• β-carotene;
  • glass transition;
  • relaxation;
  • structure;
  • water activity

Abstract:  Stability of entrapped crystalline β-carotene as affected by water activity, solids microstructure, and composition of freeze-dried systems was investigated. Aliquots (1000 mm3, 20% w/w solids) of solutions of maltodextrins of various dextrose equivalents (M040: DE6, M100: DE11, and M250: DE25.5), M100-sugars (1:1 glucose, fructose and sucrose), and agar for gelation with dispersed β-carotene were frozen at −20, −40, or −80 °C and freeze-dried. Glass transition and α-relaxation temperatures were determined with differential scanning calorimetry and dynamic mechanical analysis, respectively. β-Carotene contents were monitored spectrophotometrically. In the glassy solids, pore microstructure had a major effect on β-carotene stability. Small pores with thin walls and large surface area allowed β-carotene exposure to oxygen which led to a higher loss, whereas structural collapse enhanced stability of β-carotene by decreasing exposure to oxygen. As water plasticized matrices, an increase in molecular mobility in the matrix enhanced β-carotene degradation. Stability of dispersed β-carotene was highest at around 0.2 aw, but decreasing structural relaxation times above the glass transition correlated well with the rate of β-carotene degradation at higher aw. Microstructure, aw, and component mobility are important factors in the control of stability of β-carotene in freeze-dried solids

Practical Application:  β-Carotene expresses various nutritional benefits; however, it is sensitive to oxygen and the degradation contributes to loss of nutritional values as well as product color. To increase stability of β-carotene in freeze-dried foods, the amount of oxygen penetration need to be limited. The modification of freeze-dried food structures, for example, porosity and structural collapse, components, and humidity effectively enhance the stability of dispersed β-carotene in freeze-dried solids.