The effect of extrusion screw-speed on the water extractability and molecular weight distribution of arabinoxylans from defatted rice bran

Abstract

Arabinoxylans (AXs) are major dietary fibre in cereals. Recently, AXs have attracted a great deal of attention because of their biological activities. These activities have been suggested to be related to the content of low molecular weight (Mw) AXs, in particular those with Mw below 32 kDa. Rice bran is a rich source of AXs. However, water extraction of AXs is difficult and often gives low yield. Extrusion processing has been used to increase the solubility of cereal dietary fibre. The aim of this research was to study the effect of extrusion screw-speeds (80 and 160) rpm on the extraction yield and Mw of water extractable AXs from rice bran. It was found that the extraction of AXs increased significantly with an increase in screw speed and was accompanied by a significant decrease in the Mw of AXs from extruded rice bran. The percentage of very low molecular weight AXs (0.79-1.58 kDa) significantly increased with increasing screw speed.

Advances on the Valorisation and Functionalization of By-Products and Wastes from Cereal-Based Processing Industry

“…AXs occur in percentages of ca. 1.5 to 2.5% in wheat flour and are considered the predominant cell wall polysaccharides in rye (8-12.1% of the whole grain) [78,79] and wheat (4.1-9%) [79] but they are also present in rice bran [80]. AXs are made of a linear chain backbone of β-D-xylopyranosyl (Xylp) units linked with (1→4) glycosidic bonds.…”

“…In addition, maize bran gum contains glucose (5.1%), galactose (3.2%) and mannose (0.4%). In order to avoid negative effects on human health from the use of chemicals used for the extraction of the AXs present in maize bran, water can replace the alkaline medium, thus making their extraction more affordable and less toxic [80].…”

“…Milling reduces the molecular weight of insoluble arabinoxylans with high molecular weight and increases significantly their solubility [86]. On the other hand, extrusion affects the AX fine structure and solubility of cereal dietary fibres [80,86].…”

“…It was hypothesized that extrusion cooking increases dietary fibre solubility, probably due to the reduction in particle sizes of extruded samples [87] and/or the breaking down of covalent and non-covalent bonds in larger molecules due to the high temperature applied [88]. It was reported that extrusion of rice bran increases the extractable levels of AXs but decreases the A/X ratio and molecular weight compared with non-extruded samples [80,89]. Such an effect is likely to be owing to the high shear-stress forces and high temperatures applied that can cause the release of ferulic acid, softening of lignin and AXs depolymerization.…”

“…Arabinoxylans Fermentation [102][103][104] Ultrasonic/microwave [105,106] Enzymatic treatment [107][108][109][110] Milling fractions [111][112][113] Alkaline treatment [114][115][116][117] Nixtamalization [118] Starch removal [119] Extrusion [80,86,89,120] Subcritical water treatment [119,121] β-glucans Ultrasound [122] Roller-milling/pearling [123] Milling [124][125][126] Fermentation [127,128] Enzymatic treatment [129,130] Extrusion [131][132][133][134] Subcritical water treatment [119,135,136]…”