Pectin Changes during Freezing and Storage of Plant Products

Abstract

To keep up with the growing demand, producers of frozen plant products have to develop new freezing technologies that would preserve the sensory and biological properties of fruits and vegetables. Pectins are important structural and moisturebinding components of plant cells that improve their stability at high and low temperatures. The research objective was to study the effect of blanching and various freezing methods on the composition of pectin substances during long-term storage of fruits and vegetables. The research featured scorzonera, salsify, kohlrabi, apples, and plums. The freezing modes included two temperature modes (–24 and –35°C) and three freezing methods, i.e., blanching, air-freezing represented by natural air-freezing, artificial convection, and fluidization, and immersion in a mix of water, ethyl alcohol, sucrose, and sodium chloride. The frozen samples were stored in sealed bags at –18°C for 7–12 months. The water-soluble pectin, intermediate fraction, and protopectin obtained by extraction were determined using the colorimetric carbazole method. The qu alitative analysis relied on infrared spectroscopy. Blanching reduced the pectin content by 2–10% in vegetables and by 18–21% in apples. Fluidization and immersion freezing had the least damaging effect on pectins. Air-freezing with natural convection caused the greatest damage to protopectin. During storage, the maximal loss of pectins (66%) occurred in the salsify sample subjected to natural air convection at –24°C. The least damage (9%) was detected in the kohlrabi sample frozen at –24°C in ice environment. A higher moisture content in the native state correlated with minimal losses of pectins by the end of refrigerated storage. The research also included identification of absorption bands for pectic substances in fro zen scorzonera and salsify. In this study, pectin content depended on moisture content in tissues, blanching process, and freezing method. All frozen samples demonstrated losses of protopectin and an increase in the intermediate fraction. An intense freezing process had a positive effect on the pectin content during long-term storage. However, after six months of storage, the samples demonstrated significant fractional changes and pectin losses.