Influence of precooking on the firmness and pectic substances of three stem vegetables

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Abstract
Summary: Edible portions of the stems of sprouting broccoli, asparagus lettuce, and large‐stem mustard were compared for total pectin contents, amounts of different pectin fractions, pectinesterase activities and changes during cooking to investigate effects on the textural changes during cooking. Slices precooked for 30min at temperatures below 60°C (broccoli) or 70°C (lettuce, mustard) were firmer after 15 min recooking in boiling water than those directly cooked without precooking. Optimum temperatures for this firming effect of precooking were 50, 60 and 60°C, respectively, and coincided with the optimum temperatures of activity of pectinesterases extracted from the fresh tissues. Analysis of pectin fractions revealed that the firming effect of precooking is related to the shift from the cold water‐soluble fraction to sodium hexametaphosphate‐soluble and hot water‐soluble fractions of pectins.