High Moisture Extrusion (HME)

High moisture extrusion (HME) — also termed high moisture extrusion cooking (HMEC) — is a continuous thermomechanical food processing technology that converts plant-based protein materials into fibrous, anisotropic structures resembling animal muscle tissue. It is one of the principal industrial methods for producing structured plant-based meat analogues and hybrid protein ingredients. The process is distinguished from conventional low moisture extrusion by the use of elevated water content (typically 40–80% wet basis) and by the inclusion of an elongated cooling die that allows fibrous texture formation under controlled pressure before product exit.

Principle of operation

Extruder

The core machine in HME is a co-rotating twin-screw extruder. Dry protein ingredients and water are metered continuously into the barrel, where they are transported, compressed, heated, and subjected to intense mechanical shear. Inside the barrel:

• Temperature reaches 110–150°C
• Pressure builds to 20–40 bar
• Residence time is typically at least 150 seconds
• Specific mechanical energy (SME) input ranges from approximately 557 to 1,136 kJ/kg, depending on recipe and screw configuration

At these conditions, native protein secondary structures (α-helices and globular conformations) are disrupted. Proteins unfold and subsequently re-fold into extended β-sheet assemblies, forming an entangled viscoelastic melt.

Cooling die

Immediately downstream of the extruder barrel is an elongated cooling die — the component that is mechanically distinctive to HME and does not exist in low moisture extrusion. In the cooling die:

• Temperature is reduced from approximately 110–150°C at the inlet to 20–80°C at the outlet
• The product remains under pressure throughout
• Laminar flow, shear forces, and the temperature gradient promote phase separation and alignment of protein aggregates

The combination of directional flow and controlled cooling causes proteins to self-organise into parallel fibres with anisotropic mechanical properties — that is, the product has greater tensile strength along the fibre axis than perpendicular to it. This anisotropy is the structural basis of the meat-like bite and texture of HME products.

Process parameters

Comparison with low moisture extrusion

Protein sources

HME can process a range of protein concentrates and isolates. The functional properties of each — solubility, water-binding capacity, viscosity, gelation behaviour, and thermoplastic behaviour under shear — determine the processing window and the characteristics of the resulting structure.

Blends — most commonly soy protein concentrate with wheat gluten — are widely used to tune texture and nutritional profile. Pea protein has gained traction as a soy alternative, particularly in combination with wheat gluten.

Structural mechanism

The formation of fibrous structure in HME involves several simultaneous processes:

1. Protein unfolding and denaturation in the hot barrel disrupts native tertiary structure
2. Intermolecular crosslinking (hydrogen bonds, disulfide bonds, hydrophobic interactions) begins in the barrel melt
3. Alignment and phase separation in the elongated cooling die organises protein aggregates into parallel lamellae and fibres under the directional velocity gradient
4. β-sheet formation locks the fibre geometry as the product cools and the structure sets

The degree of anisotropy — and consequently the resemblance to muscle fibre — is governed primarily by cooling die geometry (length, temperature profile), moisture content, and protein composition. Research groups at Wageningen University & Research, notably those of Atze Jan van der Goot and Remko Boom, have contributed substantially to the mechanistic understanding of this process.

Equipment and industrial scale

Bühler PolyCool system

Bühler Group (Switzerland/Germany) markets the PolyCool twin-screw extruder with elongated cooling die as its primary HME platform:

• R&D scale: approximately 10–50 kg/h
• Industrial scale: up to approximately 1,000 kg/h
• Integrated barrel temperature control, screw speed control, and cooling die temperature zoning

Coperion ZSK

Coperion (Germany/US) offers HME-capable twin-screw extruder configurations including the ZSK series:

• Bench-scale: up to approximately 80 kg/h
• Screw diameter options: 18, 26, 40, 58, 70 mm and larger

Industrial applications and commercial producers

HME is employed by a growing number of companies across multiple product categories:

Equipment manufacturers Bühler and Coperion occupy a central role in enabling scale-up, process control, and energy efficiency across these producers.

OJAH b.v. — Dutch commercial example

OJAH b.v. is a Dutch food technology company based in Ochten (Gelderland, Netherlands) and one of the most notable commercial applications of HME technology in Europe. OJAH was founded in 2009 by Frank Giezen, Jeroen Willemsen, and Wouter Jansen, with the specific goal of producing structured plant-based meat alternatives using high moisture extrusion at industrial scale.

OJAH developed the Plenti® brand of chicken-style and beef-style HME products, sold across 21 countries and certified to BRC Grade AA and Halal standards. The facility in Ochten processes approximately 1,600 tonnes per year (as of 2017) from a 10,000 m² production site employing over 150 staff.

