CHEM215 Lecture Notes - Lecture 15: Surface Tension, Heat Treating, Pasteurization
Lecture 15 – Food Preservation 1
Traditional methods of food preservation:
• Cooking - kills microorganisms/limits their growth
• Drying - reduces water content thus inhibits the growth of microorganisms
• Concentration - similar to drying —> removes water so increase
solute concentration in cell which increases osmotic pressure, making growth
of microorganisms hard
• Salting - increase solute concentration which increases osmotic pressure
• Sugaring - increases solute concentration
‘Newer’ methods of food preservation:
• Sterilisation, pasteurisation (heating) —> kills everything that is present in the
food.
• Chilling and freezing —> microorganisms cannot grow under cold conditions
• Water activity control (Aw)
• Storage atmosphere control (eg, vacuum pack) —> most organisms are aerobic so
require oxygen to grow
• Chemical preservatives
Water activity:
• A measure of the “available” water in a system (ie water available for microbial
growth).
• Water activity is the ratio of the vapour pressure of the water present in a food
substance compared to the vapour pressure of an equivalent mass of pure water (
at same temp and pressure).
• There is a direct relationship between high water activity and microbial
spoilage —> reducing water activity hinders microbial growth.
• Factors affecting water activity:
1. Food components: foods containing sugar have a lower water activity. Sugar
molecules have many OH groups, which H-bond to water molecules, thus
containing the water molecules. Foods with a high affinity to water will have a
lower water activity
2. Physiochemical state of components
3. Porous structure
4. Temperature
5. Pressure
6. Surface tension
Food preservation is a battle against microbes:
Three strategies for dealing with food microbes
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Sugar molecules have many oh groups, which h-bond to water molecules, thus containing the water molecules. Foods with a high affinity to water will have a lower water activity: physiochemical state of components, porous structure, temperature, pressure, surface tension. Lecture 15 food preservation 1: destroy microbes > kill the outright. Treat at high pressure: remove the microbes. Sterile filtratio of liquid substance (pores are too small for microbes to pas through: inhibit their growth. Lowering water activity > drying, addition of humectants (eg salt, sugar, glycerol) Other sources of spoilage include: lipid oxidation, enzymic reactions eg. polyphenol oxidase browning, non-enzmatic browning - maillard browning, moisture changes - water uptake and loss. Modern consumers demand that foods are: high in quality but minimally processed, more natural, less severely preserved, nutritionally healthier > less sugar, salt, fats, more fibre, higher in calcium.
