Food preservation

These flashcards review key concepts from the Week 7 lecture on food preservation, including shelf life, safety vs. quality, water activity, pH, temperature control, traditional preservation methods and emerging non-thermal technologies.

What is the primary objective of food preservation?

To extend shelf life by slowing or preventing microbial, chemical, enzymatic and physical spoilage while maintaining safety and acceptable quality.

Define shelf life in food science.

The period during which a food remains safe and of acceptable quality under specified storage conditions.

Which factor always takes priority when designing a preservation method: safety or quality?

Safety, because legal and ethical responsibilities require that food must not cause harm.

Differentiate ‘use-by’ from ‘best-before’ dates.

Use-by marks the last day the food is considered safe; best-before indicates peak quality but the food may still be safe afterwards.

Give two reasons why high-moisture foods spoil faster than dry foods.

They have higher water activity and therefore support rapid microbial growth; many enzymatic and chemical reactions proceed faster in aqueous environments.

State the SI base units for mass, length and time.

Kilogram (kg), metre (m), second (s).

In which direction does heat flow?

From hot (higher temperature) to cold (lower temperature).

Target microorganism for commercial canning (retorting)

Clostridium botulinum.

Why is Clostridium botulinum a critical target in thermal processing?

It produces botulinum toxin; ~30 ng can be lethal, so destroying the cells ensures consumer safety.

What is water activity (aw)?

The ratio of the vapour pressure of water in a food to the vapour pressure of pure water at the same temperature; effectively the amount of unbound, available water.

Typical aw value of pure water

1.00 (or 100 %).

Why can jam be stored unrefrigerated for months?

≈50 % sugar ties up water, lowering aw to ~0.5–0.6, below the growth limit of most microbes.

Critical water activity below which virtually no pathogenic bacteria grow

aw ≈ 0.90 for most, and aw ≈ 0.60 for all microbes, including moulds.

List two ways to reduce aw.

1) Remove water (drying, concentration). 2) Bind water with solutes (add salt, sugar, polyols).

Explain why low-sugar biscuits (same moisture) may spoil faster than high-sugar biscuits.

Lower sugar means fewer solutes binding water, giving a higher aw that supports microbial growth.

Define hurdle technology.

Using multiple preservation factors (hurdles) together—e.g., low pH, low aw, heat—to inhibit microbes synergistically.

Give four essential growth requirements for most bacteria.

Water, nutrients, suitable temperature, and suitable pH (plus often oxygen).

Critical pH below which most pathogens cannot grow

pH 4.6.

Why are fermented foods like yoghurt generally safe after months in refrigeration?

Low pH, competitive probiotic cultures, and minimal available nutrients for spoilage organisms.

Describe the ‘botulinum cook’.

Thermal treatment at 121.1 °C (250 °F) for 2.88 min (F₀ = 3 min) to inactivate C. botulinum spores.

How does refrigeration extend shelf life?

It slows biochemical and microbial reactions by lowering temperature away from optimal growth ranges.

Why should foods be frozen quickly rather than slowly?

Rapid freezing forms many small ice crystals, causing less cell damage and texture loss than large crystals from slow freezing.

State two disadvantages of thermal preservation.

Loss of nutrients and flavour; undesirable texture or colour changes.

Give three traditional preservation methods based on lowering aw.

Drying (sun/oven), salting/curing, adding high sugar (jams, candied fruit).

Name three preservation methods based primarily on acidity.

Pickling with vinegar, lactic fermentation (sauerkraut, kimchi), acidification of canned vegetables with citric acid.

Why does vacuum packaging slow spoilage of meat?

It removes oxygen, inhibiting aerobic spoilage and oxidative rancidity.

State two antimicrobial agents legally used in foods.

Sorbates (e.g., potassium sorbate) and nitrates/nitrites for cured meats.

What sensory change often indicates lipid oxidation in fatty foods?

Development of rancid, off-flavours/odours.

Purpose of smoking in food preservation beyond flavour

Provides antimicrobial phenolic compounds and dries surface, inhibiting microbes.

Give one example each of a chilled, frozen and ambient-stable preserved product.

Chilled: pasteurised milk; Frozen: frozen peas; Ambient-stable: canned beans.

Explain how fermentation can act as a ‘hurdle’.

Fermenting microbes produce acids, alcohols and bacteriocins that suppress pathogens while competing for nutrients.

High-pressure processing (HPP) operating principle

600 MPa pressure destroys microbial cell membranes without significant heating.

Pulsed Electric Field (PEF) preservation mechanism

Short high-voltage pulses create pores in microbial membranes (electroporation), leading to cell death with minimal heat.

Why are non-thermal methods attractive to product developers?

They enhance safety while better retaining fresh flavour, nutrients and texture compared with high-heat treatments.

Ultrasound’s antimicrobial action in foods

Acoustic cavitation forms microscopic bubbles that implode, causing shear forces and cell disruption.

What is cold plasma used for in food processing?

Surface decontamination via reactive oxygen/nitrogen species that damage microbial DNA and membranes.

How does UV-C light inactivate microorganisms?

Ionising radiation (around 254 nm) induces DNA thymine dimers, preventing replication.

State one limitation common to many novel non-thermal technologies.

High capital cost or scalability challenges compared with conventional heat treatments.

Why is ‘permeate’ sometimes added back to milk, and why was it controversial?

To standardise protein/fat composition; media misinterpreted it as adulteration although it is simply milk serum components.

Function of wax coatings on apples

Reduce moisture loss and slow respiration, maintaining crispness and extending shelf life.

Why are crisp packets filled with nitrogen gas?

Nitrogen cushions chips during transport and displaces oxygen to prevent lipid oxidation.