In April 2018, OJAH was acquired by Kerry Group (Ireland) for approximately €20 million — a transaction that signalled the commercial maturity of HME-based plant protein production in Europe and validated the technology’s scalability.

OJAH was previously a portfolio company of Blue Ocean Xlerator (BOX), the Wageningen-based private incubator. Its acquisition by Kerry Group represents one of BOX’s notable exits.

Challenges

Flavour

Plant proteins — particularly soy and pea — can carry off-notes including:

• Beany or leguminous flavour (lipoxygenase-derived aldehydes)
• Bitter or astringent notes (saponins, phenolic compounds)
• Oxidation-derived off-flavours during storage

Flavour masking typically requires added seasoning and flavouring compounds, which increases ingredient complexity and challenges clean-label positioning.

Texture realism

Full structural mimicry of animal muscle tissue — including the hierarchy of myofibrils, connective tissue, intramuscular fat, and sarcomere-level architecture — remains beyond the current capability of HME. Bite, juiciness, and mouthfeel can be approximated but not replicated exactly.

Clean label

Many HME products contain methylcellulose (as a binder and fat mimetic), flavour enhancers, and colour agents. Consumer and regulatory pressure toward clean-label formulations is driving research into natural binders (psyllium husk, starch, mucilage-forming polysaccharides) and fermentation-based flavour correction.

Energy and capital cost

HME is an energy-intensive, capital-intensive process:

• High capital expenditure (CAPEX) for twin-screw extruder and cooling die systems
• Significant specific energy consumption (SME 557–1,136 kJ/kg)
• Requirement for specialised operators and process engineers
• Sensitive to raw material variability (protein content, moisture, particle size)

Research and future directions

Current and emerging research areas include:

• New protein sources: faba bean, lupin, duckweed, single-cell protein, algae
• Dry fractionation: air classification of legume flours to produce protein-enriched streams without wet extraction, reducing water and energy use
• Fat structuring: co-extrusion or injection of fat phases to improve juiciness
• Fermentation pre-treatment: enzymatic or microbial modification of protein matrices to reduce off-flavours and improve functionality
• Whole-cut analogues: producing continuous, thick-cut structures resembling chicken breast or beef steak rather than chunks or strips
• AI-assisted process optimisation: machine learning models for real-time control of SME, die temperature, and screw configuration
• 3D structuring: additive manufacturing approaches that use HME-produced paste as a substrate

Netherlands and HME

The Netherlands occupies a prominent position in global HME development and commercialisation, supported by:

• Wageningen University & Research (WUR): fundamental research on protein structuring, particularly the groups of Van der Goot and Boom
• TOP b.v.: applied process development and pilot-scale work in the Food Valley cluster
• OJAH b.v.: commercial-scale production and the first major European HME acquisition
• Blue Ocean Xlerator (BOX): incubation of early-stage plant protein companies
• Strong ingredient industry (Cosun, Avebe, Roquette Benelux) providing local raw material streams

See also

• Plant-based meat
• Pulsed electric field (PEF)
• High pressure processing (HPP)
• Modified atmosphere packaging (MAP)
• Radio frequency heating

References

• Schmid, M. et al. (2022). “High moisture extrusion: a review of the current understanding of process-structure-function relationships.” Comprehensive Reviews in Food Science and Food Safety, 21(6), 5070–5101.
• Chen, F.L., Wei, Y.M., Zhang, B., & Ojokoh, A.O. (2010). “System parameters and product properties response of soybean protein extruded at wide moisture range.” Journal of Food Engineering, 96(2), 208–213.
• Cheftel, J.C., Kitagawa, M., & Quéguiner, C. (1992). “New protein texturization processes by extrusion cooking at high moisture levels.” Food Reviews International, 8(2), 235–275.
• Grabowska, K.J., Tekidou, S., Boom, R.M., & van der Goot, A.J. (2014). “Shear structuring as a new method to make anisotropic structures from soy-gluten blends.” Food Research International, 64, 743–751.
• Palanisamy, M. et al. (2019). “High moisture extrusion of lupin protein: influence of extrusion parameters on extrudate properties.” Journal of Food Engineering, 289, 109830.
• Wittek, P. et al. (2021). “Characterization of texturized vegetable proteins: influence of the process and raw material.” Foods, 10(3), 537.
• Grayson, S. (2019). “OJAH: plant-based protein production at industrial scale.” Kerry Group acquisition announcement, April 2018.
• Van der Goot, A.J. et al. (2016). “Concepts for further sustainable production of foods.” Journal of Cleaner Production, 162, S10–S17.
• Wikipedia contributors. “Food extrusion.” Wikipedia, The Free Encyclopedia.