Drying and evaporation technology for plant-based products

2022-10-08 14:27:42 By : Mr. Wang Mike

Dry beans, lentils and peas are becoming increasingly popular in Western diets and are a well-established staple in many Eastern cultures.

As a group, pulses are one of the most nutritionally complete foods, inexpensive and widely available. High in protein, fibre and carbohydrates and low in fat, soybeans, mung beans, chickpeas, lupin, lentils and peas have also become a popular substitute for dairy and meat.

The trend towards vegan, vegetarian and flexitarian diets has created rapid growth in the plant-based industry with hundreds of new products entering the market.

Historically, ingredient manufacturers with expertise in grain fractionation focused on starch as a primary interest for its many food and industrial applications. Proteins and fibre were less important revenue streams and mostly designated for animal feed. Seizing on changing dietary habits, these manufacturers as well as new investors are now focusing their attention on valuable proteins from peas and other pulses to bolster profitability.

Most plant startups are looking at processing 35,000–70,000 tons of raw crop grind per year. These plants are interested in capitalising on the high-growth market sector of plant-based meat substitutes, producing high-value protein to be processed into texturised plant protein (TPP) — also called texturised vegetable protein (TVP) — where it is extruded as a meat substitute.

Meat substitutes are manufactured to have a range of 70–75% protein, and extruded or formed into finished plant-based meat products. Extrusion processes can be used to define the texture and shape of the products, then moulded into forms such as crumbles, strips, patties and sausages. Essentially, the cost per metric ton of this high-value protein, as sold to extruders, establishes the benchmark to determine the market value of the commodity.

But protein, being the main focus, accounts for only 20 to 25% of the entire pulse structure. The remaining 75 to 80% of the pulse ends up as co-products — starch, fibre and solubles. The challenges these plants face is identifying and evaluating the most optimum processes for separating the protein, starch, fibre and solubles, and how to use these products for maximum profitability.

Ingredient processing is almost always scale-dependent, relying on high volumes to make up for low margins. Even as demand for new protein flours, isolates and concentrates grows rapidly, the cost and complexity of building processing facilities can make ingredient production capacity slow to materialise. One of the highest priorities for plant-based protein manufacturing is identifying processing methods that offer high output, functionality benefits and efficiency.

The preparation of pulses for high-purity separation of proteins, fibre, starches and solubles relies on a wet fractionation process that incorporates drying and evaporation for final processing. These fractionation processes encompass:

The protein extraction process separates the pulse into protein, fibre, starch and solubles. After initial separation, the protein fraction is modified to yield specific properties and end-use characteristics. The most widely used process is an active precipitation method, made by adjusting the pH of the pulses, to promote extraction of the protein fraction.

Then, through functional protein modification enzymes are used to yield the protein properties desired by manufacturers, such as a nutritional or functional property, as in emulsification, gelation or solubility. The resulting protein stream can be dried to powder form in a spray dryer to yield an 80–95% protein isolate.

The fibre and starch fraction can be kept as a composite stream and dried in a ring dryer — or further separated into constituent fibre and starch products. As a high-value dietary fibre for human nutrition or other food ingredient, the fibre can be dried in a ring dryer.

The starch fraction can be dried in a flash dryer, and subsequently used in baked goods, convenience food and other food or industrial purposes.

The water discharge stream from the protein purification step contains additional nutrients that can be recovered as value-added product. Many recovery processes can be considered, but the simple implementation of an evaporator enables the concentration of a nutrient-rich liquid product while improving the environmental impact of the wastewater stream. This concentrated product can be used as a liquid fertiliser or blended with fibre as an animal feed. Additionally, the condensate stream with reduced organics concentration can be recovered as a heat source or further treated for process water recovery to reduce freshwater consumption.

As developments in the plant-based protein market evolve, process manufacturers continue to innovate new solutions to meet specialised needs.

“As a critical process in the manufacture of many powdered ingredients, such as plant-based proteins, spray drying performs a vital product drying function that must balance food safety, product quality, functionality and sustainability,” said Greg See Hoye, Market Manager at Dedert — a US company that has specialised in the design of industrial evaporation and drying systems, and partnered with ingredients manufacturers to develop custom-engineered process technologies.

Passing through an atomiser, water droplets are evaporated upon contact with hot air to release the protein as a powder which falls to the bottom of the spray dryer. The dry protein powder is pneumatically conveyed to product-collection cyclones, then discharged to a protein powder conveying line for storage in silos or packaging in bags or totes.

“Spray dryers have mostly been applied with nozzle atomisation for plant-based proteins due to manufacturers’ familiarity with dairy protein applications, specifying narrow particle size distribution ranges,” See Hoye said. “Nozzle atomisation requires the use of a high-pressure pump which has appreciable feed holding capacity and, therefore, may have substantial maintenance and sanitation considerations. Depending on the application, nozzle atomisation may be a requirement, but alternatives such as rotary atomisation could offer benefits.”

