ReviewSeed coats of pulses as a food ingredient: Characterization, processing, and applications
Introduction
“Pulses” refers to those low-fat content leguminous seeds which are harvested for dry grain (FAO, 1994). So, oilseeds (e.g. soybean and peanut), leguminous green vegetables (e.g. green peas and green beans) and leguminous fodder plants (e.g. clover and alfalfa) are traditionally excluded. Pulses are historically important in both the human diet and cropping systems as crop rotations, due to their rich-protein and biological nitrogen fixation ability. Although most pulses are not traditionally typical Western-style foodstuffs, international events like “International Year of Pulses 2016” and “Global Pulses Day” suggest that they are being promoted to be important human food world widely (Foyer et al., 2016).
As shown in Table 1, six of the 11 pulses which are covered in the FAO list, chickpea (Cicer arietinum), lupin (Lupinus), field pea (Pisum sativum), faba/broad bean (Vicia fabae), lentil (Lens culinaris) and mung bean (Vigna radiate), are the most important pulses globally, totally accounting for 79.89% of the world pulse production (81.8 million tonne) in 2016 (FAOSTAT, 2018). India is the largest pulse producer globally, followed by Canada, Myanmar and China. However, Australia is the largest lupin producer in the world, contributing an average of 58.22% of the world production in 2012–2016 (ABARES, 2018). Australian sweet lupin (ASL, L. angustifolius), which is also named “narrow-leafed lupin”, is the most important lupin specie, constituting 93% of Australian lupin production and 52% of the world production (Pulse Australia, 2016). However, chickpea has overtaken lupin as Australia's largest pulse crop since 2011-12, with a production estimated at over 2 million tonne in 2016–17 (ABARES, 2018). As a leading pulse exporter, Australia exports over 90% of its chickpeas, faba beans, lentils and mung beans, and 60% of field peas were exported, being the largest exporter of desi chickpea and faba bean in the world. Notably, although Australia exported only 50% of its lupin, this accounted for 90% of world lupin export in 2013.
Pulse seed has three distinctive parts, namely the seed coat, embryonic axe and cotyledon, which generally accounts for 8–16%, 1–3% and 80–90% of the whole seed respectively (Dueñas, Hernandez, & Estrella, 2006). However, the proportions of seed coat show great genetic and environmental variability both between and within species (Table 2). For example, lupin uniquely contains a much higher percentage of seed coat than others, with up to 24% in Australian sweet lupin and around 18% in white lupin (Clements et al., 2014). Removal of pulse seed coat (dehullinig) is a primary process to produce dehulled splits, ground flours and other fractionated pulse ingredients like pulse protein and fibre. In practice, by-product generated from the dehulling process is a mixture of seed coats, embryonic axes and brokens of cotyledons (Oomah, Caspar, Malcolmson, & Bellido, 2011; Sherasia, Garg, & Bhanderi, 2017). As a consequence, dehulling loss which is the main waste stream of pulse processing represents as much as 31% for sweet lupin in Australia (Sipsas, 2008), and up to 28% for lentil and chickpea in India (Tiwari & Singh, 2011). Currently the primary markets for pulse seed coats are low value animal feed and only very limited use in human foods such as that added to make high fibre breads and meat products (like sausage and nuggets). This by-product not only leads to a tough disposal problem for the millers, but also wastes a potential source of novel, nutritious and health-promoting food ingredient (Sherasia et al., 2017).
Growing evidence suggests that pulse seed coats have considerable amount of dietary fibre which is associated with diverse types of minerals and phytochemicals (bioactive secondary metabolites in plants e.g., polyphenolic antioxidants). Therefore, besides the well-documented physiological benefits of dietary fibre, seed coats provide potential for various physiological benefits, such as those related to antioxidant and anti-inflammatory activities. Available studies on pulse seed coats mainly focus on proximate compositions and anatomical structures, with little attention paid to their phytochemical properties and physiological functionalities. The present review brings together the current research on the characterization, processing and applications of seed coats from six selected pulses, i.e., chickpea, lupin, field pea, faba/broad bean, lentil and mung bean. This information should encourage strategies which might enable the more extended use of pulses and their seed coats in human foods.
