Applications of spray-drying in microencapsulation of food ingredients: An overview

Abstract

Spray-drying process has been used for decades to encapsulate food ingredients such as flavors, lipids, and carotenoids. During this drying process, the evaporation of solvent, that is most often water, is rapid and the entrapment of the interest compound occurs quasi-instantaneously. This required property imposes a strict screening of the encapsulating materials to be used in addition to an optimization of the operating conditions. Likewise, if the encapsulated compound is of hydrophobic nature, the stability of the feed emulsion before drying should also be considered. Thus, spray-drying microencapsulation process must rather be considered as an art than a science because of the many factors to optimize and the complexity of the heat and mass transfer phenomena that take place during the microcapsule formation. This paper reports the main process engineering information that are considered useful to the success of a microencapsulation operation by spray-drying. Besides, a summary of the most commonly used wall materials and the main encapsulated food compounds are presented.

Introduction

Thanks to microencapsulated ingredients, many products that were considered technically unfeasible are now possible. Such ingredients are totally enveloped in a coating material, thereby conferring useful or eliminating useless properties to or from the original ingredient. Microencapsulation is defined as a process in which tiny particles or droplets are surrounded by a coating, or embedded in a homogeneous or heterogeneous matrix, to give small capsules with many useful properties. Microencapsulation can provide a physical barrier between the core compound and the other components of the product. More especially, in the food field, microencapsulation is a technique by which liquid droplets, solid particles or gas compounds are entrapped into thin films of a food grade microencapsulating agent. The core may be composed of just one or several ingredients and the wall may be single or double-layered. The retention of these cores is governed by their chemical functionality, solubility, polarity and volatility. Shahidi and Han (1993) proposed six reasons for applying microencapsulation in food industry: to reduce the core reactivity with environmental factors; to decrease the transfer rate of the core material to the outside environment; to promote easier handling; to control the release of the core material; to mask the core taste; and finally to dilute the core material when it should be used in only very small amounts.

In its simplest form, a microcapsule is a small sphere with a uniform wall around it. The material inside the microcapsule is referred to as the core, internal phase, or fill, whereas the wall is sometimes called shell, coating, wall material, or membrane. Practically, the core may be a crystalline material, a jagged adsorbent particle, an emulsion, a suspension of solids, or a suspension of smaller microcapsules. The microcapsule may even have multiple walls. In this review, only “core” and “wall” will be used to refer to the encapsulated ingredient and encapsulating agent, respectively.

Most microcapsules are small spheres with diameters comprised between a few micrometers and a few millimeters. However many of these microcapsules bear little resemblance to these simple spheres. In fact, both the size and shape of formed microparticles depend on the materials and methods used to prepare them. The different types of microcapsules and microspheres are produced from a wide range of wall materials (monomers and/or polymers) and by a large number of different microencapsulation processes such as: spray-drying, spray-cooling, spray-chilling, air suspension coating, extrusion, centrifugal extrusion, freeze-drying, coacervation, rotational suspension separation, co-crystallization, liposome entrapment, interfacial polymerization, molecular inclusion, etc. (Desai and Park, 2005, Gibbs et al., 1999, Gouin, 2004, King, 1995, Shahidi and Han, 1993). Depending on the physico-chemical properties of the core, the wall composition, and the used microencapsulation technique, different types of particles can be obtained (Fig. 1): simple sphere surrounded by a coating of uniform thickness; particle containing an irregular shape core; several core particles embedded in a continuous matrix of wall material; several distinct cores within the same capsule and multi-walled microcapsules.

Although most often considered as a dehydration process, spray-drying can be used to encapsulate active material within a protective matrix formed from a polymer or melt (Dziezak, 1988). Although many techniques have been developed to microencapsulate food ingredients, spray-drying is the most common technology used in food industry due to low cost and available equipment. Microencapsulation by spray-drying has been successfully used in the food industry for several decades (Gouin, 2004), and this process is one of the oldest encapsulation methods used since the 1930s to prepare the first encapsulated flavors using gum acacia as wall material (Shahidi & Han, 1993).

The objective of this paper is to review the state of the art of microencapsulation of food ingredients by spray-drying and present necessary theoretical and practical information on this process. Thus, the present paper discusses the uses of spray-drying for microencapsulation ends from four perspectives. First, it focuses on some theoretical aspects of the spray-drying process. Next, the paper discusses the application of spray-drying in microencapsulation of food ingredients. The third section presents criteria required for encapsulating agents and describes several wall materials that have proved good encapsulation efficiency. The final part summarizes important recent applications concerning the microencapsulation of food ingredients by spray-drying.