Spray dryers for plant-based proteins are required to meet high food-grade standards according to FDA guidelines, but higher dairy-level standards such as 3-A Sanitary Standards or recommended design guidelines from the European Hygienic Engineering & Design Group (EHEDG) are not yet the norm. Dedert already manufactures spray dryers that meet stringent EHEDG hygienic design standards, which could be implemented according to user requirements. These systems incorporate a new design of removable-panel, air-gap insulation, with hinged outer-cladding doors for easy inspection access — which can be opened and closed in a fraction of the time compared to bolt-on panels. As the name suggests, there is no fibreglass or mineral wool — instead air is used for insulation between the inner and outer skin of the vessel.

Spray dryer removable air gap insulation panels.

Flash dryers are used on starch because of their thermal sensitivity. Exposing starch to high temperature can modify the structural and chemical properties, causing denaturation, gelatinisation or other functional changes. Flash drying has a low residence time, low humidity and low temperature profile, suitable for starch applications.

Flash dryers are pneumatic systems combining simultaneous air conveying and drying, where the starch solids are introduced at the feed point and dry in a single-pass arrangement, before full discharge to a product collection system.

With a short residence time of only a few seconds, the starch solids are in contact with a relatively low heat environment for a short period. As a result, flash dryers produce high-quality final starch products with uniform moisture content throughout and consistent particle size distribution.

“The flash dryer, in open-circuit configuration, operates at the lowest temperature for a given application, ensuring uniform drying and gentle treatment of the starch material,” See Hoye said. “Using ambient fresh air, the starch always remains in contact with a clean, unadulterated low-humidity drying medium to prevent risk of contamination or changes to the product’s desired state.”

The ring drying process is an extension to flash drying, and is applicable when fibre and starch require more extended drying times compared to one-pass flash drying.

Ring drying provides a significant improvement in efficiency by incorporating a manifold classifier to centrifugally return semi-dried and oversized material back to the drying system for additional drying. Internal recirculation of semi-dry solids until complete drying permits the ring dryer to operate at lower temperature compared to a flash dryer for an otherwise identical application. Nonetheless, the ring dryer maintains short residence time of only a few seconds, thereby keeping short exposure of solids to operating temperatures.

“The versatility of the ring dryer design accommodates a variety of product grades and characteristics. The manifold is critical in ensuring efficient temperature operation,” See Hoye said.

Three configurations of ring dryers are available depending on the application:

“Each of the ring dryer types can have either open-circuit (OC) or partial gas recycle (PGR) configurations to suit the product application or operational requirements. Under OC, ambient air is used as a one-pass drying medium. Under PGR, additional energy efficiency is achieved by recirculating the majority of the dryer exhaust stream back to the air heater as a preheated drying medium.”

“Recovering the nutrients in the liquid remaining from the protein purification step can be accomplished by evaporation,” See Hoye said. “In plant-based protein applications, Dedert’s falling film evaporator design offers a simple operational concept.”

Falling film evaporation — the fluid creates a thin film along the tube walls, progressing downwards (falling) by gravity to the bottom of the evaporator while water simultaneously evaporates to the tubular vapour space.

Mechanical vapour recompression (MVR) — with the addition of a mechanical compressor or fan evaporated process vapours are compressed to a higher pressure to serve as the driving steam for evaporation, minimising steam consumption.

Thermal vapour recompression (TVR) — is more energy efficient than a steam-heated system when medium- to high-pressure steam is available. The motive steam enters through the steam compressor and draws in evaporated vapour. The vapour mixes in the converging section, and the pressure is boosted to serve as the driving steam for evaporation, minimising steam consumption.

Wet fractionation for the extraction of plant protein, starch and fibre relies on solvents and intensive drying. These processes are energy- and water-intensive.

As drying is the most energy-intensive process of the processes involved in wet fractionation, a decrease in water that needs to be dried reduces the energy requirements of the whole process considerably. Process innovations for sustainability and improved energy efficiency are currently under development by Dedert, together with other like-minded process partners.

From an energy perspective, thermal drying naturally carries a cost related to fuel requirements or other heating sources due to the latent heat of vaporisation of water of about 970 BTU/lb (2256 kJ/kg). Process efficiency can be gained by shifting moisture reduction to other processes. Generally, where possible, mechanical dewatering should be implemented first, followed by concentration, then thermal drying to maximise the integrated process efficiency. This arrangement also opens the opportunity to consider alternate drying technologies with improved cost and energy efficiency for the unit operation. Success of this arrangement is dependent on a holistic approach to process design, requiring a collaborative partnership between process suppliers to ensure a seamlessly integrated process solution.

In addition to energy efficiency, freshwater consumption is another area of innovative design investigation. Process integrations between the upstream water users and downstream evaporation and drying technologies could result in water recovery for use as process water or other improved sustainability benefits.

Top image credit: iStock.com/baibaz

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