2.1 Filtration and membrane separation

Unit operations are the basic physical steps that are combined in different orders to make almost every processed food. Separation operations such as filtration, membrane separation, centrifugation, evaporation and extraction remove or concentrate one part of a food, while mixing, homogenisation, emulsification and size reduction change its structure and make it uniform. A food technologist who understands the principle, equipment and control points of each operation can design a process, set its conditions, find the cause of a fault and protect product quality and safety.

Example: a milk processing plant in Kaplamai uses a disc-stack centrifuge to separate cream, a homogeniser before pasteurisation, an evaporator to concentrate milk for its condensed products and ultrafiltration to recover whey proteins.

2.1.1 Filtration principles

Filtration is the separation of solid particles from a fluid (liquid or gas) by passing the mixture through a porous medium that holds back the solids. The clear liquid that passes through is the filtrate, and the solids collected are the filter cake or retentate. Filtration is used to clarify products such as juices, beer, edible oils and syrups, to recover solids, and to remove contaminants from water and process air.

Driving force and resistance

Flow through a filter needs a driving force, a pressure difference across the filter created by gravity, applied pressure (pumping), vacuum on the filtrate side or centrifugal force. Flow is opposed by the resistance of the filter medium and of the cake that builds up on it. The rate of filtration is described by Darcy's law:

Rate of filtration (volume per unit time) = A × ΔP ÷ [μ × (Rm + Rc)]

where A is the filter area, ΔP is the pressure difference, μ is the viscosity of the filtrate, Rm is the resistance of the filter medium and Rc is the resistance of the cake. As the cake grows, Rc increases and the rate falls unless the pressure is raised.

Types of filtration

• Cake (surface) filtration: solids larger than the pores form a cake on the surface of the medium, and the cake itself becomes the filtering layer. Used where there is a large amount of solids, as in clarifying sugar syrup.
• Depth filtration: fine particles are trapped inside a thick porous medium such as a pad or cartridge. Used for polishing liquids with few, fine particles, such as beer and wine before bottling.
• Constant-pressure filtration: the pressure is held constant and the flow rate falls as the cake builds.
• Constant-rate filtration: the pressure is increased steadily to keep the flow constant.

Factors affecting the rate of filtration

• Pressure difference: a higher pressure increases the rate, although compressible cakes pack tighter and resist more at high pressure.
• Filter area: a larger area gives a proportionally higher rate.
• Viscosity: viscous liquids filter slowly, so many are warmed to reduce their viscosity.
• Particle size and shape: fine or soft, slimy particles form a dense, low-permeability cake.
• Cake thickness and compressibility: thick or compressible cakes increase resistance.
• Filter aids: inert powders such as diatomaceous earth (kieselguhr) and perlite are added to the liquid or pre-coated on the medium to give an open, permeable cake.

2.1.2 Filtration and membrane separation equipment

Conventional filters

Membrane separation

Membrane separation uses a thin, selective membrane that lets water and small molecules pass (the permeate) while holding back larger molecules and particles (the retentate or concentrate). It works at low or moderate temperature without a phase change, so it saves energy and protects heat-sensitive nutrients, colour and flavour. The main types differ in pore size and operating pressure:

Membrane equipment and operation

• Cross-flow (tangential) operation: the feed flows across the membrane surface at speed, sweeping away deposits, while permeate passes through. This is used in nearly all food membrane plants.
• Dead-end operation: the feed flows straight at the membrane, which clogs quickly. It is used only for low-solids liquids.
• Membrane modules: spiral-wound (compact and cheap, for low-solid liquids such as whey and water), tubular (for viscous and particle-laden liquids such as juices with pulp), hollow fibre and plate-and-frame modules.
• Membrane materials: polymers such as polysulphone and polyamide, and ceramic membranes that withstand high temperature and strong cleaning chemicals.
• Fouling and cleaning: proteins, fats and minerals build up on the membrane and reduce flux, so plants are cleaned in place (CIP) with alkali, acid and enzyme detergents at set intervals and flux is monitored.

2.1.3 Evaporation

Evaporation is the removal of part of the water from a liquid food by boiling, to give a concentrated liquid. It reduces weight and volume for cheaper storage and transport, increases solids content to improve keeping quality, and prepares liquids for drying or crystallisation. Examples are tomato paste, sweetened condensed milk, fruit juice concentrates and sugar syrups.

