STARCH AND PECTIC SUBSTANCES AS AFFECTED BY A FREEZE‐THAW POTATO GRANULE PROCESS

Abstract

Changes in total content and fractions of pectic substances and starch were studied in a freeze‐thaw process for potato granule production consisting of peeling, steam cooking, hot mashing, freezing and thawing, pre‐drying, granulation, drying, cooling and sifting. Pectic substances in raw potatoes (expressed as mg uronide/netted Gem 100g dry wt) were 202.4 for water soluble and only 80 for the calgon‐soluble fractions. Water soluble pectic substances increased sixfold after cooking, and calgon‐soluble fraction about threefold. Little change was observed in either fraction due to subsequent granule processing steps. There was no indication that pectic substances present in the extracellular matrix could have influenced the processing steps in granule production. Total starch comprised 68.4%/dry wt of raw potato, decreasing slightly after cooking, increasing up to the pre‐drying step and stabilizing at a level of 83.7%. In contrast, free water‐soluble starch, depended on the processing steps, being lower in cooked unmashed potatoes than in conditioned raw potatoes. Mashing slightly increased starch content while pre‐cooling to 5.5°and freezing to −20°C with subsequent thawing to room temperature substantially decreased starch content. Mechanical forces applied in subsequent steps brought about some increase while in the final product the content was related to particle size of the granules. The percentage of broken cells in cooked potatoes mashed at various temperatures appears to be dependent on the free extracellular starch present in cell binding matrix. Broken cell counts were substantially decreased when mashing was performed immediately after cooking at high temperature or when the mash was frozen and thawed. In the latter case determination of Blue Value Index suggested that a decrease in cell binding strength of the matrix occurred due to soluble starch retrogradation.