Plant-based milk powder is increasingly finding its way into food production. The problem so far: the powder tends to form lumps when it is liquefied again.

Soy, oat or rice milk: the relevance of plant-based milk alternatives has increased significantly in recent years. Due to health, ecological or animal welfare aspects, an increasing number of consumers are turning to plant-based alternatives to conventional cow's milk. Soy milk, for example, produces 3.2 times less CO₂ emissions and uses over 20 times less water than cow's milk. In addition, plant-based alternatives can be enriched with nutrients and secondary plant substances known as dietary fiber. They have a positive effect on health.

In the food industry, there is also a trend towards plant-based milk in powder form becoming increasingly important compared to the natural liquid product. "The powder offers several advantages: Not only does it have better physical and chemical stability, it is easier to weigh, more efficient to transport and requires less storage volume than liquid milk," explains Kathrin Kramm, PhD student at the Institute of Solids Process Engineering and Particle Technology. In the plant-based milk alternatives, animal fats are replaced by vegetable oil: this ensures the formation of foam when preparing latte, for example. Another advantage of the plant-based powder alternative is that the oil has a significantly longer shelf life and does not become rancid, and no bacteria accumulate because it is encapsulated in the particles and thus protected. The plant-based powders can also be integrated into numerous product systems, such as sweets, instant products like cocoa powder or even infant formula. Kramm explains: "In this way, many finished products could be produced as a vegan alternative, and more cheaply and ecologically".

Tendency to form lumps

Until now, however, there has been one difficulty in production: compared to conventional milk powders, the plant-based alternatives pose challenges that are generally attributable to the plant-based ingredients . "If the powders are to be re-dissolved, they tend to form lumps and thus lead to incomplete rehydration of the powders, which is not acceptable for the product application," says scientist Kramm. The fundamental problem is that plant proteins are generally more hydrophobic, i.e. more water-repellent, than animal proteins. The process engineer explains how she tackles this technically: "The aim is to turn the powder back into a homogeneous liquid. To optimize dissolution, the plant-based solid particles must therefore be structured in a targeted manner. This is done by forming agglomerates. During agglomeration, primary particles are combined to form larger, porous secondary particles that have a blackberry-like particle structure. In other words, more pores are created by "gluing" many spheres together to form a structure rather than individual spheres. Much more water can penetrate this blackberry-like structure and the powder dissolves more easily. "In addition to optimizing the dissolution behavior, this structure enables improved flowability of the product and a defined stability," says Kramm.

The research project was carried out with the industrial Swiss cooperation partner Nestlé S.A.. In the laboratory, specific work was carried out to produce particles with optimized dissolution properties. "The process used for this  applied to tablets, for example, so that they only dissolve in the stomach and not in the mouth," explains Kathrin Kramm. To this end, the scientist investigated the influence of the material on the formation, structure and characteristics of the agglomerated powder. The focus was placed on the plant protein and fiber components in the complex formulation of the plant-based milk alternatives, as no literature data is available to date. To date, there are no satisfactory studies and results for the improvement of the various production processes such as powdered milk alternative productsemulsions, spray-dried alternatives in which the individual particles are produced and fluidized bed agglomeration (blackberry structure).

The perfect resolution

As the individual process steps influence the product in addition to the material properties, Kramm has analyzed these in detail and gained insights for a targeted particle design was generated. In order to understand the structuring as a function of the surface composition of the particles at the micro level, numerical methods were also used to model and analyze particle binding. The results obtained were transferred to the macro mechanism in the fluidized bed and various product structures were generated by setting defined process parameters. Through further investigations, a correlation between the particle structure, particle size and the dissolution behavior of the powders could be generated. With the help of this "formula", production in the laboratory can be transferred to an industrial scale - regardless of the production plant used. Process engineer Kramm summarizes the results as follows: "The project makes it possible to implement the results on more complex product systems of plant-based milk powder alternatives in the food industry, which are characterized by an efficient particle design with adjustments in the manufacturing process

Further information:

www.tuhh.de/spe/research