Effects of cellulolytic treatment conditions on dietary fiber content of grape pomace and use of enzyme-treated pomace in cookie making

Abstract

Grape pomace is a rich source of dietary fibers and phenolic compounds; it has been added to bakery products to enhance their dietary fiber content and antioxidant activity. Nevertheless, the Insoluble Dietary Fiber (IDF) content of grape pomace is significantly higher than the Soluble Dietary Fiber (SDF) content, the IDF/SDF ratio of bakery products was much higher than the recommended value of 3:1 from the dietetic associations. In this research, grape pomace was treated with cellulase preparation to partially convert IDF to SDF. The appropriate conditions of the cellulolytic treatment were as follows: initial moisture content of grape pomace of 7.5 g water/g dry basis, cellulase concentration of 6 U/g dry basis and treatment time of 1 h. Under these conditions, the SDF content increased by 20% and the IDF/SDF ratio decreased by 23.2%. Then, the enzyme-treated grape pomace (ETGP) and untreated grape pomace (UGPM) were separately mixed with wheat flour for cookie formulation; the grape pomace ratio was 20% of the composite flour weight. The use of ETGP or UTGP significantly improved the content of total, insoluble and soluble dietary fiber, total phenolics, anthocyanin of cookies as well as their antioxidant activity measured by DPPH and FRAP assays. Cookies supplemented with ETGP or UTGP had increased hardness and reduced brightness. The enzymatic treatment of grape pomace did not affect the total dietary fiber and antioxidant activity of cookies; however, their IDF/SDF ratio decreased to 12% and the value of 3.7 was close to the recommended ratio from the dietetic associations. Furthermore, the preference scores of ETGP added cookies and UTGP added cookies were higher than that of the control cookies without grape pomace addition.