Abstract
Legumes are typically processed by soaking and cooking before consumption. However, thermal processing can cause textural degradation. On an industrial scale, calcium is added to legumes during canning to prevent them from losing firmness and improve their visual appearance in order to meet consumer demand. The uptake of calcium can be potentially enhanced with the use of a novel emerging technology, pulsed electric field (PEF), by causing the electroporation of the cells in legumes. However, the consequences of calcium addition and PEF on the nutritional quality of cooked legumes, especially the digestibility of starch and protein, is not yet fully understood. Therefore, this thesis aimed to investigate the effect of calcium addition, combined with pulsed electric field (PEF) treatment and/or thermal processing, on the texture and in vitro starch and protein digestibility of black beans (Phaseolus vulgaris). Moreover, when processed food is ingested, oral processing, including the consumers’ mastication behaviour and resulting food particle sizes, can also play a role in its nutrient digestibility. Thus, this thesis also aimed to determine the in vivo mastication behaviour of 17 human participants when chewing black beans with the same hardness, achieved through the use of a combination of calcium addition, PEF treatment, and thermal processing. It was assessed whether the similarity of hardness, according to texture analyser (in vitro), was also perceived by consumers (in vivo). Then, the influence of the varying processing conditions and mastication behaviour on the consequent particle size, salivary α-amylase activity, and in vitro starch and protein digestibility of the in vivo masticated bolus were investigated.
The effect of calcium and thermal processing on texture and nutrient digestibility was determined by overnight soaking and cooking (100°C) black beans for 0, 1, and 2 h in increasing calcium chloride (CaCl2) solutions (0, 100, 300, and 500 ppm). Results according to the texture analyser implied that calcium chloride addition and thermal processing have a synergistic effect on improving the texture of black beans by increasing their hardness, cohesiveness, springiness, chewiness, and resilience. It was found that the higher the concentration of calcium chloride applied to the beans, the lower the extent of texture degradation in cooked black beans. While the texture parameters of cooked black beans (especially for 1 h) were improved considerably due to calcium addition, this study showed that their starch and protein hydrolysis during in vitro small intestinal digestion slowed down, probably due to limited accessibility of starch and protein to digestive enzymes. Low starch digestibility can be beneficial to consumers’ health as it lowers the blood glucose response, which is relevant for diabetic people.
To evaluate the impact of PEF on texture, black beans were treated with low and high electric field strengths (1 and 2 kV/cm, respectively) and specific energy inputs (9-13 and 91-127 kJ/kg, respectively) in the presence of CaCl2 solution (0, 100, and 300 ppm), then thermally processed for 1 h at 20, 70, 80, and 90°C. It was demonstrated that PEF has potential in facilitating the uptake of calcium into the black beans to improve texture. The use of high PEF electric field strength (2 kV/cm) and specific energy inputs (100-127 kJ/kg) and subsequent thermal processing of up to 80°C was shown to be effective in improving the texture of black beans compared to when lower PEF treatment intensity (1 kV/cm electric field strength and 9-13 kJ/kg specific energy) was applied. The former PEF parameters required a lesser amount of calcium (at least 100 ppm CaCl2) for hardness improvement of thermally processed black beans than the latter (300 ppm CaCl2). Moreover, results confirmed that calcium content was higher in PEF-treated black beans than those without PEF treatment, especially at the seed coat of the beans. Therefore, PEF could be useful in the legumes canning industry by reducing the amount of additives needed to preserve firmness and potentially shorten the calcium uptake duration.
In the in vivo mastication study, it was found that the hardness of differently processed black beans, determined to be similar by the texture analyser (in vitro), was not perceived similarly by consumers (n=17) in vivo. Further differences in particle sizes and consequent in vitro starch and protein digestibility were observed between the in vivo masticated boluses and black beans compressed in the texture analyser (90% strain) simulating in vivo mastication. Particle sizes of the in vivo masticated bolus were smaller and nutrient digestibility was higher. This implied that using humans to masticate the samples provides a closer approximation of how food is broken down and digested in vivo.
Despite the similarity in hardness, the mastication behaviour of participants varied, resulting in two distinct groups of chewers – fast and slow chewers who masticated the processed beans for < 25 and > 29 s, respectively, to achieve a bolus ready for swallowing. Nevertheless, the processing conditions involving the addition of calcium and PEF in black beans appeared to enhance the in vitro digestibility of protein based on the in vivo masticated oral bolus generated by fast chewers. Moreover, it was clearly demonstrated that in vivo mastication behaviour, specifically chewing duration, influenced the particle size distribution of the bolus, α-amylase activity in the saliva, and the in vitro starch and protein digestibility of PEF-treated black beans cooked either in the absence or presence of calcium chloride. Longer durations of chewing resulted in boluses with small-sized particles (majorly composed of a higher number of broken-down cotyledons [2 to 5 mm2 particle size], fewer seed coats [5 to 13 mm2 particle size]), and higher activity of α-amylase. This, in turn, increased the digestibility of both the starch and protein of processed black beans.
These findings clearly demonstrated that the appropriate processing conditions could preserve the texture of legumes as demanded by consumers for canned beans and have an implication on the digestibility of its nutrients such as starch and protein. The use of an emerging technology, PEF, has been shown to be promising in enhancing the uptake of calcium while reducing the required concentration to improve texture without negative impacts on digestibility. Furthermore, in this study, the rate of protein digestibility for in vivo masticated black beans for fast chewers was significantly enhanced due to PEF processing in CaCl2 solution. Lastly, chewing behaviour of human was found to be an important contributing factor in modulating the release of nutrients in the differently processed food during gastrointestinal digestion.