Preparation of low digestible and viscoelastic tigernut (Cyperus esculentus) starch by Bacillus acidopullulyticus pullulanase

https://doi.org/10.1016/j.ijbiomac.2017.04.068Get rights and content

Highlights

  • PUL modified tigernut starch had significantly increased slow digestible starch.

  • PUL modified tigernut starch had significantly decreased viscoelasticity.

  • PUL hydrolysis changed the molecular structures of tigernut starch.

  • Low digestible and viscoelastic tigernut starch is highly valued as a component in some functional foods.

Abstract

Tigernut starch is an underutilized food resource. In this study, pullulanase (PUL) hydrolysis was used to change its physiochemical properties for different food applications. The content of low digestible fractions, resistant starch and slow digestible starch, in PUL modified tigernut starch significantly increased from 2.03% to 25.08% (P < 0.05) using 100 U/g starch of PUL in the debranching reaction. The paste or dispersion of PUL modified tigernut starch had a significantly decreased viscoelasticity (P < 0.05), but the paste still exhibited a typical property of pseudoplasticity. Molecular weight, amylopectin A B2 and B3 chain of PUL modified starch were lower, while amylose content, amylopectin B1 chain were higher than those of natural tigernut starch. The low digestible and viscous tigernut starch is highly valued as a component in some functional foods.

Introduction

Tigernut (Cyperus esculentus), a monocotyledonous graminoid flowering plant of the sedge family, grows widely in tropical, sub-tropical, and temperate regions across the global land. It has been considered as an untapped source of health food with highly nutritional values and many effective pharmacological actions [1]. The most popular food application of tigernut is only found in the Nigeria. It is processed into nonalcoholic beverage Kunnu or snack. It is also used for the production of nougat, jam, beer, or ice cream as a flavoring agent. Roasted tigernut flour is sometimes added to biscuits and other bakery products. In Spain, boiled tigernut flour is mixed with sugar and water to produce Horchata, another nonalcoholic beverage of milky appearance. However, in most other countries, tigernut is still considered as a weed.

The edible tuber of tigernut contains about 30% starch [2] almost twice the quantity of starch as potato tuber. Tigernut starch was found to possess some physicochemical and functional properties comparable to those of potato or maize starch. Accordingly, it has great potentials for some industrial applications. In composite materials, tigernut starch showed high level of compatibility and binding properties, which was used in the productions of pharmaceutical tablets as a compression excipient [3], [4] and pharmaceutical suspensions as a dispersion agent [5]. In hydrate state, the gelatinized tigernut starch showed high gel strength and better freeze-thaw stability, which was suggested applicable to fruit jellies, cold drink [6], confectioneries [7].

Attempts to expand the food utilization and improve the functional characteristics of tigernut starch by chemical and physical methods have been studied. Microwave heating increased water binding capacity, gelatinization enthalpy change, peak viscosity, but reduced solubility, gelatinization temperature, amylose content, and swelling power [8]. Sodium hypochlorite oxidation reduced water binding capacity and peak viscosity. Carboxymethylation increased viscosity, freeze-thawing stability and transparency, but reduced gelatinization temperature [9]. Among these physicochemical properties, the viscosity is an important quality indicator for the food use of the starches since in most cases this will be the viscosity on the final product that arrives on the consumer's home. If a starch produces high or low viscosity paste, it can be used for different applications. For the use as the gelling agent in the pie filling, a high viscosity would be desired for preventing the spilling during the transportation [10]. But for the soups or adhering seasoning to a food product, low viscosity starches should be preferred. This kind of starch has been widely used in textile and papermaking industries.

Digestibility of starch is another factor to be considered for food utilization since it is related to the nutritional values and health functions of final products. The low digestible starches, such as slow digestible starch (SDS) or resistant starch (RS), have gained considerable interest in the past years. RS has been found to be related to improvements of lipid and cholesterol metabolisms, prebiotic effects on colon microorganism, and reduction in the risk of ulcerative colitis and colon cancer [11], [12]. SDS shows potential health benefits regarding low glycemic response, causing satiety and improved physical performance and glucose tolerance, as well as reduced blood lipid level and insulin resistance [13], [14].

Amylolytic hydrolysis by rearrangement of side chain length and distribution to change the digestibility and viscoelasticity has been applied in the modification of many cereal starches. Pullulanase (PUL, EC 3.2.1.41), an amylolytic exoenzyme produced by some microorganisms, is often used with glucoamylase in conversion of cereal starch into sugars. In the hydrolytic reactions of starch, type I PUL specifically attacks α-1,6 linkages to debranch the side chains of amylopectin, while type II PUL is also able to hydrolyze α-1,4 linkages of amylose and amylopectin besides α-1,6 linkages. These structural changes by PUL have been related to produce the modified rice [15], [16], potato [17], [18] or maize starch [19] of low digestibility and/or low viscoelasticity.

Starch, the main component in tigernut, has great influences on the nutritional values and sensory qualities of the final products. As an underutilized source, starch in tigernut is also highly valued to be developed into a new functional food component. However, the processing and utilization of tigernut starch are still limited till now, largely because of the poor understandings of physicochemical and functional properties of tigernut starch. To the best known of our knowledge, amylolytic hydrolysis, the normal modification techniques for starch, has never been used in tigernut starch to obtain some desired properties. In this study, type I PUL from Bacillus acidopullulyticus was used to produce low digestible and viscoelastic tigernut starch. The possible molecular mechanisms involved in the changes were proposed.

Section snippets

Raw materials

Tigernut (C. esculentus) was planted and harvested in the farm of Changchun University in 2009. The fresh tubers were thoroughly washed with water, naturally air-dried in room temperature, and then stored at −18 °C till use.

Chemicals

PUL at 400 U/mL from B. acidopullulyticus was purchased from Sigma-Aldrich (St. Louis, MO, USA). One unit is defined as the amount of enzyme which hydrolyzes pullulan, liberating reducing carbohydrate with a reducing power equivalent to 1.0 μmole glucose per minute at pH 5.0

In vitro digestibility

A type I PUL from B. acidopullulyticus with a specific cleaving activity towards α-1,6-glucosidic linkage was used to hydrolyze the tigernut starch. After reaction, the PUL modified starches were washed to remove the soluble short chains, and finally dried overnight.

Digestion of the PUL modified starches in vitro was determined by double enzymatical digestion based on Englyst procedure. This method has been correlated with human studies [20] and thus can be used as a reference to in vivo

Conclusions

PUL hydrolysis reduced the digestibility and viscosity of tigernut starch, which were involved in the changes of starch structures. Molecular weight and amylopectin A B2 B3 chain of PUL modified starch were lower, while amylose content and amylopectin B1 chain were higher than those of natural tigernut starch. The low digestible and viscous tigernut starch is highly valued as a component in some functional foods.

Acknowledgments

This work was sponsored in part by National Natural Science Foundation of China (NSFC-31671801 and 31371749), and “the thirteenth five” research project of science and technology (JJKH20170502KJ and 2016298) from education department of Jilin provincial government.

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