Abstract
Milk coffee beverages are a protein-stabilised emulsion and consist of dispersed oil droplets in the aqueous phase. Casein, whey proteins and mono- and diglycerides (MDGs) are surface-active agents that adsorb at the oil-water interface to form and stabilise emulsions. Protein is an important natural emulsifying agent in emulsion systems and interact with MDGs at the oil-water interface to modify oil droplet size, zeta potential, flow behaviour, physical stability and oxidative stability. Many studies have investigated the interaction between milk protein and MDG and their effect on droplet properties, flow behaviour and food structure in whippable emulsions. However, there is very little knowledge around the interaction of MDG and milk protein in beverage emulsions. MDG is an oil-soluble emulsifier and has low solubility in water, making it not suitable for direct application in many food formulations. Encapsulation of MDG could be a potential solution to convert it into a stable water-dispersible powder; however, there is very little information published about the encapsulation of emulsifiers as a functional ingredient.
This project had two main objectives. The first objective aimed to gain fundamental understandings on the effect of MDG and milk protein compositions in a protein-stabilised oil-in-water emulsion system. The second objective investigated the preparation and characterisation of an encapsulated emulsifier system, including its physicochemical properties, stability after reconstitution and functionality in model emulsions. Five experimental studies were carried out to meet these objectives.
A model emulsion prepared by microfluidisation to mimic the characteristics of a beverage emulsion was used to investigate the effect of the composition of mono- and diglycerides (unsaturation of fatty acids; monoglyceride content) and co-emulsifier (sodium stearate) content on the physical properties and stability against creaming in protein-stabilised emulsions. The experimental results demonstrated that emulsions with 0.2% MDGs produced 15-30% smaller oil droplets and 17-27% lower polydispersity indices compared to the control (no MDG). Sodium stearate (6% w/w of MDG) increased the negativity of zeta potential by 12.6-17.3 mV in emulsions containing saturated MDGs and 1.8-5.0 mV in unsaturated MDGs. Unsaturated MDGs showed better creaming stability than the control after 28 days of ageing with no improvement observed for saturated MDGs. Glycerol monooleate (GMO) demonstrated the best creaming stability among the unsaturated MDGs.
The next study investigated the effect of milk protein compositions (different ratio of sodium caseinate to whey protein concentrate) on physicochemical properties, creaming stability and oxidative stability of protein-stabilised emulsions containing GMO. The experimental results showed that the emulsion with only sodium caseinate produced smaller droplets (174.7 nm), a more negative zeta potential (-50.8 mV) and a more viscous emulsion (1.89 mPa s) compared to the emulsion with only WPC (191.4 nm; 38.8 mV; 1.65 mPa s). Protein composition had no significant effect on creaming stability. Eleven volatile compounds were identified as lipid oxidation markers, and six compounds (2-pentylfuran, octanal, nonanal, 3-octen-2-one, 2,4-heptadienal, 3,5 octadien-2-one isomers) demonstrated that emulsions with mixed protein types (sodium caseinate and WPC) had better oxidative stability than emulsions with a single protein type, i.e. either caseinate or WPC alone.
GMO demonstrated excellent creaming stability in protein-stabilised emulsions but was unsuitable for direct aqueous applications due to its low solubility in water. In order to use GMO as a functional ingredient in beverage emulsions, the feasibility of spray-drying to encapsulate oil-soluble MDG using appropriate wall materials to produce a stable water-dispersible powder with good reconstitution properties and extended shelf life was assessed. In addition, the effect of emulsion formulation (GMO concentration (33.6%, 47.0%); dextrose equivalent (DE) values of maltodextrin (DE 10, 18)) on emulsion properties, powder properties and oxidative stability was investigated. Results showed that all homogenised emulsions were suitable for spray-drying due to their high emulsion stability against phase separation, monomodal droplet size distributions (150-180 nm) and low viscosity (20-65 mPa s). All instantised powders exhibited good dispersibility (65-90%) in water and greater oxidative stability than bulk GMO. The instantised powder with low GMO and maltodextrin DE 10 showed both good dispersibility and low lipid oxidation, demonstrating that spray-drying can successfully produce an instantised GMO powder with a longer shelf life for food applications.
The application of two selected instantised GMO powders (low GMO content with maltodextrin DE 10 or DE 18) were investigated in protein-stabilised emulsions in terms of physicochemical properties, creaming stability and oxidative stability. Model emulsions with bulk GMO, two instantised GMO powders and two controls (contain either maltodextrin DE 10 or DE 18 with no GMO) were prepared using microfluidisation. The emulsion physicochemical properties were characterised by droplet size, zeta potential, viscosity and creaming index, while the oxidative stability was assessed by the formation of volatile secondary lipid oxidation products during storage (28 days at 45 °C) using gas chromatography-mass spectrometry (GC-MS). Experimental results revealed that all three emulsions with GMO had smaller average droplet sizes (180.0 nm) and narrower size distribution (polydispersity index of 0.161) compared to the two controls (197.6 nm, 0.194). All emulsions with GMO also had greater creaming stability than the control emulsions. Principal component analysis of the volatiles revealed that storage time had the greatest influence on lipid oxidation. Three lipid oxidation markers, 3-octen-2-one, 2,4-heptadienal isomer 2 and 3,5 octadien 2-one isomer 1, showed that controls had the same oxidative stability as instantised GMO, but were more stable than bulk GMO. Therefore, GMO powders can form stable protein-stabilised emulsions with good physicochemical properties and oxidative stability.
The different concentrations of bulk GMO and instantised GMO powder (low GMO with maltodextrin DE 10) was evaluated in an application of a model coffee beverage emulsion in terms of physicochemical properties, creaming stability and volatile profile at different storage time. The increasing GMO level formed fresh coffee emulsions with smaller droplet sizes and narrower size distribution that resulted in greater emulsion stability against creaming compared to the control (no GMO). However, emulsions prepared with 0.1% and 0.2% GMO were not stable at pH near to the isoelectric point of casein during storage and resulted in the growth of droplet size. These emulsions at 28 days of storage showed the presence of flocculated oil droplets due to protein aggregates when visualised using an optical microscope. The visible sediment particles in the emulsions with 0.2% GMO were associated with protein aggregate induced flocculation and could be explained by having a zeta potential below the critical level for stability ( 30 mV). The emulsions with 0.03% GMO powder demonstrated greater creaming stability than the control emulsions and had stable droplet size, zeta potential and viscosity. The chemical stability was similar to the control emulsions demonstrated by the very similar volatile concentrations.
In conclusion, the results from this thesis provide new insights into the relationship between milk proteins and a MDG-based emulsifier system and their effects on emulsion properties and volatile profile in model protein-stabilised and coffee emulsions. The knowledge from this study is useful to formulate a ready-to-drink coffee beverage with the desired emulsion properties and shelf stability. This study also presents an innovative application of spray-drying to design an emulsifier system that is not only in the right format for beverage emulsions but achieves the same functionality in the products at a lower application rate. Further development may be required to determine optimum dose rates for various food and beverage applications; but the instantised GMO powder will give better control over the dosage to ensure the desired functionality is obtained.