Microwave pretreated esterification improved the substitution degree, structural and physicochemical properties of potato starch esters

Highlights

• Acetylation with microwave pretreatment enhanced the degree of esterification.

• The starch granules maintained the integrity after treatment.

• Alterations on starch structural and physicochemical properties are noticeable.

• The current modification method provides a handy way for industrial production.

Abstract

Potato starches were pretreated with 600 W microwave and then acetylation was carried out with acetic anhydride and glacial acetic acid (1:1 in volume). The microwave-pretreated-acetylation remarkably modified the physicochemical properties of starch compared with the single acetylation method. The starch granules exhibited rough surfaces but maintained integrity and the crystal structure changed to C-type after microwave-pretreated-acetylation. Meanwhile, the microwave-pretreated starch esters exhibited the higher substitution degree, the higher infrared absorption peak intensity and the lower relative crystallinity compared to un-microwave-pretreated ones. The solubility of microwave-pretreated starches was higher at 80–90 °C than un-pretreated samples, while the swelling power of microwave-pretreated starches was lower at 60–90 °C under a same esterification level. Particularly, the solubility of microwave-pretreated samples at 90 °C was 2.11 times higher than un-pretreated ones, while the swelling power of microwave-pretreated starches exhibited 52.8% lower than un-pretreated samples at 90 °C under the 12 mL/100 g esterification level. The peak viscosity and the gelatinization enthalpy of microwave-pretreated starches were lower than un-pretreated ones under the same esterification level. Particularly, these values of microwave-pretreated starches were 2332 mPa s lower and 8.46 J/g lower than un-pretreated ones at the 12 mL/100 g esterification level, respectively.

Introduction

Being inexpensive, renewable, and fully biodegradable, starch is energy supplier in primary position of human diet and is also one of the most abundant plant polysaccharides. The potato starch is also an attractive and sustainable source for industrial application because of its inexpensive and unique properties and its high production. Native potato starches exhibit larger granules, higher purity and longer glucan chains than most of other starches as well as lower protein and fat content, higher peak viscosity, outstanding swelling power and higher transparency (Han, Zeng, Yu, Zhang, & Chen, 2009). Therefore, it is widely used in food industry products in a broad range due to its desirable gelling, thickening and food system stabilizing capacity. Additionally, it is also used in multifarious industries such as paper, textile, coating, chemical, medicine and biodegradable materials.

However, native potato starch has restrictions such as low solubility in cold water, low shear and thermal resistance, high tendency to retrograde, narrow peak viscosity range and poor process tolerance, etc. (Li et al., 2015, Xu et al., 2012). In order to eliminate the limitations and to expand the usefulness for particular industrial process, a variety of modification technologies have been developed, which can drastically modify the physicochemical properties of starches. The methods used to alter starch properties mainly include chemical modification, physical modification and enzymatic modification.

Esterification is widely used in starch industry. The introduction of ester groups into starch molecules to modify their original physicochemical properties is an important task when producing starch esters. These starch esters have lower gelatinization temperatures, and higher peak viscosity, paste clarity and freeze-thaw stability compared to native counterparts (Šubarić, Ačkar, Babić, Sakač, & Jozinović, 2014). Acetylation is one of the most common modification methods, carried out with acetic acid or acetic anhydride under alkaline condition. It increases water resistance of starch, breaks down hydrogen bonding among starch molecules and imparts acetyl groups resulting in amphiphilic property and flexibility of starch films (Bhosale and Singhal, 2006, Shogren, 1996). Acetylated starch addition can also improve the appearance of starch-based foods by providing glossy surfaces and enhancing the transparency of starch gels (Sun, Zhang, & Ma, 2016).

However, the traditional technology to produce acetylated starch has limitations such as low substitution and serious time consuming. In recent years, microwave treatments have been used to enhance acetylation and overcome these limitations (Diop et al., 2011, Zhang et al., 2014). Acetylation under microwave treatments yields starch esters with advanced qualities that can expand the application of starch for various industrial processing. Biswas et al. (2008) report a degree of substitution of 3 after microwave-assistance acetylation. Lin et al. (2017) report that amorphous regions of starches are destroyed after esterification with microwave assistance. Šubarić et al. (2014) observe a decreased gelatinization and pasting temperatures while Shogren and Biswas (2006) report enhanced cold water solubility after microwave-assistant processing. However, the microwave assisted esterification is still only under the laboratory stage because the esterification and microwave treatment are performed simultaneously. It is hardly applied into industrialization because the chemical reagents are volatile and reaction conditions are difficult to control.

In this study, microwave was used as a pretreatment method and following the acetylation was carried out with acetic anhydride and glacial acetic acid, and the structural and physicochemical properties of obtained starch samples were analyzed comprehensively through a variety of methods to figure out the advantages of the microwave-pretreated potato starch esters. This study is also expected to broaden application fields of microwave technology, to increase the diversity of starch modifications and eventually to provide information and lay the foundation for industrial production of starch modification with microwave-assistance.