables during the freeze drying process.
【Keywords】 vacuum; freeze; drying; lemon; volatile flavor compounds; the integrated freeze-drying; retention;
(Translated by LIU T)
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