Principles

Heat, usually from steam, is transferred through a heating surface to the liquid, which boils, and the vapour is removed and condensed. Most food evaporators work under vacuum, which lowers the boiling point (for example to about 50 °C to 70 °C) and so reduces heat damage, browning and cooked flavours.

Mass balance in evaporation

Solids in feed = solids in concentrate

Mass of concentrate = mass of feed × feed solids ÷ concentrate solids

Example: 1,000 kg of juice at 12% solids is concentrated to 40% solids.

Mass of concentrate = 1,000 × 0.12 ÷ 0.40

Mass of concentrate = 300 kg

Water evaporated = 1,000 − 300

Water evaporated = 700 kg

Types of evaporators

• Open pan (batch) evaporators: simple jacketed pans used for jams and small-scale products. They work at atmospheric pressure and cause more heat damage.
• Falling film evaporators: liquid flows as a thin film down the inside of heated tubes, giving very short contact time. They are widely used for milk and juices.
• Rising film evaporators: liquid boils and rises up the tubes as a film.
• Forced circulation evaporators: a pump circulates viscous liquids, such as tomato paste, through the heater.
• Plate evaporators: use plates instead of tubes, compact and easy to clean.
• Multiple-effect evaporators: the vapour from one effect heats the next effect, which runs at a lower pressure, greatly reducing steam use.
• Vapour recompression: thermal (steam jet) or mechanical compressors raise the pressure of the vapour so it can be reused as the heating medium, saving energy.

Effects on food quality

• Loss of volatile aroma: aroma recovery units condense volatiles and return them to the concentrate.
• Browning and cooked flavour: reduced by vacuum and short residence time.
• Increase in viscosity: limits how far some products can be concentrated.
• Fouling of heating surfaces: milk proteins and minerals deposit and reduce heat transfer, so evaporators are cleaned regularly.

2.1.4 Mixing

Mixing is the operation in which two or more components are combined so that each is spread evenly through the others. It is used to blend dry ingredients (flour premixes, spice blends, fortified maize flour), to combine liquids, to disperse solids in liquids and to develop dough. A well mixed product has a uniform composition in every sample, which is essential for consistent quality and for correct levels of additives and fortificants.

Types of mixers

Factors affecting mixing

• Particle size, shape and density: large differences cause separation (segregation) of the mix during mixing and handling.
• Moisture and stickiness: sticky powders form lumps and coat the mixer.
• Viscosity: viscous materials need more power and slower, heavier mixers.
• Mixer speed and time: under-mixing leaves the product uneven, while over-mixing can cause segregation, heat and damage, such as overworked dough.
• Proportion of minor ingredients: small additions such as vitamins are first pre-blended with a little of the main ingredient.
• Fill level: mixers work best at the fill level the manufacturer specifies.

Measuring mixing quality

Samples are taken from different points in the mixer at intervals and analysed for a tracer component, such as salt or a fortificant. The variation between samples falls as mixing proceeds, and the mixing time is set where the variation is within the specification.

2.1.5 Homogenization

Homogenisation is the reduction of the size of fat globules or particles in a liquid so that they stay evenly spread and do not separate. In milk, fat globules of about 1 to 10 µm are reduced to below about 1 µm, so that cream no longer rises to the top. Homogenisation also gives milk a whiter colour and fuller mouthfeel, and is used for ice cream mix, cream liqueurs, sauces, juices with pulp and infant formula.

The high-pressure homogeniser

A positive displacement piston pump forces the product at high pressure through a very narrow adjustable gap between the homogenising valve and its seat. The sudden pressure drop, turbulence, shear and cavitation break the fat globules into much smaller droplets.

• Operating pressure: typically about 100 to 250 bar for milk.
• Two-stage homogenisation: the first stage breaks the globules, and a second stage at a lower pressure (about 20 to 50 bar) breaks up clusters of small globules, which is important for cream and ice cream mixes.
• Temperature: milk is homogenised warm, about 55 °C to 70 °C, when the fat is liquid. Homogenised raw milk must be pasteurised at once, because the new fat surface is quickly attacked by the milk's own lipase, producing rancid flavours.
• Partial homogenisation: only the cream fraction is homogenised and remixed, saving energy.

Other homogenising equipment

• Colloid mills: a fast rotor turns close to a stator, shearing the product. Used for thick products such as peanut butter and sauces.
• Ultrasonic homogenisers: high-frequency sound waves create cavitation that disrupts droplets, for small batches.

Effects of homogenisation on milk

• No cream line: fat stays evenly spread.
• Whiter colour and fuller taste: more light is scattered by the many small globules.
• Softer curd: gives a softer curd that is easier to digest.
• Greater sensitivity to light and lipase: milk is more prone to oxidised and rancid flavours if not handled correctly.

2.1.6 Emulsification

Emulsification is the formation of a stable mixture (an emulsion) of two liquids that do not normally mix, such as oil and water, with one dispersed as fine droplets in the other.

• Oil-in-water (O/W) emulsions: oil droplets dispersed in water, as in milk, cream, mayonnaise, salad dressings and many sauces.
• Water-in-oil (W/O) emulsions: water droplets dispersed in fat, as in butter and margarine.

Emulsifiers and stabilisers

An emulsion is unstable on its own, because the droplets tend to join together. Emulsifiers are molecules with a water-loving (hydrophilic) end and a fat-loving (lipophilic) end that sit at the oil-water interface, lower the interfacial tension and form a protective layer around each droplet. Common food emulsifiers are lecithin (from egg yolk and soya), mono- and diglycerides of fatty acids, and proteins such as milk caseins. The balance between the two ends (the HLB value) decides whether an emulsifier favours O/W or W/O emulsions. Stabilisers such as xanthan gum, guar gum, carboxymethyl cellulose and starch thicken the continuous phase, slowing droplet movement.

Equipment for emulsification

• High-speed mixers and rotor-stator mixers: form coarse to medium emulsions such as dressings.
• Colloid mills: produce fine emulsions in thick products such as mayonnaise.
• High-pressure homogenisers: produce the finest, most stable emulsions, such as milk, cream liqueurs and nutritional drinks.
• Votators (scraped-surface heat exchangers): cool and work margarine and shortening while crystallising the fat.

Instability of emulsions

• Creaming or sedimentation: droplets rise or settle because of density difference, reversible by shaking.
• Flocculation: droplets cluster without merging.
• Coalescence: droplets merge into larger ones, leading to separation of oil (breaking), which is irreversible.
• Phase inversion: an O/W emulsion turns into W/O, as happens deliberately when cream is churned into butter.

Stability is improved by small droplet size, the right emulsifier, higher viscosity of the continuous phase and correct storage temperature.

2.1.7 Centrifugation

Centrifugation separates components of different densities by spinning the mixture at high speed, which creates a centrifugal force thousands of times greater than gravity. Heavier particles or liquids move outwards and lighter ones move towards the centre, so separations that would take hours or days by settling are done in seconds.

Principle

The settling velocity of a particle under gravity follows Stokes' law:

v = d² × (ρp − ρf) × g ÷ (18 × μ)

where v is the settling velocity, d is the particle diameter, ρp and ρf are the densities of the particle and the fluid, g is the acceleration due to gravity and μ is the viscosity of the fluid. In a centrifuge g is replaced by the centrifugal acceleration r × ω² (r is the radius and ω the angular speed), which is far greater, so separation is much faster. The equation also shows that larger particles, a greater density difference and a lower viscosity (for example warming milk) all speed up separation.

Types of centrifuges

Applications in food processing

• Cream separation: milk is warmed and separated into skim milk and cream, and the cream fat content is adjusted with the outlet valves.
• Clarification: removal of dirt, cells and sediment from milk, juice, beer and oil.
• Bacteria and spore removal: special clarifiers (bactofuges) remove spores from cheese milk.
• Oil recovery: separation of oil from water and solids in vegetable oil extraction.
• Crystal separation: spinning sugar crystals free of syrup.

Safety and maintenance

Centrifuge bowls store great energy, so they are balanced, inspected and serviced to the manufacturer's schedule, never opened while turning, and run only with all guards and interlocks working.

2.1.8 Solid-Liquid extraction and pressing

Solid-liquid extraction (leaching)

Solid-liquid extraction is the separation of a soluble component from a solid by dissolving it in a solvent. The solvent passes into the solid, dissolves the wanted component, and the solution (the extract or miscella) is separated from the spent solid. Examples are extracting coffee and tea solubles with hot water for instant products, sugar from sliced sugar beet with hot water, and vegetable oil from seeds or press cake with hexane, followed by recovery of the solvent.

Factors affecting extraction:

• Particle size: smaller particles give a larger surface and shorter diffusion path, but very fine particles slow drainage.
• Temperature: higher temperature increases solubility and diffusion, within the limits of heat damage.
• Solvent type and amount: the solvent must dissolve the wanted component selectively and be safe and easy to recover.
• Concentration gradient: fresh solvent meeting the most exhausted solid keeps the gradient high, which is why countercurrent extractors are used.
• Agitation and time: stirring and enough contact time improve yield.

Equipment ranges from batch extraction tanks to continuous countercurrent extractors (such as moving-basket and rotating-cell types) used in large oil and sugar plants.

Pressing (expression)

Pressing is the separation of liquid from a solid by applying mechanical pressure. It is used to extract fruit juices, sugarcane juice and vegetable oils, and to dewater materials.

• Screw press (expeller): a rotating screw pushes the material through a barrel with a tapering space, squeezing out oil or juice continuously. Used for sunflower, cottonseed and avocado oil.
• Roller mills: sets of heavy grooved rollers crush sugarcane with water sprayed on the bagasse (imbibition) to extract more sugar.
• Hydraulic presses: batch presses, including rack-and-cloth presses, used for fruit juice.
• Belt presses: pulp is squeezed between perforated belts passing over rollers, used for fruit juice.

Pre-treatment improves yield: fruit is crushed or pulped and may be treated with pectinase enzymes, and oilseeds are cleaned, dehulled, flaked and heat-conditioned. Cold pressing at low temperature gives higher quality, more flavourful oils but lower yield than hot pressing or solvent extraction.

2.1.9 Size reduction

Size reduction is the breaking of solid food into smaller pieces by mechanical force. It increases surface area, which speeds up drying, extraction, cooking and heat transfer, makes mixing more uniform, produces the required particle size (as in flours and sugar) and makes food easier to handle and eat.

Forces used in size reduction

• Compression: crushing between two surfaces, as in roller mills.
• Impact: a sharp blow from a fast-moving hammer or pin, as in hammer mills.
• Attrition (shear): rubbing between surfaces, as in plate and stone mills.
• Cutting: slicing with sharp blades, used for fruits, vegetables and meat.

Size reduction equipment

Energy for size reduction

Three classical laws estimate the energy needed:

• Rittinger's law: energy is proportional to the new surface created, applied to fine grinding.
• Kick's law: energy depends on the reduction ratio (feed size ÷ product size), applied to coarse crushing.
• Bond's law: used for intermediate sizes, E = 10 × Wi × (1 ÷ √d₂ − 1 ÷ √d₁), where E is the energy in kWh per tonne, Wi is the work index of the material, and d₁ and d₂ are the feed and product sizes in µm (the sizes through which 80 percent of the material passes).

Effects of size reduction on food

• Heat generation: friction heats the product, which may cause loss of volatile flavours (spices) and fat rancidity. Cryogenic grinding with liquid nitrogen is used for some spices.
• Enzymic browning: cut fruits and vegetables brown quickly unless treated with acid or blanched.
• Faster deterioration: greater surface area increases oxidation and moisture uptake, so ground products need good packaging.
• Dust hazards: fine flour and sugar dust can explode, so mills need dust extraction, earthing and spark control.
• Sieve analysis: product size is checked with a stack of standard sieves to confirm it meets specification.

2.1.10 Application

Unit operations are combined into process lines. The table shows how the operations in this chapter are applied in common Kenyan food industries.

Selecting a unit operation

When designing or improving a process the food technologist considers:

• The product requirement: the particle size, clarity, solids content or stability needed.
• Effect on quality: heat, shear and oxygen exposure and their effect on nutrients, colour and flavour.
• Food safety: hygienic design, ease of cleaning in place and removal of contaminants.
• Yield and losses: how much product is recovered.
• Energy and water use: for example membrane concentration and multiple-effect evaporation save energy.
• Capacity and cost: throughput, capital cost, running cost and maintenance.
• Worker safety: guarding, noise, dust and pressure hazards.