Influence of shelf temperature on pore formation in garlic during freeze-drying

The formation of pores in garlic was examined during freeze-drying at different shelf temperatures (-5, -15, and -25 °C). The moisture content of dried garlic and its temperatures at different locations within the sample were measured as a function of drying time. The apparent porosity of fresh garlic was 0.25 (±0.03), while 72 h dried samples showed about 0.700 for the drying temperatures used in this study. Apparent porosity increased with the decrease of moisture content showing varied curvatures depending on the shelf temperatures. The estimated shrinkage-expansion coefficients from Rahman's model were 0.313, 0.429, and 0.409 for shelf temperatures -5, -15 and -25 °C, respectively. Samples dried at -5 °C showed significantly lower open pore porosity compared with the samples dried at -15 and -25 °C, respectively, although all samples showed similar apparent (i.e. total) porosity. Pore-size distribution and scanning electron micrographs also supported that characteristics of pores are different for samples fried at -5 °C when compared with samples dried at -15 °C and -25 °C.