Engineering & Technology · Agricultural, Food & Biosystems Engineering

Food processing, heat and mass transfer and preservation principles

A drying operation reduces a product’s mass substantially. That does not mean the same proportion of its solids has disappeared, and the remaining water percentage does not by itself establish whether the product will keep. Food processing combines physical transfers with biological and chemical behaviour.

Use process balances, heat transfer and fluids for further detail; this chapter defines its own elementary accounts and units. Plant biology and chemistry provide useful context. No crop yield, soil value, food-processing endpoint, storage limit or machine operating setting in an example is an instruction for a real system.

Back to the chapter

After reading, you can

Use elementary mass and heat accounts while distinguishing a physical processing calculation from evidence of food preservation.

Identify what is transferred or changed

Processing can separate, mix, reduce size, heat, cool or remove water from a material. Heat transfer concerns energy moving because of temperature differences; mass transfer concerns material movement such as water migrating to a surface and leaving as vapour. Internal transfer and surface conditions can limit a process differently. A uniform-temperature or uniform-moisture assumption needs justification.

Preservation asks additional questions about microbial behaviour, enzymes, chemical change and the product’s intended condition. A thermal process must establish the relevant effect throughout the product, including a limiting location and time history. A calculated energy input alone does not establish that effect. Product composition, contamination and subsequent handling remain relevant.

A moisture percentage needs a declared basis

Wet-basis moisture is water mass divided by total wet mass. Dry-basis moisture instead divides water mass by dry-solid mass. For the original 100 kg feed, 80 kg water and 20 kg solids mean 80% wet-basis moisture, but 400% on a dry basis. The two percentages describe the same composition with different denominators.

Assume the dry solids remain 20 kg and a hypothetical output is 50% water by wet mass. Its dry fraction is also 50%, so total mass is 20/0.50=40 kg. It contains 20 kg water; 60 kg water has been removed. This fraction is chosen only for mass-account practice and is not a preservation endpoint for a food.

Feed 100 kg: 20 kg dry solids and 80 kg water, 80% wet-basis moisture. Assumed output 50% water by mass with no dry-solid loss: 20 kg solids/.50dry fraction=40 kg total, containing 20 kg water. Removed water 60 kg. All segments use 5 SVG units/kg. The 50% fraction is only an arithmetic assumption, not a stable or safe-food moisture target.

Water content and water activity answer different questions

Water activity aw is an equilibrium vapour-pressure ratio relative to pure water at the same temperature. It describes a thermodynamic relationship, not the fraction of product mass that is water. Binding and composition mean products with the same moisture fraction can have different water activities. Measuring one does not automatically supply the other; temperature and equilibration matter to the method.

For a supplied constant-heat-capacity model, heating 10 kg by 20 K with cp=3.5 kJ/(kg·K) requires 700 kJ of sensible energy. That account excludes phase changes, reactions, nonuniformity and transfers elsewhere. Removing water as vapour also requires energy and transport, with real losses and changing conditions. Keep these balances as components of a process model; preservation claims need relevant product/process evidence and defined limits.

Check two meanings of a percentage

The feed’s 80 kg water divided by 100 kg total gives 80% wet basis. Dividing the same 80 by 20 kg dry solids gives 400% dry basis. The 40 kg output’s 20 kg water gives 50% wet basis and 100% dry basis. The mass account remains 100=40+60 kg.

The separate heating calculation is Q=mcpΔT=10×3.5×20=700 kJ. It gives a model energy change, not a heating time or a validated food process. No microbial reduction, shelf life or suitability follows from these invented inputs alone.

Try the reasoning

Does a 50% water fraction mean aw=0.50?
Show the answer and reasoning
No. One is a mass fraction; the other is an equilibrium vapour-pressure ratio. Their relationship depends on the particular material and conditions.
Why can dry-basis moisture exceed 100%?
Show the answer and reasoning
Its denominator is dry solids, so water mass can exceed that denominator.
Does matching the total calculated heat establish uniform processing?
Show the answer and reasoning
No. Spatial transfer, time history and the relevant biological or chemical effect still require evidence.

What to carry forward

Track conserved constituents and energy carefully, then check the product-specific behaviour that a preservation decision actually requires.

References

Teaching resources and research supporting this lesson’s explanations and examples.