2. Traditional preservation methods (DONE)

Three traditional ways of food stabilization

• Reduction of water activity, antimicrobial composition

• Thermal preservation

• ‘Non-thermal’ preservation

ai vs xi in solutions without specific interaction

Effect of solute on chemical potential

• There are two chemical potentials one for the solute and one for the water

• a is the activity of the water/solute

• The chemical potential of a solute in a solution increases with higher solute concentration.

• The presence of a solute lowers the chemical potential of the solvent

Differences in chemical potential

How to calculate the water activity of a solution

1. Calculate mole fraction of solute.

2. Calculate mole fraction of water

3. Calculate mole fraction with water on top and the rest on bottom.

Is addition of salt to a product effective for preservation?

• A lot of salt has to be added to actually prevent the growth of microorganisms

• This usually also changes the taste and therefore is not worth it.

Is sugar or salt more effective in reducing water activity?

• Sucrose is less effective than NaCl

• Additionally measured and calculated aw deviate for sucrose: measured < calculated. 

What does it mean when 𝛾_𝑤 is smaller than 1?

• Meaning it is not an ideal solution, as there is an interaction between solute and solvent.

• In sugar this happens due to strong interaction between the sugar OH groups and water → there is less ‘free water’

• Which is good because it prevents microbial growth.

Effect of fermentation on water activity

• Ethanol is produced, which lowers the water activity significantly

Sorption isotherm

• Equilibrium between air and product

• Creates controlled relative humidity

• RH/100=aw

• Measure change in mass

• Temperature dependent

Axis for sorption isotherms

Moisture content (gH2O/100g DS) vs Water activity

Why does acidification prevent growth of microbes

Most microbes do not grow below 4.5

What acid is usually used for acidification?

Acetic acid

• Cheap

• Strong acidification

• Also antioxidant (will reduce browning and oxidation of oils)

Other options:

• Citric acid

• Fumaric, lactic and malic acid

• Phosphoric, tartaric acid

Preservative action of weak acids

• Charged, dissociated ions can not penetrate the hydrophobic cell membrane

• Un-dissociated acids (a-polar) can penetrate cell membrane passively

• In cell, acids dissociate and establish lower intracellular pH

• Cell tries to re-establish pH by actively expelling ions

• Energy sources are depleted, cell gets exchausted and may die

Positive redox potential and spoilage

Usually spoiled by aerobic bacteria and fungi

How can spoilage be prevented from a redox perspective?

• Creating a reducing atmosphere (anoxic, using nitrogen or CO2)

• Or by adding reductors

  • Typical reductors: ascorbic acid, ascorbate, nitrite and sulphur dioxide.

Examples reductors

• Ascorbic acid

  • Can reduce quinone into catechol and therefore stops enzymatic browning

  • Is antimicrobial even to facultatively anaerobic organisms.

• Sulphur dioxide

  • Used in wines to stop growth of yeast

  • Used in dried fruits

• Nitrites

  • Active against anaerobic bacteria

  • Can create ions that inhibit bacterial growth

Caustic

• Also known as lye (hydroxides of Na, K or Ca)

• has a high pH which will saponify lipids thus rendering bacterial membranes defective, and also partially hydrolyze proteins. 

Preservatives against gram + microorganisms

• Lysozyme

• Nisin

Four types of smoking used as a preservation method

• Cold smoking

• Hot smoking

• Roast smoking

• Application of liquid smoke

Steps of cold smoking

Steps of hot smoking

• Curing = drying by means of salt, nitrates, sugars, etc.

Roast smoking

• Barbeque

• Product roasted

• In some cases: product desiccated (e.g. Bakkwa)

• Can be stored without refrigeration

Smoking with smoke condensate

• Normal smoke contains toxic components

• Smoking takes time: expensive

Alternative:

Smoke condensate or ‘liquid smoke’: condensed/dissolved and refined

• Controlled pyrolysis of sawdust

• Vapor led through cold tube with water (‘stripping’) - components condense and dissolve/disperse in the water

• Some volatiles stripped away

• Phase speparation (purification); removes tarry phase (such as PAHs), particles settle

• Aqueous phase may be concentrated. 

Making liquid smoke

Tarry: The tarry phase in smoke is the solid and liquid particulate matter that results from the combustion of substances like tobacco or biomass. This is a toxic residue containing a variety of harmful chemicals, including carcinogens, and is a major cause of smoking-related diseases by coating the lungs and damaging them over time.

Hurdle technology

Using several mild preservation strategies at the same time

1. Disruption of homeostasis

   • Conditions allow metabolic activity, but are not ideal (pH, T, E)

2. Metabolic exhaustion

   • Spore germination is possible, growth is hindered by metabolic exhaustion

   • m.o. live but do not grow

3. Several simultaneous stress factors avoid build-up of tolerance by micro-organisms.

Compartmentalization as preservation method

• Water-in-oil emulsions

• Smaller droplets

  • Better microbiological stability, local/isolated growth only

  • More difficult to physically stabilize

• Physical barrier against outgrowth of microorganisms

• Butter, margarine, oil-continuous sauces, are stable, do not need refrigeration.

How does smoke preserve foods?

• Contains antimicrobial component (e.g. phenolics, formaldehyde & acetic acid)

• Acetic acid also provides lower pH

• High temperatures

Example of hurdle technology

Curing of meat is a traditional form of hurdle technology, combining drying, salting, pH reduction (fermentation), and nitrites to inhibit C. botulinum and enhance color

Antimicrobials spices

Herbs and spices such as thyme and rosemary contain potent antimicrobials.