Physical and mechanical properties of plant-derived soft-shell capsules formulated with hydroxypropyl starches from different botanical sources

Highlights

• Soft-shell capsules were prepared using starches to replace animal sources.

• Physical properties of starch films and soft-shell capsules were investigated.

• Mechanical properties of starch films and soft-shell capsules were investigated.

• Starch-based soft-shell capsules were disintegrated within 1200 s.

• These soft-shell capsules could be applied for manufacturing commercial products.

Abstract

Soft-shell capsules are prepared herein using hydroxypropyl starches from different botanical sources (maize, waxy maize, potato, and cassava) as a replacement for animal-based materials such as gelatin. The physical, mechanical, and morphological properties of the starch films are characterized to investigate the possibility of manufacturing soft-shell capsules. Starch films originating from tubers, including potato and cassava, exhibit higher tensile strength, resulting in higher hardness of the soft-shell capsules compared to those originating from maize and waxy maize starches. None of the starch-based soft-shell capsules broke in a brittleness test, and there are no distortion defects in the seams that seal the capsules. Disintegration and stability tests over six months show that although the soft-shell capsules manufactured from maize and potato starch disintegrate faster than those from waxy maize and cassava starch, all of the capsules disintegrate within 1200 s, which is acceptable for commercial application.

Introduction

Soft capsules have been used in industry since the mid-19th century. The first was made by constructing leather frameworks, which were made one at a time. The contents were dropped into the frame, which was then charged. The capsules were made manually by covering the frame with gelatin and enclosing it. With advancements in technology, the individually constructed leather frames were converted to molds with multiple holes. The manufacturing process also evolved into a plate process with a die pocket, and in 1933, R.P. Scherer invented the rotary die process and commercialized the technology [1]. The current methods of producing soft capsules are roughly divided into two categories, i.e., the rotary die method [2] and the falling-drop method [3]. Among them, the rotary die method has the advantages such as the more precise dosage and greater reproducibility as compared with the falling-drop method [4].

Gelatin is a polymer with a linear structure, obtained from collagen derived from pig skin, bovine leather and bones, fish bones, and fish scale. It contains a mixture of water-soluble proteins (84–90%), mineral salts (1–2%), and water (8–15%). The protein fraction mainly comprises amino acids linked by amide bonds, forming a linear polymer with molecular weights ranging from 15,000 to 250,000 Da [5]. The main component of the soft capsule shell, gelatin, is heated to a sol state and then molded to form the shell of the soft capsule by means of a sol-gel transition. Therefore, it can be said that the characteristics of the soft capsule depend on the chemical, physical, and physiological properties of the gelatin. Because of these properties, gelatin is widely used as a capsule material [6]. In spite of the many advantages of gelatin such as nutritional excellence as a food, film transparency and gloss, film forming ability, and heat adhesion, it is derived from animals, and therefore there is a limit to its intake for religious reasons and for vegetarians. Furthermore, there is a growing demand for vegetable films to replace gelatin due to concerns about swine influenza (SI) and bovine spongiform encephalopathy (BSE) and the increasing demand for a naturalistic image for health functional foods [7].

Studies on gelatin substitutes have been reported. Due to the drawbacks of gelatin capsules and the water instability of gelatin, attempts have been made to use alternative substances such as hydroxypropyl methylcellulose cellulose, hydroxypropyl starch, and starch [8]. Hydroxypropylation is commonly used to modify starch for the food industry. Hydroxypropyl groups are hydrophilic in nature, and when introduced into starch granules, weaken the internal bond structure holding the granules together. The substituent disturbs the association of the polysaccharide chains, thus preventing retrogradation due to the hydrogen bonds. The improved functional properties of hydroxypropylated starches, such as the extended shelf life of cold storage products (freeze–thaw stability), higher peak viscosity and paste clarity, and decreased gelatinization temperatures are well documented [9]. The requisite properties for alternative soft capsule substrates are high water-solubility, extensibility and film formability, sol-gel thermal stability, and adhesion to the formed sheet. Hard capsules were first commercialized, focusing on hydrocolloids capable of sol-gel transition; the industry was later expanded to include soft capsules. Hydrocolloids are a heterogeneous group of long chain polymers (polysaccharides and proteins) characterized by their ability to form viscous dispersions and gels when dispersed in water. The presence of a large number of hydroxyl (–OH) groups markedly increases their affinity for binding water molecules, rendering them hydrophilic [10]. Native starches such as maize, potato, waxy maize, tapioca, sweet potato, and mung beans have been studied for a long time because of their low price and availability; however, it is difficult to develop soft capsules from these materials [11]. Starch films suffer from some disadvantages, such as high water-solubility, poor water barrier properties, and poor mechanical properties, limiting their applications in the food industry [8]. Various chemical approaches have been employed to overcome these disadvantages [12]. In order to further improve the physical properties of vegetable soft capsules, hydroxypropyl modified starch is more suitable because it binds water more strongly and gelatinizes at low temperature [13].

Soft capsules using animal gelatin as the main raw material for the film still have the largest market share. However, due to changes in consumer perceptions of safety and the expansion of markets that require halal and kosher certification due to religious reasons, interest in vegetable soft capsule materials has increased [14]. Although public attention has increased, actual research on soft vegetable capsules is insufficient, and there is a lack of materials for developing stable soft vegetable capsules. In this study, we develop starch-based soft-shell capsules using hydroxypropyl starch from various botanical sources including maize, waxy maize, potato, and cassava. The physical and mechanical properties of the starch-based films and soft-shell capsules are also investigated and a disintegration test is performed to evaluate the feasibility for manufacturing soft-shell capsules.