Section snippets
Seed coat morphology and physical properties
The pulse seed coat (often referred as hull or testa) is a protective outer layer of the pulse seed. Structures of pulse seed coats have been overviewed by Moïse, Han, Gudynaitę-Savitch, Johnson, and Miki (2005) and Smykal, Vernoud, Blair, Soukup, and Thompson (2014). Anatomical structures of seed coats of field pea (Van Dongen, 2003), faba/broad bean (Youssef & Bushuk, 1984), chickpea (Wood, Knights, & Choct, 2011), lentil (Hughes & Swanson, 1986), lupin (Clements, Dracup, Buirchell, & Smith,
Pulse seed coat composition
The nutritional composition of whole pulse seeds have been reviewed in the FAO/INFOODS global food composition database for pulses (uPulses 1.0) (FAO, 2017). The composition of seed coats of the selected six pulses are summarized (Table 3). Generally, pulse seed coats have about 8–10% moisture, 3% ash, 1–3% lipids and 2–8% protein, with a major carbohydrate components (60–90%), mainly insoluble non-starch polysaccharides (NSPs) (Tiwari & Singh, 2011). Of the macronutrients, we focus on
Mycotoxins contamination
Pulses are vulnerable to be contaminated by fungus and the resulting mycotoxins (e.g., alfatoxins, ochratoxins and phomopsins) during pre- or post-harvest (CAST, 2003). A further increase in human exposure of them by consuming products containing contaminated pulses may occur. However, recent systematic surveys on mycotoxins in pulses based human food are lacking. Here, phomopsins in contaminated lupin seeds, a highly representative example of mycotoxins contamination of pulses, will be
Bio-availability of nutrients in pulse seed coats
Bio-availability refers to the extent that nutrients can be released from food matrix into digestive fluid, and thereby available for intestinal transport, biotransformation, absorption and metabolism (Versantvoort, Van de Kamp, & Rompelberg, 2004). There is strong evidence that structure and composition of a food matrix will govern the bio-availability of many nutrients in the gastrointestinal tract (Wahlqvist, 2016).
A few published clinical studies have suggested that pea seed coat
Effect of processing on pulse seed coats
Generally, pulses are dried in the field to achieve the target moisture of 9–20% for threshing (i.e. removal of pods), then cleaned, graded and further dried to approximate 13% for storage. Storage conditions (e.g. seed moisture, relative humidity, duration and temperature) significantly affect the seed coat characteristics. For example, the seed coat colour of faba bean has been observed to darken from beige to dark brown depending upon the storage conditions (Nasar-Abbas et al., 2009).
Application of pulse seed coats in human food
Pulses have been historically important sources of energy, protein and dietary fibre in human diet. Currently, pulse seed coats have only limited use in human food such as in high fibre breads and meat products. However, the high content of dietary fibre in pulse seed coats, along with considerable amounts of minerals, phytochemicals (e.g. polyphenols) suggests they could be more widely utilised as novel functional dietary fibre ingredients (Macagnan, da Silva, & Hecktheuer, 2016). There are
Conclusions
To date, pulse seed coats are little utilised in human food. However, there is potential for the seed coat to be used as a natural “nutritious dietary fibre” which could (1) fill the “fibre intake gap”, (2) provide considerable levels of minerals and antioxidants, and (3) achieve greater safe and sustainable utilization of pulses by exploiting value-added applications of their by-products (Saura-Calixto, 2012; Sharma et al., 2016). However, in-depth studies on biochemical, and nutritional
Acknowledgements
The author Liezhou Zhong wishes to thank the support from Curtin International Postgraduate Research Scholarship (CIPRS)/Health Sciences Faculty International Research Scholarship (HSFIRS). The salary of Jonathan Hodgson was supported by a National Health and Medical Research Council of Australia Senior Research Fellowship [grant number 1116973], and a Royal Perth Hospital Medical Research Foundation Fellowship. This research did not receive any specific grant from funding agencies in the
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