Major oxidative reactions in pistachio shell dry foods are due to fat peroxidation. Oxidation of fats in foods is almost exclusively related to unsaturated fatty acids and is often self-catalytic, and the oxidation products themselves catalyze the reaction so that the rate increases with time.
Pistachio is a seed with high fat content and rich in unsaturated fatty acids. This makes pistachios very sensitive to the possibility of the product being pickled.
Hydrolysis of fats leads to a gradual increase in food acidity, which is due to the formation of fatty acids. Therefore, fat hydrolysis prefers fat oxidation because the fatty acids formed can be substrates for the oxidation reaction.
Hydroperoxides, which are major products of fat oxidation, can break down by-products such as aldehydes, alcohols, ketones, or acids to cause bad odors.
These by-products, as well as peroxides and fat-free radicals, can react with proteins and vitamins, causing the nutritional value and important properties of pistachios to be lost.
The effect of oxidation on pistachio kernel quality
Importance of favorable conditions for drying pistachios
The quality of the dry food product and its price are greatly affected by the drying operation. Products that are dried at lower temperatures have good storage stability but require longer processing times.
Activity at low humidity reduces or kills the growth of microorganisms, but increases the rate of fat oxidation. Therefore, creating optimal drying conditions to prevent damage to the quality of pistachios is very important.
Compared to other food products, studies on the drying of pistachio nuts and its effect on the quality of pistachios are very limited. The drying temperature of pistachios affects its sensitive properties and its roasted taste increases when dried at high temperatures. (138-116 degrees Celsius)
The effect of oxidation on pistachio kernel quality
Moisture content in determining the quality of pistachios
Drying in a certain amount and at the right humidity (4 to 6% of base humidity) is one of the important factors of good pistachio quality. The moisture content of dried nuts is up to 4% (base moisture) in terms of transparency, but in sweetness, bitterness and dryness it is lower than the moisture content of dried nuts up to 6 or 11% (base moisture).
The moisture content of nuts with 6% (base moisture) is also higher in sweetness and lower in bitterness and dryness than those at 11% (base moisture). Drying affects the ingredients of pistachios, but it is less effective than washing and roasting.
Unsaturated fatty acids are prone to changes during processing. Studies have shown that the drying temperature of pistachios has a lesser effect on the quality of pistachios, which is measured based on the percentage of edible seeds with enclosed shells or the percentage of nuts isolated from the shell in the sample.
Many studies have been done on pistachio drying. Drying the grains at temperatures above 50 degrees is not recommended because the reaction speed is stiff and as a result the grain quality decreases.
The optimum temperature for drying grains is 40-50 degrees Celsius, because lower temperatures require longer processing times. Temperatures above 50 ° C of fat oxidation in the grain, especially in husked grains, are associated with an increasing trend of K232 and K270 indices and a decrease in oxidation stability values.