Boysenberry pomace as a source of antioxidant and dietary fibre in an extruded products

Abstract

Boysenberry, a Rubus hybrid (R.ursinus × R.idaeus), was processed to acquire its pomace. Pomace was used to prepare extruded breakfast cereal products with maize and rice bases. In total 8 types of products were made: maize and rice extrudates with 4 levels of pomace substitution (control, 5 %, 10 % and 15 % (w/w) pomace mix). Each of the products was analysed for its physio-chemical, textural, availability of carbohydrates for digestion and antioxidants activity. The expansion of the products decreased significantly (p < 0.05) with an increase of pomace substitution. There was no distinct linear trend for the other properties such as moisture, crispiness and hardness for maize products. Although, in the case of rice products, crispiness appeared to decrease and moisture increased with increasing levels of pomace in the products. No significant difference was observed with the hardness of the products, the mean hardness of the products ranged from (2981 g – 3241 g). L*a*b* colour analysis was used to calculate chroma and hue angles for each of the products and raw mixes. Colour of the products tended to darken as the replacement with pomace increased. Mean L* value of pomace was 19.55 which is darker in appearance, whereas mean L* values for control maize and rice were 96.38 and 96.35 and was whiter in apperance. In vitro starch hydrolysis of the extruded products showed that the inclusion of the pomace to the extruded products significantly reduced the rate of sugar hydrolysis. The area under the curve (AUC) significantly decreased with the increase of pomace contents in extruded products, which indicate a potential glycaemic reducing action. The inclusion of pomace in extruded products significantly increased the product’s total phenolic content and antioxidant activity. The extrusion process significantly decreased the antioxidant activity of extruded products in comparison to its raw mixes, although extruded product was able to retain an apprecable level of antioxidant activity. Extruded products containing 15 % pomace (w/w) showed 300 % surge in the antioxidant activity when compared to the control for both maize and rice. This research illustrates that the introduction of boysenberry pomace to extruded products was able to decrease potential glycaemic action and increased antioxidant activity of the extruded products.