Plantain starch granules morphology, crystallinity, structure transition, and size evolution upon acid hydrolysis

Highlights

• Amorphous fractions were firstly hydrolysed following zero-order kinetics.

• Crystallinity content increased in the first 7 days of the hydrolysis process.

• Short crystalline chains were harder to hydrolyse at intermediate hydrolysis times.

• After 20 days of acid hydrolysis crystallinity was similar to that of native starch.

• Structure transitions during acid hydrolysis can be detected by available techniques.

Abstract

Plantain native starch was hydrolysed with sulphuric acid for twenty days. Hydrolysis kinetics was described by a logistic function, with a zero-order rate during the first seven days, followed by a slower kinetics dynamics at longer times. X-ray diffraction results revealed a that gradual increase in crystallinity occurred during the first seven days, followed by a decrease to values similar to those found in the native starch. Differential scanning calorimetry analysis suggested a sharp structure transition by the seventh day probably due to a molecular rearrangement of the starch blocklets and inhomogeneous erosion of the amorphous regions and semi crystalline lamellae. Scanning electron micrographs showed that starch granules morphology was continually degraded from an initial oval-like shape to irregular shapes due to aggregation effects. Granule size distribution broadened as hydrolysis time proceeded probably due to fragmentation and agglomeration phenomena of the hydrolysed starch granules.

Introduction

Starch, a natural homo-polysaccharide of glucose monomers extracted from diverse botanical sources (tubers, cereals, legumes and unripe fruits), is an abundant, renewable, biodegradable and inexpensive material with a wide variety of applications in food and non-food industry (Bello-Pérez & Paredes-López, 2009). Starch is organized in small granules, where size and shape as well as physicochemical and functional properties are specific of botanic origin (Biliaderis, 1991). These features depend of the internal organization and relative composition of the main starch components given by amylose and amylopectin. Starch is considered as semi-crystalline polymer since amorphous and crystalline zones are combined within growth rings with complex geometric arrangements with nano-sized blockets (Oates, 1997). Crystalline lamellae in the blockets are organized as amylopectin double helices formed by intertwining chains of more than ten glucose units. Branching points are found in the amorphous lamellae (Gallant, Bouchet, & Baldwin, 1997). It is commonly accepted that blockets in B- and C-type starches are of the order of 400–500 nm, while in A-type starches of the order of 20–100 nm (Gallant et al., 1997).

Fabrication of nano- and micro-particles from biopolymers can be utilized as delivery systems or to modulate the physicochemical or sensory characteristics of food products. Such is the case of starch, which is rarely used in its native form. In fact, different chemical and physical methods have been designed in order to modify starch structural and functional properties. Digestibility, biodegradability and thermal stability are important starch properties that are commonly focused for modification purposes. Acid hydrolysis has been used for obtaining nanoparticles of various starches (Putaux et al., 2003, Chen et al., 2008, Kim et al., 2012). The idea underlying acid hydrolysis is to exploit the difference in acid susceptibility of amorphous structure and semi-crystalline starch lamellae. Starch hydrolysis with sulphuric and hydrochloric acids produces Nageli amylodextrin and linearized starch, respectively. Acids yield fast hydrolysis of the starch amorphous zones with negligibly slow hydrolysis of crystalline areas. Limited starch acid hydrolysis produces nanocrystals that can be used as fillers in polymeric matrices to improve their mechanical and barrier properties or for stabilizing emulsions (Li, Sun, & Yang, 2012). Starch nanocrystals have been produced from waxy maize starch (Angellier et al., 2004, Angellier et al., 2006, Le Corre et al., 2011, Le Corre et al., 2012, Li et al., 2012).

This work considers the acid hydrolysis of plantain starch. The motivation for using plantain starch for this study relies on the increased interest in using native commodities for developing raw materials for industry, either for producing new products or to replace those products now made from non-renewable sources, for spearheading regional development. Starches from different botanical sources and acid or enzyme hydrolysed modified starches obtained from them are promising candidates for contributing to the achievement of this goal owing to their complete biodegradability, low cost, ready availability and renewability (Lu, Xiao, & Xu, 2009). In this way, the aims of this work were to study: (i) the acid hydrolysis of plantain starch and to determine the kinetics dynamics of the process; (ii) to monitor morphology, crystallinity, structure transition and particle size changes occurring in the hydrolysed granules during hydrolysis time; (iii) to establish an interrelationship between the progression of these parameters with hydrolysis kinetics, in order to gain insights about the underlying phenomena taking place in this complex process, so that eventually a better control may be achieved in the production of plantain starch derivatives with specific functional properties.