Factors Affecting Product Quality in TVP and HMMA Processing

There are many different challenges to plant-based protein processing. Some protein powders like pea and lupine-based ingredients are notorious for their difficult flow characteristics. Static electricity or high fat content can cause powders to stick together and cling to the surface of the conveying lines and feeders. In addition, the thermo-mechanical extrusion process of plant-based proteins depends on various process parameters. Moisture, pressure, temperature, and shear forces all contribute the quality of the desired end product.

In addition to process know-how for the entire process, the challenges of plant-based protein production can be divided into four main factors along the extrusion process: recipe, process moisture, energy input and discharge.

Of the four, the recipe is the most influential. With the appropriate recipe, including the quality of raw materials, it is possible to optimize the extrusion process for high-quality plant-based protein products. However, it is impossible to produce high-quality products without the right recipe, no matter how much you adjust the other factors. It is also vital to note that changing any one of the factors affecting quality also influences the other factors, making quality control a challenging process.

In this article, we show which factors should be given special attention in the production of plant-based proteins. Because their manufacturing process and the parameters responsible for their quality differ, we consider TVP (Texturized Vegetable Proteins) and HMMA (High Moisture Meat Analogues) separately.

While the ZGF centric food pelletizer is attached to the ZSK food extruder’s process section for manufacturing TVP in order to cut the product directly at the nozzle plate, HMMA is discharged using a specialized cooling die which produces a product strand exhibiting a texture closely resembling that of genuine meat.

Factors affecting TVP quality
Several characteristics of the product define TVP quality. Fiber texture, length, and strength should all be as similar as possible to the meat equivalent. Bulk density, shape, and extrudate surface are also important features.

The properties of the protein source will impact the achievable quality of TVP. Protein content should be between 50 and 80 percent. Higher protein content can create a gummy product.
Protein powders also have a limited capacity to bind with water. Adding too much moisture to the extrusion process causes a lighter color, a softer and mushy texture, and less visible fibers. This happens because the mixture has a lower viscosity. The temperature and pressure in the extruder also decrease due to the extra water. However, the correct amount of moisture gives a firm texture and visible fibers.

Mechanical energy comes from screw speed and screw configuration. Changing these parameters alters the energy input into the process. Higher energy input means the product gets warmer and expands. But too much heat can also brown the product, which affects its appearance. If the energy input is too low, there is no expansion in the product, and the TVP texture is poor.

The TVP product exits the extruder through a die plate which aligns the product fibers and through a centric pelletizer which cuts the melt strands into granules. The die plate orients the fibers and creates a pressure drop that contributes to expansion and increased porosity of the product.

Fiber length and strength are features of HMMA quality, as well as the texture of the product and the firmness of extrudate. Flavoring and colors are also quality parameters for HMMA as these products are used in ready-to-eat meals.

HMMA contains 50 to 70 percent water and 30 to 50 percent protein, with less than 10 percent fat content. Higher fat content causes mushy fibers with a low fiber strength. Process moisture is a critical parameter for HMMA quality, which is determined by water addition, raw material moisture content, and water binding. If the protein source does not bind well with water, adding starch or fibers to the recipe could help to improve product quality. Too much water gives the product a brittle texture and lighter color.

Mechanical energy for HMMA production comes from the screws and thermal energy from heating in extruder barrels. Screw speed and configuration are controllable variables that affect the fiber quality and texture. Thanks to the wide range of possible screw speeds of Coperion’s ZSK Food Extruders, raw materials with different functionalities can be processed. Depending on the properties of the ingredients, slow speeds (~300 min^-1) can create HMMA with a soft texture that is dough-like and brittle. Its fiber layers break easily. Medium speeds (~600 min^-1) can create products with a consistent texture and fiber layers that are easy to separate. High speeds (~1200 min^-1) can create a strong HMMA product texture with hard to separate layers. If the raw materials change, the screw speed can be adjusted to meet the desired product structure.

HMMA discharges through a cooling die, which defines the formation of fibers in the final product. Flowrate through the cooling die as well as temperature profile play a role in the product quality. HMMA products are not porous like TVP. They have a dense fibrous texture similar to their meat equivalents.

HMMA exits the cooling die as an endless strand. Post extrusion such as cutting allows a wide variety of forms and shapes.

Basically, in the production of TVP and HMMA, both the four individual factors and the seamless integration of all process steps into the overall process are crucial for product quality. With its comprehensive process know-how, Coperion implements complete systems for plant-based proteins from a single source, in which the extruder is first-class matched to the conveying system, feeders, pelletizer or cooling die. This is a major advantage compared to production plants that are assembled from aggregates from different suppliers.