Food Safety
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Spring is in the air! 

Which means ambient temperatures will be on the rise. What does this mean in terms of keeping your food safe?


Most microorganisms which can cause illnesses in humans have optimal growth at temperatures from 25-37°C - the same temperature range of a typical summer's day! This means that consumers need to be extra precautious when handling and transporting perishable foods outside of the fridge or freezer, as the higher outside temperatures can cause food to warm up quicker once taken out of the fridge or freezer.  

Perishable food is food that only lasts a few days in the refrigerator, such as lettuce, grapes and cheese.

Most harmful microorganisms are not able to grow and multiply at refrigerated temperatures (your refrigerator should be below 5°C). This is why it is important to keep perishable foods as cold as possible, so that harmful microorganisms cannot multiply to reach the required 'infective dose', which is the amount of bacteria required to successfully cause illness in the average person. For example, Shigella only require about 10 cells to start an infection, whereas S. aureus can require up to 108 cells.​​

The table gives an estimation of how long perishable food will remain safe to eat when stored at certain temperatures and what temperatures must be reached to kill harmful microorganisms.

Death of bacterial spores
75°C and aboveDeath in a few seconds
65°CDeath in a few minutes
55°CDeath after a few hours
±21-50°CDangerous in a few hours
±13-21°CDangerous in several hours
5-13°CDangerous in a few days
1-4°CDangerous in a few weeks
<0°CMicrobial growth slowed

What can you do?

  • ​When grocery shopping, select the frozen and refrigerated food items last. This minimises the time the food spends outside of the fridge or freezer. Take a cooler bag to transport the perishable food in.
  • Minimise the time spent between the grocery shop and your fridge or freezer at home.
  • If you go on a picnic, make sure all perishable food is transported in a cooler bag with ice packs to keep its temperature as low as possible for the duration of the picnic. Keep the cooler bag under the shade!
  • When thawing frozen food, do it in the refrigerator and not at room temperature.

Learn more about keeping your food at the correct temperature:


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​ ​
How do you grow bacteria in the lab?

When a scientist wants to know what kind of bacteria is in a sample, they pour a very small amount of their sample onto a petri dish which contains selective agar. The selective agar contains various ingredients which allows some types of bacteria to grow, while other types are not able to grow. These specialised formulations are based on the specific growth parameters of each bacteria, which differ among different types of bacteria. Together with this selective agar and specific growth conditions such as temperature and oxygen availability, only certain types of bacteria end up growing on the selective agar from a sample which originally contained many different types of bacteria.

For example, the picture shows Escherichia coli colonies (the tiny pink-red dots) grown on a petri dish containing Violet Red Bile Glucose agar. A colony of bacteria, typically about 0.5-5mm in diameter, is a group of bacteria cells which is visible to the naked eye and contains between one million to one billion bacterial cells.

Ecoli_petridish.jpgThis agar is specific for a group of bacteria called Enterobacteriaceae (Gram-negative), which E. coli belongs to. All species in the Enterobacteriaceae family ferment glucose to produce an acid, which turns the colour indicator to a red-purple colour, producing the red-pink colonies (E. coli aren't naturally coloured pink!). 

Scientists can work out how much bacteria is in a sample by knowing how much sample they added to the petri dish and by counting how many colonies have grown on the plate. The amount of bacteria present in a sample is described as colony forming units per gram of sample, CFU/g for short.

Although using selective agar gives a good indication of whether a certain bacteria is present or not, additional confirmatory tests are recommended to confirm the type of bacterial species indentified.

Typical ingredients of Violet Red Bile Glucose agar and their functions:

Yeast extract & peptone​To supply carbon, nitrogen, essential minerals, and B-complex vitamins to stimulate the growth of bacteria
Sodium chlorideSupplies essential electrolytes for transport and osmotic balance
Bile salts & crystal violetInhibits the growth of Gram-positive bacteria
Glucose All species in the Enterobacteriaceae family ferment glucose
Neutral red pH indicator
Agar Solidifying agent

Learn more on how it's done:


​​The importance of pasteurising raw milk.

Unpasteurised milk (raw milk), and other diary products which contain raw milk, can contain dangerous microorganisms, such as Salmonella, E. coli, Listeria and Campylobacter, which can cause serious illnesses.

Milk is a highly nutritious food but also provides an ideal environment for microbial growth due to its high nutrient content, near neutral pH and high water activity.

Raw milk has a very short shelf-life due to the presence of bacteria and enzymes. Pasteurisation makes it possible to distribute milk to the public and reduces the risk of consuming pathogenic bacteria.

Raw milk, as secreted by healthy cows, generally contains low numbers of microorganisms (± ≤ 1000 total bacteria per ml). Microorganisms, whether pathogenic or not, can enter the raw milk either from the animal's teat or from other external contaminants including the animal's skin, soil, bedding, manure, milking equipment and during transport and storage. Therefore, collected raw milk can contain levels of a few to several thousands of bacteria per ml, depending on animal health and farm hygiene. 

The process of pasteurising is based on the simple fact that heat kills microorganisms. Pasteurisation is a process that heats a food product to a certain temperature for a certain amount of time. Milk is typically pasteurised at 72°C for 15 seconds, known as 'High Temperature Short Time' (HTST) or at 135°C for 2-5 seconds, known as 'Ultra-High Temperature' (UHT).

This type of processing does not have any significant negative effect on the nutritional quality of milk. The heat treatment during pasteurisation does not affect mineral content, carbohydrates or fats. With regards to proteins, caseins are heat stable and are thus not affected. Whey proteins are not affected by HTST but UHT temperatures have shown to cause some damage to the heat sensitive amino acids of whey proteins but does not have an overall significant effect on protein quality.

There is a small reduction of some vitamins due to their sensitivity to heat during pasteurisation. Vitamin B1 is typically reduced from 0.45 to 0.42 mg/L, vitamin B12 from 3.0 to 2.7 μg/L and vitamin C from 2.0 to 1.8 mg/L. Other vitamins, such as B2, B3, B5, folic acid, A, D, E, K  are not significantly affected.

Pasteurisation also inactivates some enzymes present in milk which are sensitive to heat. This helps to extend the shelf-life of the milk as some enzymes cause degradation of the milk. For example, the enzyme lipase degrades fats, resulting in off odours and flavours. On the other hand, the enzyme lactoperoxidase is heat-stable and is not destroyed during pasteurisation.

The enzymes present in milk do not make a major contribution to the digestion of milk, as they are not the same enzymes used by humans required to digest the major components of milk; which are complex sugars, fats and proteins. These enzymes which digest the components of milk are naturally present in the human stomach and small intestine. Those who are lactose intolerant, do not make enough of the enzyme lactase which is needed to digest lactose and thus the side effects of indigestion occur.​

Learn more about the pasteurisation process: