Upcycling of food side streams: the benefits of twin-screw granulation of apple pomace

Significance of the topic

The valorization of food-processing side streams is a high-priority objective for sustainable food production and circular economy strategies. Apple pomace, a voluminous by-product of juice manufacturing, contains valuable polysaccharides and fibers but suffers from poor techno-functional properties (low solubility, poor flowability) that limit its reuse in food formulations. Developing continuous, scalable processing routes that modify pomace structure and functionality can convert waste into usable ingredients, reduce disposal costs, and lower the carbon footprint of food production.

Objectives and study overview

This application note investigates twin-screw granulation as a continuous, one-step route to functionalize apple pomace from commercial juice production. The work evaluates how process parameters (throughput, screw speed, added water, barrel temperature profile) influence the microstructure, water-holding and gelling behavior of the resulting granules, and identifies operational limits where thermo-mechanical degradation reduces functional performance.

Methodology

• Feed and formulation: Apple pomace solids were fed gravimetrically at mass throughputs of 0.2, 0.4, or 0.6 kg/h. Water was added by peristaltic pump to maintain a constant added-water fraction of 23 wt% (0.06, 0.12, or 0.18 kg/h respectively).
• Extrusion conditions: Trials used a Thermo Scientific Process 11 co-rotating twin-screw extruder (11 mm, L/D = 40) equipped with a granulation kit and open discharge to produce granules directly. Barrel temperature sections were set to a stepped profile (barrel2 40 °C; barrel3 60 °C; barrel4 80 °C; barrels5–7 120 °C). Screw speeds of 400 and 600 rpm were tested.
• Rheological evaluation: Dispersions were prepared by mixing 1 g sample with 20 mL tap water, briefly stirring and immediately measuring oscillatory time sweeps on a Thermo Scientific HAAKE Mars iQ Air rotational rheometer using plate–plate geometry, 1.5 mm gap, 20 °C, 1% strain, 1 Hz over 20 minutes (60 time points) within the linear viscoelastic region.
• Microstructure: Particle surface and porosity were observed by scanning electron microscopy (Phenom XL) in high vacuum; samples mounted on aluminum stubs with carbon adhesive and imaged at 15 kV.

Used instrumentation

• Thermo Scientific Process 11 co-rotating twin-screw extruder with granulation kit and open discharge.
• Gravimetric twin-screw feeder and peristaltic pump for water addition.
• Thermo Scientific HAAKE Mars iQ Air Rheometer (plate–plate geometry) for oscillatory time-sweep measurements.
• Phenom XL scanning electron microscope for surface and porosity imaging.

Results and discussion

• Granule morphology and porosity: Twin-screw granulation produced a porous powder composed of agglomerated particles. SEM showed that lower throughput (longer residence time) yielded looser, more porous granules, whereas higher throughput produced more compact particles likely due to higher fill and reduced expansion.
• Water binding and gelation: All granulated samples, when rehydrated, formed gels/pastes characterized by storage moduli (G') exceeding loss moduli (G''), indicating formation of a gel network. Porosity and internal structure influenced swelling and gelation kinetics: more porous granules swelled faster and developed stronger gel networks.
• Effect of residence time and throughput: Residence time in the extruder is inversely related to throughput. Longer residence times (lower throughput) increased the degree of cell-wall rupture and release/solubilization of intracellular polysaccharides responsible for water binding and gel formation. Short residence times (higher throughput) reduced this effect, producing less swellable, more compact granules.
• Effect of screw speed and shear/heat input: Increasing screw speed (from 400 to 600 rpm) raised shear stress and local temperature via friction, which did not significantly change nominal residence time but increased thermo-mechanical input. Excessive shearing/thermal exposure caused partial degradation of polysaccharides, reducing both storage and loss moduli and decreasing gelling/water-binding capacity.
• Operational window: The data indicate an optimal thermo-mechanical energy window where sufficient cell-wall disruption enhances functionality; beyond that window, degradation of structure-forming polysaccharides reduces performance.

Benefits and practical applications of the method

• Direct granulation: Twin-screw granulation with open discharge eliminates a separate milling step by delivering immediately usable granules with improved flow and handling.
• Enhanced functionality: Processed apple pomace exhibits increased water-holding capacity and the ability to form gels upon hydration, enabling its use as a functional ingredient (thickener, binder, water-retention agent) in food formulations.
• Upcycling and circularity: The method provides a continuous, scalable route to valorize a low-value by-product into functional ingredients for human food, animal feed, or ingredient streams for pectin extraction.
• Tunable properties: By adjusting throughput and screw speed, manufacturers can tune porosity, particle compactness, and gelation behavior to match target applications.

Future trends and possibilities for utilization

• Integration with biochemical pretreatments: Combining granulation with mild enzymatic or chemical pretreatments could selectively solubilize pectins and fibers while minimizing thermal degradation.
• Process optimization and scale-up: Development of scale-up strategies, real-time monitoring (torque, barrel pressure, temperature profiling) and process analytical technology (PAT) to control residence time and energy input.
• Tailored granule design: Engineering granule porosity and size distributions to optimize release, swelling, and sensory properties in specific food matrices.
• Formulation development: Incorporation of granulated pomace into bakery, meat analogues, sauces and dairy alternatives as fiber-rich functional ingredients; sensory and regulatory evaluation will be required.
• Combined valorization chains: Coupling granulation with downstream fractionation (pectin extraction, polyphenol recovery) to maximize resource recovery from pomace streams.

Conclusion

Twin-screw granulation is demonstrated as a promising continuous technology to functionalize apple pomace by inducing cell-wall disruption and producing porous granules with enhanced water-binding and gelling properties. Process parameters—especially throughput (residence time) and screw speed (thermo-mechanical input)—control microstructure and functional outcomes. An optimum energy input window exists: insufficient input limits functionality, whereas excessive thermo-mechanical stress degrades polysaccharides and diminishes gelation. The approach enables direct production of handleable granules and offers a scalable path for upcycling pomace into value-added food ingredients, provided processing is optimized to avoid degradation.

References

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