Abstract
Different extrusion temperatures (90, 100, 110, and 120 °C) were used to investigate changes in the expansion ratios, die pressures, phytochemical contents and antioxidant activities of extrusion products of black rice. The results showed that the die pressure significantly decreased with the increasing extrusion temperature, and the expansion ratio reached a peak value at 100 °C. The soluble-free and total phenolic acid contents gradually increased, whereas portions of soluble-free and soluble-conjugated phenolic acids transformed into insoluble-bound phenolic acids. The soluble-free (52.45) and insoluble-bound (73.59 mg GAE/100 g DF) total phenolic contents (TPC) reached peak values at 110 °C. The soluble-conjugated TPC values remained similar. Antioxidant activity occurred at higher levels in the range from 100 °C to 120 °C. The anthocyanin content decreased after extrusion possibly because some anthocyanin remained in the residue after extraction and could not be completely extracted. The content of free sterols increased from 90 °C to 110 °C and decreased at 120 °C. However, the content of bound sterols showed an opposite trend and reached a minimum value at 110 °C.
Highlights
Rice is a staple food for almost half the world's population and is cultivated in over 100 countries
The results indicate that the die pressure signi cantly (P < 0.05) decreased, with a slight reduction from 90 C to 100 C and a large decline from 100 C to 120 C
The expansion ratio can re ect the degree of expansion, which mainly depends on higher die pressure and vapor pressure
Summary
Rice is a staple food for almost half the world's population and is cultivated in over 100 countries. Whole grain rice de-husked from rice, is a typical whole grain that plays an important role in good health and a balanced diet; it is becoming popular in western countries and is being more gradually accepted in developing countries.[1] Black rice is a special type of whole grain rice. Extrusion technology has been used extensively to produce snake foods, baby foods, breakfast cereals, and pet foods because this process cannot only improve digestibility and nutrient bioavailability, but can be used to develop a range of products with distinct textural properties including crispiness, expansion, and mouthfeel.[6] In food extrusion processing, various parameters, such as extrusion temperature, screw speed and the moisture content of the material, play important roles in retaining functional compounds and improving textural properties; these parameters can affect the extrusion responses, which can in uence the quality of the extrusion products. Wang et al found a negative linear correlation between extrusion temperature and die pressure, and the increase of screw speed resulted in the decrease of die pressure and motor torque.[7]
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