Synthesis and characterization of citric acid esterified canna starch (RS4) by semi-dry method using vacuum-microwave-infrared assistance
Introduction
Starch is the most vital energy reserve of nature and the most important carbohydrate energy source of human diet. It occurs as granules in the amyloplast of storage organs like seeds, tubers and the chloroplasts of green leaves (Ellis et al., 1998). For nutritional purposes, starch is generally classified by the rate and extent of digestion as rapidly digestible starch (RDS), slowly digestible starch (SDS), and resistant starch (RS) (Englyst, Kingman, & Cummings, 1992).
Unlike RDS and SDS which can be completely digested and absorbed, RS resists digestion in the small intestine, and can almost totally be fermented by microbes in the colon to produce short-chain fatty acids (SCFAs) (Zaman & Sarbini, 2016; Zia ud, Xiong, & Fei, 2017). It has been reported to show physiological functions of the RS similar to dietary fiber, including increase of satiety, prevention against colorectal cancer, reduction of glycaemic response, reduction of fat accumulation, lower the risk of heart disease, promotion of the growth of probiotic microorganisms and increase of the absorption of minerals (Fuentes-Zaragoza et al., 2011; Lockyer & Nugent, 2017; Sajilata, Singhal, & Kulkarni, 2006). Therefore, RS has drawn much attention in recent years.
RS has been categorized into five types, among these chemically modified starches are classified as RS4 (Lockyer & Nugent, 2017). The resist digestion capabilities of chemical modified starch were generally accomplished through inhibiting enzyme access and forming a typical linkages such as etherification, cross-linking, and esterification (Liu et al., 2014). However, chemical modified techniques were limited due to issues such as environmental concerns about the discharge of the excess use of reagents, and consumers concerns about the food safety of chemically modified ingredients being added to their foods (Dupuis, Liu, & Yada, 2014).
Lately, esterification with citric acid is one of the most widely used chemical modification method for producing RS4, and was proved to be an effective mean of producing RS with high safety and thermal stability (Lee, Lee, & Lee, 2018; Mei, Zhou, Jin, Xu, & Chen, 2015; Remya, Jyothi, & Sreekumar, 2018; Xia, Li, & Gao, 2016; Xie & Liu, 2004; Ye et al., 2019). Compared with other chemical substances to produce RS4, citric acid is often preferred because is Generally Recognized as Safe (GRAS) for food substances, according to the US Food and Drug Administration, and it is widely applied in the food industries (Fiume et al., 2014; Gautier-Luneau, Bertet, Jeunet, Serratrice, & Pierre, 2007; Kapelko-Żeberska, Buksa, Szumny, Zięba, & Gryszkin, 2016). In short, the esterification of starch with citric acid offers advantages, including environmentally friendly, nutritional safety, inexpensiveness and easy to obtain.
Microwave radiation is an efficient source of thermal energy, providing fast and uniform heating, and is becoming a standard technique in various fields of chemical synthesis including modification of starch (Liu, Ming, Li, & Zhao, 2012). Microwave dielectric heating dissipates heat inside the medium and raises the energy of the molecules rapidly. Consequently, under microwave dielectric heating, more molecules become energized, and this usually results in higher reaction rates under microwave dielectric heating (Li et al., 2001). One of the most valuable advantages of using controlled microwave dielectric heating for chemical synthesis is the dramatic reduction in reaction times, promote reactions (Colman, Demiate, & Schnitzler, 2014; Kappe, 2008).
Many studies have reported the microwave-assisted chemical modification of starch such as carboxymethylated starch (Liu et al., 2012), ethyl starch (Singh & Tiwari, 2008), starch acetates (Shogren & Biswas, 2006), starch maleate (Xing, Zhang, Ju, & Yang, 2006), starch carbamate ester and phosphate ester (Lewandowicz et al., 2000). For starch citrates, Jyothi, Moorthy, Sreekumar and Rajasekharan (2007) prepared citrates of cassava starch by using a domestic microwave oven (heating for 3−7 min, temperature 120−160 °C) showed low degrees of substitution in the range of 0.005−0.063. However, microwave heating is unable to provide desired quality characteristics in the high DS citrate starch esterification due to altered heat and mass transfer, high temperature, and long-time microwave heating will lead to rapid loss of water and sample over cooked. Therefore, combining microwave heating with other rapid surface heating mode such as infrared radiation (IR) would yield the desired quality.
