Influence of pH and cinnamaldehyde on the physical stability and lipolysis of whey protein isolate-stabilized emulsions Enmin Chen a , Shan Wu a , David Julian McClements d , Bin Li a, b, c , Yan Li a, b, c, * a College of Food Science and Technology, Huazhong Agricultural University, Wuhan 430070, China b Key Laboratory of Environment Correlative Dietology (Huazhong Agricultural University), Ministry of Education, China c Functional Food Engineering &Technology Research Center of Hubei Province, China d Department of Food Science, University of Massachusetts, Amherst, MA 01003, USA article info Article history: Received 21 October 2016 Received in revised form 21 January 2017 Accepted 24 January 2017 Available online 27 January 2017 Keywords: Whey protein isolate Cinnamaldehyde Protein Lipid digestion Schiff base reaction Chemical compounds studied in this article: Cinnamaldehyde (PubChem CID: 637511) abstract Cinnamaldehyde (CA), a common hydrophobic flavor, was encapsulated in oil-in-water emulsions that were stabilized by whey protein isolate (WPI). The impact of CA content and pH on the physical stability and lipolysis of the emulsions was then investigated. The presence of CA gave the emulsions a creamy yellow color, which became darker during storage. Emulsions formed using only CA as the oil phase contained large droplets that were physically unstable to particle growth and phase separation. The addition of medium chain triglyceride oil (MCT) improved the stability of emulsions containing CA, which was attributed to inhibition of Ostwald ripening effects. Fluorescent microscopy indicated that the adsorption of the protein to the droplet surfaces led to a thicker adsorbed layer in the presence of CA. The stability of the emulsions to droplet flocculation and coalescence depended on the CA level in the oil phase and the pH of the aqueous phase. An in vitro model was used to assess the impact of oil phase composition and pH on lipid hydrolysis and emulsion microstructure under simulated gastrointestinal tract conditions. The rate of lipid hydrolysis was highly dependent on CA level and pH. These results may facilitate the fabrication of emulsions with controlled GIT fate that are suitable for use in functional foods and beverages. © 2017 Elsevier Ltd. All rights reserved. 1. Introduction Essential oils, usually extracted from plants, are increasingly being utilized as functional ingredients in foods because of their potent antimicrobial, antioxidant, and antiradical activities (Asbahani et al., 2015). These oils are complex mixtures of both non-volatile and volatile compounds that vary in their lipophilicity and water-insolubility. Those compounds include alkaloids, flavo- noids, isoflavones, monoterpenes, phenolic acids, carotenoids, and aldehydes (Bakkali, Averbeck, Averbeck, & Idaomar, 2008; Seow, Yeo, Chung, & Yuk, 2014). The relatively low water-solubility of essential oils means that they need to be incorporated into colloidal delivery systems before they can be introduced into aqueous-based foods. However, these delivery systems must be carefully designed to maintain the biological activity of the active components, and to ensure that they do not adversely impact the desirable sensory characteristics of foods and beverages, such as appearance, texture and flavor (Buranasuksombat, Kwon, Turner, & Bhandari, 2011). Emulsion-based delivery systems, which can easily be formu- lated from food-grade ingredients, are a popular vehicle used to encapsulate, deliver and protect lipophilic functional components (McClements & Li, 2010b). The selection of an appropriate oil phase and emulsifier is critical to creating an emulsion-based delivery system with the desired functional attributes. One of the major challenges that needs to be overcome when fabricating essential oil-loaded emulsions is the fact that the oil phase contains many components that are sufficiently soluble in water, promoting droplet growth through Ostwald ripening, which then accelerates creaming and phase separation (Chang, McLandsborough, & McClements, 2012; Tian, Lei, Zhang, & Li, 2016). Ostwald ripening occurs due to the diffusion of oil molecules from small droplets to large droplets through the intervening aqueous phase. The incor- poration of water-insoluble oils (such as medium or long chain triglycerides) into the oil phase can suppress Ostwald ripening through an entropy of mixing effect known as compositional * Corresponding author. No 1 Shizishan Road, Hongshan District, Wuhan 430070, China. E-mail address: yanli@mail.hzau.edu.cn (Y. Li). Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd http://dx.doi.org/10.1016/j.foodhyd.2017.01.028 0268-005X/© 2017 Elsevier Ltd. All rights reserved. Food Hydrocolloids 69 (2017) 103e110