The effects of laccase and xanthan gum on the quality of glutten-free amadumbe bread

Abstract

Celiac disease (CD) is an auto-immune disorder that is triggered by the consumption of gluten in predisposed individuals. The only remedy that has been proposed thus far is total exclusion of gluten from the diet. This may be the most difficult task to most celiac disease patients for most of the convenient and widely consumed baked products such as bread are prepared using ingredients that contain gluten. The replacement of gluten in the baking industry comes with some implications on the overall quality of the baked products, especially bread. It has been observed that gluten-free baked products currently on the market are of poor texture, less volume, not visually appealing and have a bad taste. Hence, the need for polymeric substances that can mimic gluten properties, yielding baked products with similar characteristics as the gluten-containing counterparts. Various crops such as rice, sorghum, sweet potato and cassava have been used and additives such as hydrocolloids, protein-based ingredients, emulsifiers and enzymes included to improve gluten-free bread quality.

The use of carbohydrate-rich tubers and protein-rich legumes as gluten-free ingredients shows great potential in the food industry. Amadumbe (Colocasia esculenta) is a carbohydrate rich tuber which is highly underutilized in South Africa and contains vast amounts of mucilage, a hydrocolloid which can be of great help to improve dough rheology. Hydrocolloids have been reported in literature to have the ability of improving dough water holding capacity and improving dough viscosity hence facilitating gas retention and impacting on the overall quality of the baked product. However, despite the presence of mucilage, amadumbe is very low in protein and it is difficult to produce bread with properties that resemble gluten-containing bread. Hence the need for protein supplementation which may also potentially facilitate protein cross-linking during bread making. Legume proteins from crops such as soy bean and bambara groundnuts contain abundant quantities of lysine, tyrosine and cysteine which could potentially be manipulated through the use of enzymes such as laccase in order to initiate the formation of a network similar to gluten.

The project investigated the effect of laccase and xanthan gum (a hydrocolloid) on the quality of gluten-free bread supplemented with bambara groundnut flour and soy protein isolate as protein sources. Flour blends were prepared using a ratio of 70:30 (amadumbe flour: bambara groundnut flour) and 88:12 (amadumbe flour: soy protein isolate) based on a targeted protein content of 16 g/100 g and the quality properties were determined. Colour analysis showed that

amadumbe flour had a higher L* value compared to the other flours and the blends, showing that amadumbe can be used in applications where food colour contributes to food perceptions. However, when bambara groundnut flour and soy protein isolate were added the L* value decreased. The nutritional profile of the individual flours and the blends showed that amadumbe flour protein content was improved with the addition of bambara groundnut flour and soy protein isolate in the above-mentioned ratios. The protein content of amadumbe increased from 2.36 g/100 g to 15.87 g when bambara groundnut flour was added and to 16.10 g/100 g when soy protein isolate was added, values that were close to the targeted protein content. Incorporating bambara groundnut flour and soy protein isolate in amadumbe flour resulted in improved water absorption capacity, foam capacity and stability as well as emulsion capacity and stability of the amadumbe flour. However, there was no significant difference in oil absorption capacity between amadumbe flour and the blends. The blends were then used to formulate different bread samples incorporating the enzyme laccase (25 nkat/g flour) and a hydrocolloid, xanthan gum (1%).

Laccase-mediated treatment of gluten-free amadumbe dough resulted in a 30% decrease in the free sulfhydryl groups and a 40% decrease in phenolic content indicating that crosslinking had occurred. Laccase action resulted in a 64% increase in bread specific volume and a 32% decrease in bread crumb hardness. Sensory analysis showed that laccase-treated bread samples were more acceptable compared to the non-treated bread samples in terms of appearance, texture, aroma and taste. The acceptability index varied between 46% and 86.2%. This study showed that there is great potential of laccase in gluten-free bread making.

The addition of 1% xanthan gum to amadumbe dough supplemented with bambara groundnut flour and soy protein isolate resulted in gluten-free amadumbe bread with improved crumb texture and specific volume, and decreased the rate of moisture loss. Sensory analysis also revealed that gluten-free amadumbe bread with added xanthan gum was more acceptable compared to the bread samples without xanthan gum. The acceptability index of the bread samples ranged between 40% and 85%. The resulting bread with xanthan gum showed that hydrocolloids such as xanthan gum can be successfully used in the development of gluten-free baked products.

Overall, this study has shown that the incorporation of laccase and xanthan gum to gluten-free amadumbe bread results in bread with improved and acceptable bread properties.