FOOD 4150 Lecture 6

Foods as Ecosystems

• ecosystems are composed of the environment and the organisms that live therein

• food environment is composed of intrinsic factors inherent in the food and extrinsic factors external to it

Intrinsic Factors

• characteristics of the food itself are called intrinsic factors

• pH

• Aq

• oxidation reduction potential

• nutrient content

• antimicrobial constituents (naturally and added preservatives)

• biological structures

pH

• log scale measurement of its acidity

• the lower the pH, the more energy the cell has to spend to maintain its intracellular pH near neutrality and the less energy it has to grow and produce toxins

• most microorganisms grow best in a pH range of 6.6-7.5

Optimum pH

• for most foodborne microbes is near neutrality

• most foodborne pathogens cannot grow below pH 4.4

• spoil causing bacteria (LAB/fungi) can grow well in acidic products

• food with pH less than 3 rarely have microbial problems

Water Activity

• major factor in controlling microbial growth and chemical reactions in food

• water can be chemically bound to food molecules

• the amount of unbound or available water determines weather microbes can grow

• most foods have water activity above 0.95 and that will provide sufficient moisture to support growth of bacteria, yeasts, molds

Microbial Growth and Aw

• most fresh food: Aw>0.99

• lowest aw for pathogen growth: 0.86 (S. aureus)

• How S. aureus does not make toxins below aw of 0.93

• 
  

Changes in aw within Food

• drying: water activity is decreased by physically removing water (jerky)

• solutes: water activity is decreased by adding solutes such as salt or sugar (jams)

• freezing: water moves outside of the cell, solute conc. within the cell increases

• combination: one or more of the above can be combined for a greater influence on water activity

Oxidation - Reduction Potential

• loss of electrons - oxidized (aerobes)

• gain of electrons - reduced (anaerobes)

• note: when electrons are transferred from one compound to another, a potential difference is created between the 2 compounds, this difference is expressed in millivolts (mV)

Antimicrobial Constituents

• naturally occuring substances that possess and express antimicrobial activity

• essential oils: cloves, garlic, thyme, rosemary

• cow milk - lactoferrin, eggs - lysozyme

Biological Structures

• the natural covering of some food provides excellent protection

  • outer covering of fruits and veggies, shell of nuts, shell of eggs

  • livestock: skin, feathers, hide

Extrinsic Factors

• factors in the environment and external to the food, which affect both the microorganisms and the food itself during processing and storage

• temperature

• relative humidity

• gases in the environment

• presence of other microorganisms

Temperature - Extrinsic

• phychrotrophs

• mesophiles

• thermophiles

• thermoduric

Relative Humidity - Extrinsic

• can change aw within foods and the growth of microorganisms

• food with low aw ———— storage environments low RH

• foods with low aw —— storage environments high RH: pick up moisture until equilibrium is restored

Gases in the Environment - Extrinsic

• CO2 and O2 most important gases in modified atmosphere package (MAP)

• ozone (O3): strong oxidizing agent

• MAP and O3: extend shelf life

Hurdle Technology

• fundamental of food preservation:

  • controlling microbial spoilage, food poisoning, fermentation processes

• hurdles must keep the normal population of microorganism in food under control

Injured Cells and VNC

• injury is defined as the inability of cells exposed to sublethal stress to grow on selective media, while retaining culurability on nonselective media

• viable but nonculturable (VNC) cannot be cultured on any medium

• injured and VCN cannot be detected by colony formation

• they can make people sick and can be recovered by standard methods once they recover from the injury