Functions of Milk: Do plant-based varieties stack up?


Plant-based products are all the rage right now – from protein beverages to meat analogs – and milk is no exception. However, when we think of milk coming from a nut, does it really make it… milk?

Almond milk by raw pixel via Pexels

Since my body reacts poorly to dairy, I’m no stranger to the likes of almond milk. But as much as I try to use it the same way I would with milk in my cereal and in baking, it never amounts to quite the same experience. To understand why these milk substitutes aren’t truly identical, let’s dissect the fundamental components that constitute milk and explore its functions.

What is milk made up of?
Milk is an emulsion, specifically an oil-in-water dispersion. Milks from different animals differ in their compositional ratios but cow’s milk is approximately 87% water, 4.8% lactose, 4% fat, 3.5% protein and 0.29% inorganic salts, primarily calcium and phosphate (1).

Milk fat contains over 400 fatty acids and are found mostly in the form of triglycerides. These triglycerides exist as globules surrounded by other polar phospholipids and proteins to keep them suspended in the water phase since triglycerides themselves are non-polar. With 3 fatty acids on a glycerol backbone, there can be many possible arrangements of fatty acids (400^3) which produces a multitude of triglyceride species (if it really only is 400, that’s 64, 000, 000) making milk fat highly complex (2).

Milk proteins are comprised of relatively small molecules, called caseins, which account for 80% of the total protein, while the remaining 20% are globular whey proteins (1, 3). Casein micelles scatter light which gives milk its opacity and white color (1).

Functions of Milk
Milk is a staple ingredient in many of our favorite food products like cheese, ice cream, chocolate and baked goods, as each of its components lend to a different function.

Liquid is often added in any recipe for baked goods for a couple of reasons. They help suspend the particles of different ingredients for interaction, and provide hydration of starch granules for gelatinization.

Proteins have multiple functions. Upon heat denaturation, the re-association of protein allows for a gel matrix to form, which provides the structure in baked goods. They also act as a stabilizer for foams and emulsions as they are surface-active, reducing surface tension of two phases. When you think of mousses or anything whipped and airy, sodium caseinate, a protein of milk, lends body and texture, and improves its whipping properties (4). And in cheese making, caseins and whey proteins are essential for coagulation, a very important step to get your cheese (1)!

Fats determine the mouthfeel and texture of food depending on its melting and crystallization behavior. Melting points affect the plasticity, while crystallization affects the type of crystals that form – whether it is fine or coarse (5). Milk fat crystallizes in the beta-prime (ß’) form which is uniform and needle-like, lending to a smooth texture. Milk fat is predominantly solid but not entirely, between -5ºC and 5ºC, which makes it the ideal fat for making ice cream (1).

Like all other fats too, milk fat largely contributes to flavor in food applications. In this case, the characteristic flavor of milk is attributed to the volatile lower- and medium-chain fatty acids – C4:0, C6:0, C8:0 and C10:0 (6). The complex nature of milk’s fatty acid profile is also responsible for its unique flavor.

While lactose may provide some sweetness, it is a relatively low sweetness sugar with low solubility, making it the least valuable constituent in milk (7). However, it is an essential carbon source for lactic acid bacteria during fermentation in cheesemaking (3).

By Mike via Pexel

Non-dairy milks are the aqueous extracts of plant sources, for example, nuts, legumes, grains or cereals. They are obtained by soaking the plant material in water, blending it – forming a slurry, and then straining it to remove the solid particles. From this process, they are technically not “milks” by definition as they don’t derive from mammary glands.

In today’s market, soy milk is the most common milk substitute and has received the most amount of research, but almond milk has quickly caught up and become popular among consumers (8).

How do milk alternatives fare against cow’s milk?
It probably comes as no surprise that soy milk is the most common, because of the alternative milks, soy milk is reported to be most similar in physicochemical properties to cow’s milk. In a study by Mäkinen and colleagues, the two milks exhibited similarities in flow index, initial pH and volume mean particle diameters. Furthermore, transmission electron microscopy (TEM) images show both their particle sizes are small while oat, quinoa and rice milk had larger aggregates (9).