IR heating offers many advantages over conventional heating under similar conditions, including more heating efficient, shorter start up time, less processing time and less energy cost, reduced quality losses, versatile, simple and compact equipment (Krishnamurthy, Khurana, Jun, Irudayaraj, & Demirci, 2008; Rastogi, 2012). However, infrared heating has some disadvantages such as only surface heating and low penetration depth (Datta & Rakesh, 2013; Krishnamurthy et al., 2008). However, these drawbacks can be used positively when combined with microwave heating mode, combinations of IR heating with microwave heating can also increase energy throughput (Rastogi, 2012).
In this research, canna starch extracted from the rhizome of locally grown edible canna (Canna edulis Ker.) was selected as a starch matrix for modification. Canna edulis Ker. is mainly cultivated in South America, Thailand, Vietnam and China. Its dry rhizome contains 70–80 % starch, which is reported for one of the most potential raw materials for starch industry production (Zhang & Wang, 2009).
Some studies have been carried out on the physicochemical properties and modification of canna starch (Chuenkamol, Puttanlek, Rungsardthong, & Uttapap, 2007; Juansang, Puttanlek, Rungsardthong, Puncha-arnon, & Uttapap, 2012; Lan, Liu, Yang, Wu, & Wang, 2017; Puncha-arnon, Puttanlek, Rungsardthong, Pathipanawat, & Uttapap, 2007; Zhang & Wang, 2009; Zhang, Chen, Liu, & Wang, 2010; Zhang, Zhang, Deng, Wu, & Zhong, 2018). In our previous work, oxidized starches were successfully prepared from Canna, cassava, and corn starch using a green method of vacuum-assisted microwave heating (Zhang et al., 2018). In our study, canna starch citrate was prepared by a semi-dry method with vacuum-assisted, microwave and IR heating processes, and no such report had been published. The objective of this investigation was to produce esterified citrate starch (with high content of thermally stable RS4) by a green new method with simple processes, low output cost, and limited amounts of waste water and gas.
Section snippets
Materials
Canna starch, food grade, was obtained from Yujiang Starch Co. Ltd (GuiZhou, China) (The amylose, protein, lipids and moisture contents of Canna starch were 31.49 %, 0.92 %, 0.28 %, and 8.28 %, respectively). Pancreatic α-amylase (10080, 50U/mg) and Amyloglucosidase from Aspergillus niger (EC3.2.1.21, 49291, ≥120 U/mg) were purchased from Sigma-Aldrich (St. Louis, MO, USA). CA and all other chemical reagents utilized in this study were of analytical grade and water was doubly distilled.
Synthesis of citrate starch composite by vacuum-microwave-IR treatment
Citric
The degree of substitution (DS)
The influence of the different contents of CA and vacuum treatment on the DS values of treated starches are presented in Fig. 1. In this study, when the amount of CA increased from 20 % to 40 %, the DS of the treated starches significantly increased from 0.123 to 0.273 with vacuum treatment and from 0.079 to 0.239 without vacuum treatment. The DS values significantly increased along with the increment of the amount of CA and increased by vacuum treatment. This result was in agreement with Xia
Conclusion
CA-treated canna starch samples with DS values ranging from 0.123 to 0.273 were synthesized by a semi-dry method with the aid of vacuum treatment, microwave and infrared radiation. The maximum thermo stable resistant (RS) content reached 86.5 %, which suggest it as an effective method for preparing high content of thermally stable RS4. Vacuum treatment may increase the internal pressure of the starch granules, causing the granules to expand and become brittle, and microwave radiation was used
CRediT authorship contribution statement
Chunmei Wu: Methodology, Investigation, Data curation, Writing - original draft. Rui Sun: Investigation, Formal analysis. Qi Zhang: Data curation. Geng Zhong: Conceptualization, Methodology, Supervision, Resources, Writing - review & editing.
Acknowledgements
This study was supported by Chongqing high quality grain project (No. 40812314), Chongqing Nutrition Society project (No. 2019010).
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