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The slowest-heating location in a container, where lethality accumulates least during thermal processing.
The point inside a can, jar or retort pouch that a food processor measures temperature at, because it's the last place to reach a safe temperature during sterilisation.
The first stage of drying, in which free surface water evaporates at a steady rate and the surface stays at the wet bulb temperature.
The early, fast stage of drying fruit, grain or milk powder, before the surface dries out and the process slows down.
See also: Wet bulb temperature, Critical moisture content
The moisture content at which drying shifts from the constant rate period to the falling rate period.
Tells a biscuit or snack manufacturer what packaging barrier is needed to stop the product going soft and stale on the shelf.
See also: Constant rate period
A process combining mixing, shearing, heating and forming in a screw barrel, forcing material through a die. High-temperature short-time extrusion cooks and expands.
The process behind pasta, breakfast cereals and puffed snacks like Kurkure — dough forced through a shaped die under heat and pressure.
See also: Specific mechanical energy, Starch gelatinisation
The deposition of protein, mineral or burnt-on residue on heat exchanger surfaces during processing, adding a thermal resistance and restricting flow.
The reason a milk pasteuriser or evaporator needs regular shutdowns for cleaning — burnt-on protein and mineral deposits reduce heat transfer over time.
See also: Overall heat transfer coefficient, Pasteurisation
The temperature at which an amorphous solid changes between a rigid glassy state and a rubbery state.
Why crackers and papad turn soft and lose their snap when left open in humid weather.
See also: Freeze drying, Water activity
The section of a continuous pasteuriser that provides the required residence time at process temperature.
The pipe section in a milk pasteuriser that keeps every drop hot long enough to kill harmful bacteria before it's cooled.
See also: Laminar flow, Reynolds number, Pasteurisation
Forcing a liquid at high pressure through a narrow valve to break dispersed droplets into smaller ones, stabilising the emulsion.
Why packaged milk doesn't separate into a layer of cream on top, the way fresh, unprocessed milk does.
See also: Emulsion, Stokes law
Smooth, ordered flow in parallel layers, occurring at low Reynolds number, with a parabolic velocity profile in a pipe.
Used in cleanroom and sterile filling lines at infant-formula and pharma-grade food plants to keep the air free of contamination.
See also: Reynolds number, Holding tube
The logarithmic average of the temperature differences at each end of a heat exchanger, used because the difference varies along its length.
Used by process engineers sizing a heat exchanger for a dairy or juice plant, to work out how much heating surface is actually needed.
See also: Overall heat transfer coefficient
A fluid whose shear stress is directly proportional to shear rate, so viscosity is independent of shear.
Water and thin syrups behave this way, unlike ketchup or honey — which is why ketchup needs a shake to start flowing but water doesn't.
See also: Rheology, Thixotropy
The combined conductance of all resistances between two fluids — convective films, wall conduction and fouling — used as Q = UAΔTlm.
Used by dairy and beverage plants to design pasteurisers and evaporators that heat or cool product efficiently at scale.
See also: Log mean temperature difference, Fouling
A chart relating dry bulb temperature, wet bulb temperature, humidity ratio, relative humidity and enthalpy of moist air.
Used by engineers designing food dryers, cold stores and bakeries to control humidity and prevent condensation or over-drying.
See also: Wet bulb temperature, Relative humidity
The ratio of the partial pressure of water vapour in air to its saturation pressure at the same temperature, as a percentage.
What a food warehouse or spice storage room controls to stop products absorbing moisture and clumping or growing mould.
See also: Water activity, Psychrometric chart
A pressure vessel used to sterilise food after sealing in its container, allowing temperatures above 100 °C.
The industrial pressure cooker used to sterilise canned foods and retort pouches, like packaged ready-to-eat curries.
See also: Cold spot, Commercial sterility, Aseptic packaging
The dimensionless ratio of inertial to viscous forces, Re = ρvD/μ.
Used by process engineers to predict whether fluid moving through a pipe in a dairy or juice plant will flow smoothly or turbulently, which affects mixing and cleaning.
See also: Laminar flow, Holding tube
The mechanical energy delivered to unit mass of material in an extruder, in J/kg.
Used by snack-food engineers to control how puffed or dense an extruded product, like a corn puff or pasta shape, turns out.
See also: Extrusion
Atomising a liquid feed into a hot air stream so droplets dry almost instantly into powder.
How milk powder, instant coffee and infant formula are made — liquid sprayed into hot air, drying to powder almost instantly.
See also: Freeze drying, Wet bulb temperature
The relation giving the settling velocity of a sphere in a fluid, proportional to the square of particle diameter and the density difference, and inversely proportional to viscosity.
Explains why cream rises to the top of unhomogenised milk over time, and is used to design the centrifugal separators dairies run.
See also: Homogenisation, Emulsion
The direct transition of a solid to vapour without passing through the liquid phase, occurring below the triple point of water — 0.0098 °C and 611 Pa.
The physics behind freeze-dried coffee and camping food — ice turns straight to vapour without melting, leaving the food's structure intact.
See also: Freeze drying, Glass transition temperature
A curve relating equilibrium moisture content to water activity at constant temperature, typically sigmoidal.
Used to decide the right packaging material and shelf-life claim for dry products like biscuits, namkeen and spice mixes.
See also: Water activity, Moisture content
The temperature reached by a wetted surface in a moving air stream at steady state, where evaporative cooling balances heat gain from the air.
Used in food drying and cold storage design to understand how effectively air can cool or dry a product through evaporation.
See also: Psychrometric chart, Spray drying, Constant rate period
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