Oil distribution, microstructure and in vitro starch digestibility of bran-enriched magwinya

Abstract

Magwinya, a cereal fried dough, is a popular traditional snack widely consumed across various ethnic groups in Sub-Saharan Africa, but little is known about its production and consumption in a scholarly context. Furthermore, its oil distribution, starch digestion and microstructure have not been documented. It is against this background that this research was carried out. Firstly, a survey of the production process, ingredient formulation, sales, characteristics and consumption of magwinya in Thohoyandou were investigated. Out of the 30 magwinya production sites visited and 650 consumer questionnaires distributed, data were obtained from 29 sites and 634 consumers: a response rate of 97% and 98%, respectively were realised. Results revealed, details on the formulation, ingredients, processing methods, sales, consumption patterns and consumer preference of magwinya; and proposed considerations for the development of a healthier magwinya. Cake wheat flour (55%) was the main ingredient used. Production process was non-automated as evidenced by manual mixing and fermentation processes (93%), coupled with low usage of electronic equipment (14%). With a daily turnover between ZAR500 – ZAR3000 ($35 - $210), there is a need to improve magwinya production through an automated production line, especially for large-scale producers of this product. The daily turnover indicated that magwinya production is a lucrative business contributing immensely to the livelihoods of, and financially empowering the producers, who were all females (100%). Consumer data revealed magwinya to be a moderately liked food (46%) consumed at least twice a week (32%), as either a snack or main meal; with taste as the most favoured characteristic (79%). About 93% of consumers fell within <20 and 21-30 age groups. A greater percentage of consumers (75%) disliked the oiliness of magwinya, and with increasing awareness of the health implications of frequent consumption of fried foods; 87% of consumers affirmed purchase of low-fat magwinya if made available. Development of low-fat, nutrient-rich magwinya is therefore recommended to offer consumers a healthier variety. The experiments in this study were constructed using a 2x2x7 factorial experimental design. Factors considered were two (2) bran types - wheat and oat bran; two (2) product types and seven (7) bran amounts (0, 1, 5, 8, 10, 15 & 20 g/ 100 g). Oil fractions of the total oil content of magwinya was categorized as surface oil (SO), penetrated surface oil (PSO) and structural oil (STO) using spectrophotometric method. Moisture loss reduced (P < 0.05) from 23.35% in control to 15.19% fried batter with 20% oat bran (OB), while a reverse trend was observed in fried dough. Reduction of total fat from 0.43 g/g in control to 0.38 g/g at 20% OB and 8 g wheat bran (WB) was observed. At 15% OB and 20% WB, total fat reduced from 0.41 g/g in control to 0.26 g/g of FD. An STO<SO<PSO trend was observed in fried dough while fried batter followed a SO<STO<PSO trend. Furthermore, oil penetration in crust and crosssectional properties of fried dough were elucidated using confocal laser scanning microscopy (CLSM). Penetrated oil by image analysis (POia), porosity and pore features were quantified from the cross-section micrographs; while crust surface roughness was measured using fractal metrics. Crumb porosity ranged between 54.94 – 81.84% and reduced (P < 0.05) with bran addition. Crumb pore sizes ranged from 0 – 475 µm with <1 circularity, indicating elliptical shape. POia values were notably higher (P < 0.05) than PO by soxhlet extraction (POsox) except for WB fried dough where the values of POia and POsox were closely ranked. Linear effect of initial moisture content and bran concentration showed significance on image properties. Mean fractal dimension (FD) decreased as initial moisture increased. addition of WB caused a significant reduction of FD of fried dough while an opposite effect was noted for its oat bran counterpart. Due to non-collinearity of image properties (FD, POia & porosity), data were fitted to cubic polynomial regression with r2 values >0.70. Confocal microscopy and image analysis were effective in measuring oil absorption and interpreting crumb properties of fried dough. Glucose, starch fractions and estimated glycemic index (eGI) of bran-enriched magwinya were estimated using in vitro starch digestion assay. Rapidly available glucose (RAG) of control fried dough (60.31 g/ 100 g) product was 33% less than fried batter (90.07 g/ 100 g) and more unavailable glucose (UG) less than fried batter. OB and WB significantly reduced RAG and increased UG of fried products. However, OB showed about 9% more reduction than wheat bran. A similar trend was observed for rapidly digestible starch and resistant starch of fried products. The eGI results showed control fried batter to be a high GI food (eGI = 80.02) and control fried dough to be medium GI food (eGI = 58.11). WB fried dough, fried batter and OB fried dough can be categorised as medium GI foods at eGI of 56.46 – 58.39, 65.93 – 68.84 & 56.34 – 57.27 respectively; while OB fried batter at 73.57 – 80.03 as high GI foods. RS showed negative significant correlation with eGI (r= - 0.866, -0.932, P < 0.01) and fat content (r = -0.618, - 0.671, P < 0.01) for OB and WB fried products. Finally, the distribution of chemical components of magwinya were visualised using NIR hyperspectral imaging (HSI). Principal component analysis (PCA) was applied to mean-centred data for pixel-wise classification using spectral scattering (standard normal variate) and 1st and 2nd derivatives of Savisky-Golay method. There was little separation observed in the PCA score plots in the results due to a large similarity between classes. Prominent bands related to oil were featured at 1400, 2305 and 1716 nm, while those related to starch were featured at 1449, 1776, and 2261 nm. 1941 (related to moisture), and 2117 nm (related to protein). Bands related to protein were featured at 1509, 1994, 2223, 2229 nm and water at 1450, 1934 and 1940nm. Aromatics, phenols (1422 nm) and benzene were also identified, albeit minimum. The processing methods and some principal components assisted in mapping the chemical components of magwinya. Visualisation of magwinya chemical constituents using HSI shows good promise for further research in modelling and predictions. The protocols for oil distribution measurement and CLSM used herein can be applied to other thick deep-fried products for qualitative observation and quantitative measurement of specific physical or chemical property. These results reveal that ingredient modification and processing is effective for oil uptake reduction, regulation of starch digestion and related eGI of deep-fried dough/ batter foods.