Numerous plant-based products are on the market, developed using various plant sources, such as soy, pea, and other pulse sources, almond, hazelnut, cashew, coconut, oat, rice, hemp, quinoa, etc.
While animal milk is naturally rich in nutrients that support the growth of microbial cultures, plant extracts are not necessarily equivalent in this regard. Making tasty, cultured plant milk is much more complex, and formulae for cultured products must be specially designed.
Plant bases are formulated from various ingredients and contain different levels and types of protein, carbohydrate including sucrose and/or alternative sweeteners, fat, minerals, stabilizers, and other ingredients.
Formulation of the base
Plant bases for yogurt alternatives must be designed to contain an adequate amount of nutrients to allow optimal growth of the cultures. Unlike in cow’s milk, the taste and flavor of plant bases are mainly determined by the type of plant or nut, source, and quality of raw materials, any enzymatic digestion applied, and the extraction and purification process of the protein isolate to remove undesired taste and flavors.
The processes of plant base preparation can differ depending on the quality of the raw materials, which causes variability in performance regarding acidification time and the flavor of fermented products.
The dominant flavor of coconut, cereal, and nut bases comes from the source itself. However, some plant bases can have off-flavors such as beany, fishy, bitter, soapy, earthy, and unclean notes. Depending on the compounds that cause off-flavors, the culturing process can eliminate, mask, or enhance them.
The protein levels in plant bases are arbitrary and vary widely from 0.2-6%. The composition, structure, and functionality of the different plant proteins at low pH also differ significantly from what is seen with caseins and whey proteins. Therefore, it is to be expected that the texture of the cultured plant bases will be different. For example, a firm set at pH 4.6 or below is rarely seen in cultured plant bases compared to dairy yogurt.
Increasing the protein level can increase the viscosity, but unless plant bases are formulated with thickeners/stabilizers, they rarely have the appearance of the set and stirred dairy yogurt. Typical stabilizers are starch (modified corn starch, tapioca, rice), pectin, locust bean gum, guar gum, agar, and carrageenan .
The fat levels vary from 1-2% except for coconut base, which can contain higher fat levels (up to 20%). Fat does not significantly impact culture performance but can influence taste and texture. Texture and mouthfeel can be affected by the fat level, composition, and melting point of predominant fatty acids fractions.
Fermentable carbohydrates and sugar
Depending on the formulation and ingredients, plant extracts or ‘milks’ are formulated from pulse protein, cereals, and/or nuts may or may not contain sufficient amounts of fermentable carbohydrates to allow acidification to pH 4.60. Sucrose/cane sugar is the most common sugar used for sweetness, but not all species and strains that are components of the cultures can ferment it. All lactic acid bacteria metabolize glucose (dextrose), and it should be added if any additional fermentable sugar is needed.
The sources of glucose can be, for example, dextrose powder or agave syrup. The level of added glucose can vary from 0.5-2.0% and should be optimized. It depends on the overall composition of the plant base, the initial pH, the level of protein, and the minerals present and added.
To match the nutritional quality of dairy milk more closely, plant-based beverages (‘milks’) are usually supplemented with calcium (usually as CaCO3), up to the calcium level in milk or exceeding it by up to 50%. Cultured plant-based products can be successfully made without added calcium, but some may be required if a stabilizer such as pectin is used.
Calcium is added as CaCO3, Ca-phosphate, and Ca-citrate, but other food-grade calcium salts can be used as well. The salt and glucose levels should be optimized as some calcium salts may change the initial pH or increase the base buffering capacity, resulting in extended incubation time or prevention of the fermentation from reaching its target pH.
The recommended initial pH is 6.6-6.8, which is similar to ruminant milk. It can be lower, e.g. 6.0, but it should not be higher. It is based on the physiology and metabolism of lactic acid bacteria and yogurt cultures. In a rich nutrient base, they can start growing at a higher pH, but this will require more fermentable carbohydrates and more time to reach the target pH, e.g. 4.60 or lower (Figure 1).
Heat treatment of the base
Pasteurization and heat treatment of ruminant milk (92°C for 3-5 min) is designed to eliminate microflora of raw milk and denature whey protein to increase the texture of yogurt and increase the texture water-binding capacity. The temperature and time of the heat treatment of plant bases (UHT or HTST) should be designed to fulfill other functions, mainly to eliminate microflora present in the raw material. It should also correspond to the source of protein and other ingredients (stabilizers) used in the formula.
The cultures, inoculation, and incubation
Composition of the culture
The presence of both yogurt species St. thermophilus and Lb. delbrueckii ssp. bulgaricus in the culture is not mandatory, as plant-based products do not fall under the standard of identity of yogurt.
Taste and flavor
In dairy products, cultures have a crucial effect on texture, flavor, and aroma. In plant bases, the role of the cultures is to produce lactic acid and lower the pH. This changes the taste and flavor of the base.
In plant bases, the effect of culture on texture is different. The isoelectric point of plant proteins is different from milk proteins, and a fermented plant base will rarely form a firm acid gel at low pH.
Some strains can produce exopolysaccharides (EPS). It is well known that some EPS contribute to enhanced viscosity, creaminess, smoothness, and reduced syneresis in fermented dairy products. Different hypotheses have been formulated to explain their performance in dairy products. It is generally known that the role of the different milk proteins and their interactions between the different components in milk is crucial for the final textural properties. In dairy alternative yogurts, the EPS formed by the cultures can contribute to an overall product smoothness that might be sufficient for a cultured beverage in certain bases. Still, to match the texture of set and stirred yogurt, the plant base must be formulated with stabilizers.
The inoculated base is left undisturbed in cups or a fermentation vat until the target pH is reached.
There are no rules regarding the acidification rate and time to pH 4.60-4.55 or different target pH. It ranges from 3.5 – 14 hours. Fermentation time depends on the overall composition of the base, protein level, present, and added minerals, i.e. buffering capacity.
Figure 2 shows an example of acidification profiles in several plant beverages that are commercially available on the US market.
There is no legal requirement on minimal pH of cultured plant bases. It is typically designed to match dairy yogurt at pH 4.60 and lower. The decision should be made based on food safety, target taste, and flavor at the beginning of and throughout the shelf life.
Set-style cultured product: The cups are transferred from the incubation room to the cooler when the target pH is reached.
Stirred and drinking style: When the target pH is reached, the product is stirred and then typically pumped through the smoothing valve filter or backpressure valve to obtain a smooth appearance—higher mechanical shear results in a thinner texture.
Greek-style dairy alternative yogurt is formulated to have a high protein level or to have the texture of a dairy Greek-style yogurt. The process is the same as stirred yogurt.
The product is then poured into retail containers. The cooling profile and timing depend on the choice of stabilizers.
Fruit preparation and/or flavor may be added by in-line mixing before packaging.
Dairy vs. dairy alternative culture
Fermented plant-based dairy alternative yogurts cater to consumer segments that have serious allergies to dairy proteins or prefer vegan options without the use of milk ingredients. These consumers rely upon controlled production processes, which means that dairy protein allergens should be below specific food safety limits to ensure the safe consumption of fermented products. Therefore, Chr. Hansen’s culture for plant-based products are produced using non-dairy ingredients.
If there is no specific requirement from a customer or expectations from consumers, any yogurt culture can be used to ferment a plant base.
Source: Chr. Hansen