Water uptake and its impact on the texture of lentils (Lens culinaris)
Introduction
Lentil (Lens culinaris M.) is one of the prominent sources of plant protein after soybean in the world. Lentils are an excellent source of complex carbohydrates and dietary fiber as well as a good source of high quality protein, vitamins and minerals (Adsule et al., 1989). Despite being nutritious and healthy, legumes including lentils are underutilized in developed countries like Europe, North America and Australia (Schneider, 2002). The main factors limiting their utilization are their long hydration (up to 16 h) and cooking times (up to 1 h). The longer cooking time requires higher fuel energy input which is a constant impediment in lentil-consuming regions of the world. Furthermore, the longer hydration time also encourages proliferation of harmful microorganisms (Buckle and Sambudi, 1990).
Hard-to-cook (HTC) legumes are those which take longer time to soften during normal cooking procedures. The HTC characteristics have been noticed mostly in stored pulses and legumes as well as some freshly harvested seeds (El-Tabey Shehata, 1992). Traditionally, pulses are soaked 12–16 h beforehand to facilitate cooking and in the process, water penetrates through the seed coat to the cotyledons and distributes among starch and protein fractions. When water is uniformly distributed within the seed, the cotyledons become soft and uniform in texture. The water thus imbibed facilitates starch gelatinization and protein denaturation during cooking (Deshpande and Bal, 2001, Abu-Ghannam, 1998). Soaking also prevents the anisotropic heat transfer in dry seeds. Seed hardness is an important quality attribute because it reflects water uptake, seed coat permeability and overall texture and quality. The seed hardness is affected by water uptake, which in turn affects cookability of legumes (Chen et al., 1993). It has been reported that varieties with faster water uptake rate required less cooking time (Deshpande and Cheryan, 1986). However, the effects of soaking time and soaking temperature on the lentil hardness, so far, have not been reported.
Germination of pulses before consumption or cooking is known to bring important changes in biochemical, nutritional and sensory characteristics (Ahmed et al., 1995). Most of the published research on germination of pulses has focused on investigating its effect on the nutritional and antinutritional factors (Alvarez and Guerra, 1985, El-Adawy, 2002, Ganesh and Venkataraman, 1975, Sathe et al., 1983). Previous studies indicated that during germination the reserved nutrients (carbohydrates, proteins and lipids) stored in the cotyledon are degraded by enzymes and used for the respiration and development of the embryo (Bryant, 1985).
The objectives of this study were two-fold. Firstly, we aimed at quantifying the effect of soaking temperature and time on the hardness of different cultivars of lentils. Secondly, we aimed to quantify the effect of germination on the textural hardness. Physicochemical properties as well as microscopic and mercury porosimetry studies were also carried out in order to explain the above mentioned effects.
Section snippets
Materials
Three varieties of lentils viz., Boomer, French-green and Nugget were used in this study. The lentils were obtained from AWB Seeds Limited, Victoria, Australia. Prior to use, the samples were cleaned and immature and broken seeds were removed. The samples when received from supplier was analyzed as the “Fresh” samples, and the same samples were also stored for 18 months then reanalyzed to study the effect of ageing (storage) on the texture and water uptake characteristics. All samples were
Proximate composition
Table 1 lists the chemical composition and physical characteristics of the lentils. The chemical compositions of lentils were similar to other lentil cultivars previously reported by Jood et al. (1998). Among the chemical constituents, protein content was found significantly different (p < 0.01). French-green had the highest protein content (28.92%) while Boomer and Nugget had similar protein contents, (26.73% and 26.34%, respectively). The percentage of extracted fat and total ash content of
Conclusions
The study has shown that the maximum water uptake capacity and soaking time required to attain the softness was related to the soaking temperature used. Boomer had the highest water uptake capacity (74.60 g water/100 g of seeds) of the three cultivars during soaking at 20 °C. This can be attributed to its pore system properties such as high permeability, low tortuosity and longer characteristic length. It implies that the water uptake characteristic is a reflection of the pore structure of the
Acknowledgements
The authors acknowledge and thank Bruce Armstrong, School of Science and Engineering, University of Ballarat for the seed count test, David Page, Chemical Engineering, University of Queensland for the mercury porosimetry test and Adrian Warmington, St John of God Pathology, Ballarat for preparation of the microscopic slides. The authors also wish to thank Louis Van Autryve for conducting some of the experiments. The first author gratefully acknowledges the AusAID for granting a scholarship.
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