Effect of processing variables on quality of freeze-dried carrots

Abstract

The effects of processing variables such as freezing rate, freeze-drying rate, piece size and added sugar on quality of freeze-dried carrots were studied. Quality was measured in terms of rehydration ratio, waterholding capacity, Allo-Kramer shear analyses, Gardner color analyses, GLC volatile analyses and sensory evaluation. Rapid freezing by immersion in liquid nitrogen produced a less desirable freeze-dried product with a lower rehydration ratio; a lighter, more yellow color; and lower sensory scores for appearance, texture, flavor and overall acceptance. Histological examination revealed greater cell disruption in samples which were frozen more slowly which led to a more rapid and complete rehydration. GLC volatile analyses indicated that added sugar was the processing variable which exerted greatest influence on volatile retention. Total volatile retention ranged from 7.8 to 91.2%. Cooking in 15% sugar solution resulted in more than two-fold increase in total volatile retention. On the basis of this study, the optimum process for preparing freeze-dried, cooked carrots would consist of preparation of 3/8 or 3/4 in. slices, cooking in a 5% sugar solution, freezing at 0°F and freeze-drying at a shelf temperature of 80°F.