Zeta potential (also written as ζ-potential and sometimes referred to as electrokinetic potential) is a valuable technique that can be used to predict possible instability of an emulsion or a suspension. This case study outlines some work that we have undertaken comparing the zeta potential and particle sizes present within plant-based and animal-based milks. We found that plant-based milks tend to have a larger relative zeta potential compared to the animal-based milks, so are arguably more stable. The animal-based milks, soya milk and oat milk had similar particle sizes but almond milk was an outlier and the particle sizes were considerably larger than those in the other milks tested.
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Milk is a naturally occurring emulsion where fat droplets are dispersed within a continuous aqueous phase. In order to prepare a stable emulsion and ensure that the oil and aqueous phases maintain homogeneity there also need to be surfactants (also called emulsifiers) to stabilise the system. Within animal-derived milks these include lipids, whey proteins and phosphoproteins, such as casein.
The recent explosion of plant-based foods as a substitute for animal-derived products has led to a range of plant-based milks including almond, oat, soya, rice and coconut. These milks are sometimes naturally occurring, e.g. coconut milk, or may need to be prepared, in the case of soy milk. Preparation of plant-based milks from beans or nuts usually includes cleaning and dehulling then grinding to make a paste. Any enzymes that may affect the flavour or lead to unfavourable oxidation are denatured by heating and any sedimented solids are removed by filtration. Finally, flavours or sugars are added if required and pasteurisation of homogenisation is undertaken. Sometimes emulsifiers need to be added to the formulation to ensure stability and that separation does not occur before the milk reaches the customer.
Zeta potential an interfacial property
A zeta potential is established whenever a liquid comes into contact with a surface (e.g. liquid–liquid or solid–liquid), therefore zeta potential is classified as an interfacial property. Within an emulsion, droplets of one phase are distributed within another phase. Emulsions are generally oil-in-water or water-in-oil, with the latter phase dictating the continuous medium. The suspended particles may have a surface charge that will have an impact upon the surrounding liquid medium. This surface charge leads to a reorganisation of ions that are nearby; oppositely-charged ions are attracted and particles of the same charge are repelled.
With increasing distance from the surface of the particle the influence of the charged surface on the ion reorganisation decreases. This therefore leads to a gradient, or change in potential, over distance. The layer of ions directly interacting with the surface is called the Stern layer. These ions are bonded fairly strongly to the surface of the particle and have opposite charge to the particle surface. Outside this layer there is a second layer of more diffuse ions; the outer limit of this layer is known as the slipping plane. This is the point at which the central particle no longer affects the ions in the surrounding medium. The inner shell of charge (Stern layer) and the outer ionic atmosphere (diffuse ion layer) is known as the electrical double layer.
During zeta potential testing the sample is loaded into a cell with conductive points. A charge is applied to the sample and a laser monitors how fast the particles move when they are charged. It is important that the sample is permeable to light, so sometimes samples need to be diluted. The faster the particles move, the higher the value of zeta potential. Zeta potential can be either negative or positive and can range from –200 to 200 mV, depending upon the electrochemical behaviour of the particle interface. Generally speaking, a large value (i.e. over 30 mV or below –30 mV) indicates that a sample is stable and unlikely to aggregate or coalesce. In addition, a large value indicates that the surfaces of particles are highly charged and repel each other. An Anton-Paar Litesizer 500 was used to measure zeta potential and particle size differences between the following own-brand supermarket milks:
- Full fat cow milk
- Semi-skimmed cow milk
- Skimmed cow milk
- UHT cow milk
- Almond milk (unsweetened)
- Oat milk (unsweetened)
- Soy milk (unsweetened)
All samples were diluted with deionised water to ensure data collection was possible. The samples were not filtered prior to the data being collected.
The differences in zeta potential were collected and plotted, Figure 1. The animal-derived milks were all fairly similar in zeta potential value. Of these samples, the full fat milk had the largest magnitude of zeta potential and the skimmed milk the smallest. The UHT milk used was semi-skimmed, and the data correlate to the semi-skimmed cow milk. The results from the cow milk suggest that the higher the fat content, the higher the zeta potential and the more stable the emulsion.
Considering the plant milks, all samples had a zeta potential higher than those for the cow milk samples, therefore the plant milks are likely to be more stable suspensions. Oat milk had the largest magnitude zeta potential and almond milk the smallest, suggesting that oat milk formed the most stable colloidal system. The reason for this could be that the plant-based milks tend to contain emulsifiers and stabilisers, which are likely added during processing to ensure stable emulsions are formed. For example, the oat milk and soya milk both contained gellan gum and the almond milk contained gellan gum and locust bean gum.
Comparison of the hydrodynamic diameter between plant and animal derived milks
Particle size data was also collected for the samples. Using the Anton-Paar Lightsizer 500, particles ranging from 0.3 nm to 10 μm can be measured. It should be noted that both full fat cow milk and almond milk had a bimodal distribution of particle diameters, and only the dominant hydrodynamic diameter is provided here. Considering the animal-based milks, the average hydrodynamic diameter correlated to decreasing fat content; the full fat milk had the largest hydrodynamic diameter and the skimmed milk the smallest. The semi-skimmed UHT correlated well with the fresh semi-skimmed milk, Figure 2. Looking at the plant-based milks, the almond milk had a considerably larger hydrodynamic diameter compared to the other samples investigated, with an average hydrodynamic diameter of 1335 nm. Oat milk was next largest, and soya milk was the smallest with a hydrodynamic diameter similar to that of the cow-derived milks.
In summary, we have been able to compare the zeta potential values and hydrodynamic diameters of animal-based milks with plant-based milks. The zeta potential values show that the plant-based milks tend to form more stable emulsions than the animal-based milks. The reason for this could be that the plant-based milks tend to contain emulsifiers and stabilisers, which are likely added during processing to ensure stable emulsions are formed. For example, the oat milk and soya milk both contained gellan gum and the almond milk contained gellan gum and locust bean gum.
The hydrodynamic diameters measure also showed interesting similarities and differences between the plant-based and animal-derived milks. For the most part, the samples all had a similar hydrodynamic diameter. The outlier was almond milk, which had significantly larger particle sizes than the other milks. The reason for this may be that the almond milk contains two stabilisers, whereas the other samples tested only contained one.
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