Characteristics of fractural properties and microstructural aspects of three kinds of frozen vegetables (pumpkin, carrot and asparagus)

Abstract

  Edible parts of three vegetables with different tissue characteristics, i.e. pumpkin flesh, carrot root and asparagus stem, were frozen at -40°C in a refrigerator regulated at -45°C. After thawing, their fractural properties were examined. The ice crystal spaces formed in the tissues during freezing were observed using a light microscope and a transmission electron microscope. The pumpkin flesh with a higher specific gravity formed smaller ice crystals in its extracellular spaces. Most of intracellular ice crystal spaces in the pumpkin flesh were formed at -5°C. In the frozen and thawed carrot pieces, large ice crystal spaces were observed in the extracellular spaces and the rupture strain values were increased. In the frozen and thawed asparagus stems, large ice crystal spaces were observed in the area surrounding the vascular bundles which were scattered in the pith parenchyma. As a result, the rupture strain values were increased both in the upper and lower parts. The cells of the collenchyma tissue surrounding the pith in the upper part were cracked by the ice crystal formation, but no crack was observed in the lower part of the stem, and the rupture stress values in the lower part were higher than those in the upper part. The differences in the amounts of vegetable components, such as starch and soluble substances, and the varieties of the tissue structures also affected the ice crystal formation in the frozen vegetables, which eventually influences their fractural properties after thawing.