Flavor release from lactose/protein matrix during storage: Effects of lactose crystallization and powder microstructure

Highlights

• Ethyl acetate (EA) encapsulated by lactose/WPI (4:1) was unstable.

• The powders with higher lactose content caused the most EA flavor escaping.

• Lactose crystallization led to structural modification of microparticles.

• EA particles tended to accumulate onto the surface of powders at high RH.

Abstract

Glass-forming carbohydrates are widely used to encapsulate volatile flavor substances to avoid excessive flavor release and increase storage stability. The objective of this study was to investigate the effects of the crystallization of lactose on the microstructures of powders, as well as subsequent influences on flavor release from lactose-containing matrices. Mixtures of lactose and whey protein isolate were used to constitute the wall materials in ratios of 0:1, 1:4, 1:1 and 4:1 (w/w). Ethyl acetate (EA) was used as a model core material and microparticles were prepared by freeze-drying. Microparticles with higher lactose contents had lower glass transition temperatures and exhibited increased water adsorption behaviors at various water activities (0.33, 0.54 and 0.75 a_w). A rapid decrease in flavor retention was found in matrices with high lactose contents, especially during storage at 0.75 a_w. X-ray diffraction and crystallinity analysis proved that the degrees of crystallization of lactose in the encapsulation systems differed, which was induced by differences in water activities. Images of microstructures showed that the formation of crystals caused structural modifications of the powder. Consequently, EA particles were prone to accumulate and migrate to the surface of the powder, which resulted in considerable release of EA flavor.

Introduction

Flavor release issues during food storage have attracted increasing attention in recent studies. Such issues play a critical role in consumer food preferences and the storage stability of food products. Most flavor compounds are volatile and are prone to escape from a solid food matrix, especially as a result of the effects of ambient temperature and relative humidity (RH) (Bortnowska & Goluch, 2018; Wang, Doi, & McClements, 2019). Hence, encapsulation is the most widely used method of protecting the core materials in a powdered food system against evaporation, oxidation, or interactions between ingredients and is expected to improve the chemical and physical stability of these materials during processing and storage (Saifullah, Islam Shishir, Ferdowsi, Tanver Rahman, & Van Vuong, 2019). The wall materials are usually made of natural or modified polysaccharides, proteins, lipids, or other polymers in order to obtain an appropriate structure for the better control of flavor release (Shishir, Xie, Sun, Zheng, & Chen, 2018). Recently, several studies have demonstrated that the addition of low-molecular-weight compounds to protein-based microparticles can improve their encapsulation ability (Li, Roos, & Miao, 2016a; Zhou, 2013). These studies found that the addition of low-molecular-weight carbohydrates increased the density of amorphous mixtures and formed a compact shell, which improved the barrier properties of the wall materials. Because of the better barrier properties, the encapsulation ability of the microparticles was improved and the loss of volatile materials would decrease.

Lactose is a principal component of milk and is also used as a drug excipient in the pharmaceutical industry (Huppertz & Gazi, 2016). It is also commonly used as a wall material to embed volatile or active substances. Lactose crystals exhibit significantly greater thermal and mechanical stability and reduced stickiness during storage (Carpin et al., 2016). However, low-molecular-weight carbohydrates, such as lactose and trehalose, naturally exist in various crystalline and amorphous forms (Fu et al., 2019). It is commonly believed that the crystallization of lactose affects the microstructure of the matrix and also the subsequent encapsulation performance, including the barrier properties of the wall materials, as well as the release rate of the core material in certain conditions (Li et al., 2016a, 2016b). In that case, the phase transitions of lactose, which are generally triggered by the storage environment, are one of the most important factors that determine the encapsulation of a lactose-containing powder system (Li, Roos, & Miao, 2016b; Potes, Kerry, & Roos, 2012).

A number of studies have declared that the transition from amorphous lactose to the crystalline form occurs when the storage temperature approaches or exceeds the glass transition temperature, and the amorphous structure exhibits dramatic modifications in terms of free volume, molecular mobility and dielectric coefficient (Fu et al., 2019; Li et al., 2016a). Once the glassy phase of lactose undergoes a transition to the rubber-like phase, the diffusion of volatile core materials may change rapidly, which corresponds to the structural modification of the powder system (Zhou, 2013). This kind of significant transformation may contribute to structural changes in the microcapsule, which are closely associated with the release of core flavor compounds (Huppertz & Gazi, 2016). Furthermore, some studies found that the diversity and complexity of the wall materials may also affect the crystallization performance of lactose after spray-drying and freeze-drying (Carpin et al., 2016; Fu et al., 2019). For example, Fan and Roos (2016) reported that the addition of protein to the wall mixture could delay or inhibit crystallization after freeze-drying, for which possible reasons were based on bond hindrance theory (Lopez-Diez & Bone, 2000), steric hindrance theory (Adhikari, Howes, Bhandari, & Langrish, 2009) and diffusion limitation theory (Das, Lin, Sormoli, & Langrish, 2013). Although several studies have tried to determine the effects of the crystallization of sugars on powder structures (Li et al., 2016a; Zhou, 2013), there is still a lack of further studies of structural modification and flavor release behaviors.

The objective of this study was to investigate the effects of the crystallization of lactose on the structural modification of encapsulation systems and flavor release from freeze-dried lactose/whey protein isolate (WPI) powders during storage. Different ratios of lactose and WPI used as wall materials were compared at various values of RH. This study can be used to determine critical physical parameters for the optimization of the processing conditions and storage stability of lactose-containing encapsulation systems.