Soy milk is also closest in protein content (2.95%) to cow’s milk. Other plant milks varied greatly, with almond milk being higher than cow’s at 4.3% and rice milk having as low as 0.07% protein. Lower protein can lead to a weaker gel formation, which poses a problem for structure stability. Despite soymilk having a similar protein content to cow’s milk, its gel strength was still found to be weaker, likely due to different protein structure and composition (9). Jeske et al. also cited that the difference in functional properties is due to a larger molecular size and complex quaternary structure (10).

Additionally, while casein and whey are the main proteins in dairy milk, other proteins are present in plant sources. For example, glycinin and ß-conglycinin make up the bulk of soybean protein, and amandin for almonds. Since casein is necessary for cheesemaking, the absence of casein in plant-based milk alternatives will affect the texture of dairy-free cheese analogs. The textural differences found when incorporating more vegetable protein than casein were lower elasticity, a sticky consistency and poor flavor (10). However, analog cheeses in the form of spreads are a way to avoid these textural problems and are reported to come close to dairy cheese.

To put it simply, although these plant-based milk alternatives share enough similar properties with dairy milk to be considered a substitute, every plant source is going to have a different compositional make-up in terms of ratios and types (of carbohydrates, proteins, fats and minerals). This is what causes even the slightest variation in taste, texture and functionality. For example, although soy milk is attributed to be the closest to cow’s milk, what might seem like a small variation on the molecular level will still produce a difference in applications. However, processing methods and additives such as gum thickeners are ways to go around this and improve these properties.

Replacing milk in any recipe may not produce the exact same results; nonetheless, they come close. We should also note that they may work better in certain applications than others, for example, the cheese spread instead of hard cheeses, or denser cakes that don’t require a stronger structure or where a nuttier flavor is sought after.

Final verdict
Distinctions in taste and texture don’t necessarily mean they are bad distinctions. In fact, many of them have high overall acceptability. I mean, there’s a reason why almond milk is becoming so popular! In fact, I prefer the taste of almond milk yogurt over dairy yogurt.

And for those of us who are lactose intolerant or health concerned, plant “milks” are definitely our best bet! They are nutritionally dense with much lower calories. With plenty of novel options emerging – from pistachio milk to walnut milk (my personal favorite) – you’re bound to find one that matches your preferences!

1. Clarke C. “Ice Cream Ingredients”. Science of Ice Cream (2nd Edition), by Clarke C. Royal Society of Chemistry. 2012.
2. Metin S, and Hartel RW. “Milk Fat and Cocoa Butter.” Cocoa Butter and Related Compounds, by Garti N and Widlak NR, AOCS Press, 2012.
3. Kindstedt PS. “The Basics of Cheesemaking.” Cheese and Microbes, by Catherine DW. American Society for Microbiology (ASM). 2014.
4. Ennis MP and Mulvihill DM. “Milk Proteins.” Handbook of Hydrocolloids by Phillips GO and Williams PA. Woodhead Publishing. 2000.
5. Scheeder MRL. “Lipids from land animals”. Modifying Lipids for Use in Food by Gunstone FD. Woodhead Publishing. 2006.
6. Kilara A. “Low fat ice cream”. Food Texture and Design Optimization by Yadunandan Lal D and Joseph ML. John Wiley and Sons. 2014.
7. O’Mahony JA and Fox PF. “Milk: An Overview.” Milk Proteins – From Expression to Food (2nd Edition) by Singh H et al. Elsevier. 2014.
8. Alozie YE and Udofia US. 2015. Nutritional and Sensory Properties of Almond (Prunus amygdalu Var. Dulcis) Seed Milk. World J. Dairy & Food Sci., 10 (2): 117-121. DOI: 10.5829/idosi.wjdfs.2015.10.2.9622
9. Mäkinen OE. et al. 2015. Physicochemical And Acid Gelation Properties Of Commercial UHT-Treated Plant-Based Milk Substitutes And Lactose Free Bovine Milk. Food Chemistry, 168: 630-638. doi:10.1016/j.foodchem.2014.07.036.
10. Jeske S et al. 2018. Past, Present And Future: The Strength Of Plant-Based Dairy Substitutes Based On Gluten-Free Raw Materials. Food Research International, 110: 42-51. doi:10.1016/j.foodres.2017.03.045

Science Meets Food

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