Physicochemical characterization and oxidative stability of fish oil encapsulated in an amorphous matrix containing trehalose

Abstract

Fish oil with 33% omega-3 fatty acids was microencapsulated by spray-drying in a matrix of n-octenylsuccinate-derivatized starch and either glucose syrup or trehalose. Samples showed no difference in physicochemical properties as determined by measurement of particle size, oil droplet size, true density and BET surface. Upon storage at low relative humidity, lipid oxidation was decreased in trehalose containing samples indicating that in the amorphous state trehalose is a more suitable wall material for microencapsulation than glucose syrup. The retarded oxidation of trehalose containing samples may be attributed to the unique binding properties of trehalose to dienes. At 54% relative humidity, a rapid oxidation of the microencapsulated oil was observed upon crystallization of trehalose, which limits the range of applications to products to be stored at low humidity.

Introduction

For several decades, microencapsulation by spray-drying has been applied in the food industry and is still the predominating technology as it is rather inexpensive and straightforward (Gouin, 2004, Ré, 1998). Typical wall materials for microencapsulation by spray-drying are low molecular weight carbohydrates like maltodextrins or saccharose, milk or soy proteins, gelatine and hydrocolloids like gum arabic or mesquite gum (Fäldt and Bergenståhl, 1995, Hogan et al., 2001a, Hogan et al., 2001b, Hogan et al., 2001c, Keogh et al., 2001, Kim and Morr, 1996, Lin et al., 1995). Recently, the suitability of n-octenylsuccinate-derivatized starch (n-OSA starch) for microencapsulation of fish oil and the influence of type of n-OSA starch and drying conditions on microcapsules characteristics have been described (Drusch & Schwarz, 2006).

Problems associated with the use of low molecular weight carbohydrates in microencapsulation are caking and structural collapse as well as re-crystallization of the amorphous carbohydrate matrix upon storage. Le Meste, Champion, Roudat, Blond, and Simatos (2002) concluded that caking can be explained by the formation of inter-particle bridges between adjacent particles when surface viscosity reaches a critical value. Caking and collapse at high relative humidity were described for microencapsulated sea buckthorn oil, orange peel oil or linoleic acid (Beristain et al., 2002, Partanen et al., 2005, Ponginebbi et al., 1999) and dairy powders (Roos, 2002). Crystallization is a two step process with the phase of initial nucleation and subsequent crystal growth. Crystallization of lactose in dairy-based powders has intensively been studied (Joupilla et al., 1997, Joupilla et al., 1998, Knudsen et al., 2002, Thomsen et al., 2005). Apart from a negative impact on handling properties, both, caking or collapse and crystallization may lead to a release of the encapsulated substance from the matrix.

In this context, physicochemical properties of trehalose appear to be very promising concerning its use in microencapsulation. Trehalose possesses a uniquely high glass transition temperature. Data on the glass transition temperature range from 79 °C to 115 °C and are attributed to polymorphism in the crystallization pattern (Willart et al., 2002). If trehalose crystallizes, the predominant form is the trehalose dihydrate, thus immobilizing water and keeping the water activity at a low level. For these reasons, trehalose remains in the glassy state at temperatures higher than other sugars and has a greater capability of stabilizing proteins, lipids or carbohydrates embedded in trehalose glasses (Richards & Dexter, 2001).

Aim of the present study was to investigate the physicochemical properties of microcapsules with trehalose vs. glucose and whether substitution of glucose syrup by trehalose leads to an increase in oxidative stability of microencapsulated fish oil.