Flashcards in Food technology Deck (100)
What are the main intrinsic factors of food that affect the microbial behaviour?
- Nutrient availability
- pH and buffering capacity
- Natural antimicrobials
- Redox potential and poising capacity
- Food matrix
- Water activity (Aw)
What nutrients are required in order for microorganisms of food to grow?
- Source of energy
- Source of nitrogen
- Vitamins and related growth factors
Outline the water requirements of food microorganisms
- Essential for like
- Higher requirement for moulds
- Then Gram -ve, then yeasts, then Gram +ve
What are potential sources of energy for food microorganisms?
- Amino acids
- Fat (small number of bacteria)
What are sources nitrogen for food microorganisms?
- Amino acids
- Other nitrogenous compounds
Give examples of natural antimicrobial compounds in food
- Essential oils in some plant species (eugenol in cloves, allicin in garlic)
- Lactoferrin, conglutinin, lactoperoxidase system and a rotavirus inhibitor in cow's milk
- Lysosyme in eggs and milk
Outline how pH affects microbial growth in food
- Specific ranges in which different microorganisms grow best (most between 6.6-7.5, few below 4.0)
- Some foods better able to resist change than others e.g. meats better buffering capacity
Why is it useful to know the pH range of different microorganisms?
- Hazard analysis for different microorganisms being present in a certain food
- Some foods inherently resistant due to inherent acidity e.g. fermented milks, pickles
What is redox potential (Eh)?
- The ease with which the substrate loses (is oxidised) or gains (is reduced) electrons
- The potential difference measured and expressed as mV (milli-volts)
What are the specific redox requirements of anaerobic and aerobic microbes?
- Aerobic require positive Eh values
- Anaerobic require negative Eh values
What is the redox potential of a food determined by?
- Characteristic Eh potential of the original food
- Poising capacity of the food
- Oxygen tension of the atmosphere around the food
- Microorganisms effect on the Eh of the food
- The pH of the food
What is meant by Water Activity (Aw) of a food?
A measure of how efficiently the water present can take part in a chemical (physical) reaction
Outline the importance of Water Activity in food preservation
- Reducing water available for microbial growth important
- Can have high moisture but low water e.g. solution saturated with salt
- Below Aw 0.79, few pathogens are of concern
What is an important mechanical barrier of food to microbial growth?
Skin, often left on fermented meat products
What are the main extrinsic factors that affect microbial behaviour in foods?
- Relative humidity
- Heat (D-value, Z-value)
Outline what is meant by psychotrophs and give examples and their importance
- Grow well at or below 7degreesC
- Optimum between 20-30degreesC
- e.g. Listeria, Yersinia, can grow at refrigeration temperature
Outline what is meant by psychrophiles
Optimum growth temperature is 15degreesC or lower
Outline what is meant by mesophiles and their importance
- Grow well between 20-45degreesC
- Optimum between 30-40degreesC
- Most bacteria are mesophiles
Outline what is meant by thermophiles and their importance
- Grow well at an above 45degreesC
- Optimum between 55-65degreesC
- Important for food stored at high temperatures or cooked
What is the D-value?
Decimal reduction time, the time taken for the population to pass through a log cycle (i.e. 90% of population is killed) at a certain temperature.
What is D121?
The time required to kill 90% of a population of microorganisms at 121degreesC
What affects D-value?
- The pathogen
- The food type
Give an example of how food type can affect D value
Salmonella is 90x more resistant in chocolate vs milk, due to presence of fats and proteins in chocolate that protect the cell
How is the number of cycles at the D value determined?
- Need to remove 99.99% of the microorganism
- For spores need to get down to 0.1 spore/gram of food
What is the Z value?
The temperature change that is rquired to change the D value by a factor of 10
- i.e. treating food at 90degreesC for 30 min is the same as 100degrees for 3 min or 110degrees for 0.3min
Why might the Z-value not always be useful?
Increasing the temperature may affect the quality of the food or may damage the equipment
What are the main gaseous atmospheres used in food preservation?
- Carbon dioxide (aka modified atmosphere packaging)
- Vacuum packing
Outline the use of ozone in gaseous atmosphere packaging of food
- Strong oxidising agent
- Not used on high-lipid foods as would cause increased rancidity
Outline the relationship between relative humidity and gaseous atmosphere of a food
By altering the gaseous atmosphere, it is possible to retard surface spoilage without lowering the RH
Removal of oxygen increases preservation