Effect of milk protein composition on physicochemical properties, creaming stability and volatile profile of a protein-stabilised oil-in-water emulsion
Graphical abstract
Introduction
Protein-stabilised oil-in-water emulsions are the basis for many food products, such as milk, yogurt, whipping cream, ice-cream, gravies, mayonnaise and salad dressings (Loi, Eyres, & Birch, 2019b; McClements, 2015). The oil droplets of these emulsions are formed and stabilised by milk protein molecules, which consist of hydrophilic and hydrophobic groups. Milk proteins are surface active molecules, which adsorb to the oil-water interface, lower the interfacial tension and form a protective layer surrounding the oil droplets (Walstra, 2002). These proteins at the interface also provide repulsive forces such as steric and electrostatic forces between droplets, which extend oil droplet stability in the emulsions (Tcholakova, Denkov, Sidzhakova, & Campbell, 2006). The repulsive forces prevent oil droplets coming together, hence prolonging creaming, flocculation, coalescence and the eventual phase separation of emulsions.
Milk proteins such as casein and whey proteins are widely used as food ingredients with excellent emulsifying properties in food formulations (Dickinson, 1997; Sliwinski, Roubos, Zoet, van Boekel, & Wouters, 2003; Wilde, Mackie, Husband, Gunning, & Morris, 2004). It is also common in food products to include an additional low molecular weight emulsifier, such as monoglycerides (Fredrick et al., 2013), to enhance physicochemical properties and emulsion stability of a protein-stabilised emulsion (McClements, 2004).
The most common destabilisation phenomenon in protein-stabilised emulsions is creaming, which is an upward movement of oil droplets due to their lower density compared to the aqueous phase (Euston & Hirst, 2000; McClements, 2015). In our previous work (Loi, Eyres, & Birch, 2019a), the interaction between various monoglyceride compositions and milk proteins showed that glycerol monooleate (GMO) improved stability towards creaming after 28 days of ageing. The improved creaming stability was due to the formation of homogeneous and small oil droplets facilitated by GMO inclusion at the interface (Dickinson & Hong, 1994; Matsumiya, Takahashi, Inoue, & Matsumura, 2010).
Oxidative stability of lipid is an important quality parameter for food products. Protein-stabilised emulsions are generally known to protect oil droplets from oxidation (Donnelly, Decker, & McClements, 1998; Elisia & Kitts, 2011; Gumus, Decker, & McClements, 2017; Mei, McClements, & Decker, 1999). Milk proteins such as casein and whey proteins have been reported to inhibit lipid oxidation via chelation of transition metal ions or scavenging free radicals (Donnelly et al., 1998; Faraji, McClements, & Decker, 2004). The metal chelation of unadsorbed casein in the aqueous phase will increase oxidative stability (Berton, Ropers, Bertrand, Viau, & Genot, 2012; Gumus et al., 2017). However, it has been reported that metal chelation occurring at the interface may reduce oxidative stability (Villiere, Viau, Bronnec, Moreau, & Genot, 2005). Milk proteins can also form a barrier at the interface to isolate the interaction between oil and prooxidants (Donnelly et al., 1998) to increase oxidative stability. Different milk protein compositions may also exert different oxidative stability during ageing (Berton-Carabin, Ropers, & Genot, 2014). One of the methods to determine oxidative stability in emulsions is to measure volatile secondary products by headspace solid-phase microextraction (HS-SPME) with gas chromatography–mass spectrometry (GC–MS). This method is widely used to study secondary products of lipid oxidation (Damerau, Kamlang-ek, Moisio, Lampi, & Piironen, 2014).
While milk proteins and monoglycerides adsorb to the oil-water interface, their interaction at the interface can influence physicochemical properties, emulsion stability and oxidative stability. There have been many studies looking at the effect of milk proteins or monoglycerides independently on physical and oxidative stabilities in emulsions, however, there is little information about the interaction of milk protein composition and glycerol monooleate and the effect on emulsion properties and stability. This study aims to understand the effect on milk protein composition on physicochemical properties, creaming stability and oxidative stability in protein-stabilised emulsions. This study is useful to food technologists to optimise the use of ingredients in formulations to achieve good stability against creaming and increased oxidative stability in products.
Section snippets
Materials
Glycerol monooleate (Radiamuls MG 2905K) (containing at least 90% monoglyceride content) was a gift from Oleon (Klang, Malaysia). Sodium caseinate and whey protein concentrate 80% (WPC) were provided by Tatua Co-operative Dairy Company Ltd. (Morrinsville, New Zealand). Refined canola oil and caster sugar were purchased from the local supermarket. Sodium azide was purchased from Sigma-Aldrich (St. Louis, MO, USA).
Preparation of model emulsions
Five model emulsions with different milk protein compositions (5 different ratios
Droplet size and polydispersity index of model emulsions
Table 2a shows the droplet size of model emulsions prepared with different milk protein compositions over 28 days of ageing at 45 °C. Droplet sizes of fresh emulsions prepared with different milk protein compositions were significantly different (p < .05). Fresh emulsions with only sodium caseinate (Cas-100) had a significantly smaller droplet size (176.1 nm) than the fresh emulsions with only WPC (Cas-0; 191.3 nm). This effect is due to the smaller colloidal structure of sodium caseinate,
Conclusions
This study evaluated the effect of different milk protein compositions on physicochemical properties, emulsion physical stability and oxidative stability of protein-stabilised emulsions. Emulsions with sodium caseinate showed a smaller droplet size, higher zeta potential and higher viscosity, which corresponded to an increase in sodium caseinate content. All the emulsions had good emulsion stability against creaming and different milk protein compositions did not affect creaming stability
Acknowledgements
C.C.L. would like to acknowledge University of Otago Doctoral Scholarship towards his PhD study.
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2023, Food ChemistryCitation Excerpt :As widely reported in literature, the smaller the droplet size, the larger the oil surface area available for interaction with oxygen and pro-oxidant compounds, and thus, the higher the rate of oxidation. Indeed, Loi and co-authors (2019) reported the highest volatile development for o/w emulsions with the smallest droplets size formulated with 100 % of sodium caseinate. The last PLS1 was carried out using NaCl as response variable and VOCs as explanatory variables.