The ratio of chelate-soluble fraction to alcohol insoluble residue is a major influencing factor on the texture of lotus rhizomes after cooking
Introduction
Lotus rhizomes (Nelumbo nucifera Gaertn.) belongs to the nymphaeaceae family of perennial aquatic vegetables, and is widely grown as a non-conventional vegetable in China, India, Japan and Australia (Thanushree, Sudha, & Crassina, 2017). At present, there are various types of products from lotus rhizomes, which can be generally classified into two categories according to their taste quality (Xie, Zhao, Yan, Wang, & Li, 2018). One is the mealy type, which is mainly cooked through stewing and steaming, and the representative varieties include Elian 9, Wild Variety of Hubei, and Miancheng Variety; the other is crispy type, which is suitable for stir-frying or cold dressing with sauce, and the representative varieties include PiaoHua of Henan and Lulinhu of Hubei.
Food texture is the sensory and functional manifestation of the structural, mechanical and surface properties of foods detected through the senses of vision, hearing, touch and kinesthetics (Szczesniak, 2002). With the global increases in aging, the special needs of the older people and patients (such as dysphagia) in food texture are attracting increasing concerns (Funami, Ishihara, Nakauma, Kohyama, & Nishinari, 2012), which bring great challenges as well as opportunities for the development of modern food industry (Aguilera & Dong, 2016).
Plant-based foods are usually subjected to thermal processing before consumption, which will unavoidably change their texture (hardness) (Sila, Doungla, Smout, Van Loey, & Hendrickx, 2006). Many reports have indicated that after heat treatment, the changes in the texture of plant-based foods are mainly related to the changes in the structure of pectin (Greve et al., 1994, Lecain et al., 1999, Le et al., 2001;). For example, after heating, the water-soluble fraction (WSF) and chelate-soluble fraction (CSF) increased a lot, and digestion of WSF with RG-I degrading enzymes showed that the Arabian of WSF became more linear in carrot (Broxterman, Picouet, & Schols, 2017). Favaro, Beleia, Junior, and Waldron (2008) studied the differences in cell wall components and texture of three varieties of cassava after short-time, middle-time and long-time heat treatment. Their results showed that after short-time heat treatment, the contents of Ca2+ and Mg2+ were low and those of phytic acid and monovalent cations were high. Besides, long-time heat treatment of cassava resulted in more CSF.
In our previous research, we found that the CSF/WSF ratio determines the texture of lotus rhizomes after cooking by analyzing the main cultivars and landraces of lotus rhizomes in Hubei (Xie et al., 2018). By expanding the scope of sampling to include 18 varieties of lotus rhizomes collected in 2015, this study aims to explore the effects of CSF on the texture change of the lotus rhizomes after cooking through examining the texture and cell wall pectin characteristics before and after thermal processing. Principal Component Analysis (PCA) and regression analysis were used to further determine which components have greater influence on the classification of samples. This work provides the basic data for further studying the impact of cooking on lotus rhizomes.
Section snippets
Materials
Eighteen varieties of samples were collected from different regions of China in December 2015. The letters, names and origins of the 18 samples are shown in Table 1. Supplementary Fig. 1 demonstrates the distribution layout of the sample collection.
The lotus rhizomes were transferred to lab after harvesting and then washed under running tap water, hand-peeled and cut into 1.5 cm-thick slices. Then, the samples were thermally treated in boiling water for 30 min and then cooled to room
Effect of heat treatment on the hardness of various samples
The hardness of part of samples with different cooking time is presented in Fig. 1A. Heat treatment significantly decreased the hardness of all the samples. It is worth noting that the samples with higher hardness before cooking did not necessarily show higher hardness after cooking and vice versa. The differences in hardness among samples after heat treatment reached the maximum at 30 min. Therefore, to determine the main factors affecting the hardness, 30 min cooking time was chosen for
Discussion
The experiments presented here directly demonstrate the relationships between the cell wall components and the texture of lotus rhizomes after cooking of 30 min. Many recent studies have shown that the variations in the texture of plant-based food after cooking are mainly attributed to the changes in pectin (Ross et al., 2011, Njoroge et al., 2016). However, because of the small sample sizes in these studies, the variations of cell wall components after cooking may not be representative, or the
Conclusions
By studying the variations of cell wall components of 18 varieties of lotus rhizomes after cooking for 30 min, we found that CSF has a remarkable relationship with the texture of lotus rhizomes after cooking. The hardness after cooking varied greatly among different lotus rhizomes. The results show that the 18 varieties are significantly different in the contents of pectins and monosaccharides but all show a high content of Man. We speculate that there is a large amount of Mannan
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the Research Center of Aquatic Vegetable Processing and Preservation (2017ABA152-4).
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