Water uptake and its impact on the texture of lentils (Lens culinaris)

https://doi.org/10.1016/j.jfoodeng.2010.03.028Get rights and content

Abstract

Water uptake behavior of three cultivars of lentils (Boomer, French-green and Nugget) was studied at three different hydration temperature regimes (room temperature, 50 °C and 85 °C). Boomer had the highest amount of water uptake capacity (74.60 g water/100 g of seeds) at room temperature (20 °C) which can be linked with its pore properties. French-green lentils imbibed the largest amount of water at elevated soaking temperatures (50 °C and 85 °C) and can be attributed to its higher seed surface area to volume ratio, high protein content and relatively thinner seed coat. Water uptake at elevated temperatures (50 °C and 85 °C) were predicted by a two parameter Mitscherlich model (R2 > 0.99, χ2 < 0.5) within 1.36–3.07% average absolute error. At room temperature, only the water uptake of Boomer was reasonably predicted by this model. The texture (hardness) of the soaked lentils was found to be related the amount of water uptake and soaking temperature used rather than seed size or seed mass alone. The hardness values of Boomer, French-green and Nugget at 85 °C after 75 min soaking were reduced to 5.66%, 3.53% and 6.77%, respectively compared to their respective initial seed hardness values. The water uptake capacity of the aged Boomer was found to be significantly reduced compared to the fresh seeds (p < 0.01) while French-green and Nugget do not exhibit significant change (p < 0.05). During 72 h of germination, hardness of all lentil cultivars showed a typical pattern which decreased in the first 24 h followed by an increase compared to the hydrated seeds. A peak hardness value was obtained within 36–48 h before finally declining in all types of lentils. Structural changes within the cotyledon, such as depletion of starch granules, new nuclei formation and development of tissues in vascular bundles and rupture of the seed coat were observed during germination.

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.

References (26)

  • S.S. Deshpande et al.

    Effect of soaking time and temperature on textural properties of soybean

    Journal of Texture Studies

    (2001)
  • S.S. Deshpande et al.

    Microstructure and water uptake of Phaseolus and winged beans

    Journal of Food Science

    (1986)
  • T.A. El-Adawy

    Nutritional composition and antinutritional factors of chickpeas (Cicer arietinum L.) undergoing different cooking methods and germination

    Plant Foods for Human Nutrition

    (2002)
  • Cited by (38)

    • Combination of germination and innovative microwave-assisted infrared drying of lentils: effect of physicochemical properties of different varieties on water uptake, germination, and drying kinetics

      2022, Applied Food Research
      Citation Excerpt :

      It is concluded that Greenland with the highest water uptake showed the most decrease in bio-yield force among these three varieties. The obtained results are comparable with the results by Joshi et al. (2010) and Gorji et al. (2010). The other remarkable difference in the force-time curve in raw, soaked, and germinated seeds is the delay in bio-yield point.

    View all citing articles on Scopus
    View full text