What we do not see: The importance of microbial food safety

The Impact of Microorganisms in Food

The food we eat is rarely, if ever, sterile; it carries various microorganisms whose composition depends on which organisms gain access to the food and how they grow, survive, and interact throughout the entire supply chain. The microorganisms present will originate from the natural microflora of the raw material and those organisms introduced in the course of harvesting/slaughter, processing, storage, and distribution. The numerical balance between the various types of microorganisms will be determined by the properties of the food, its storage environment, the properties of the organisms themselves, and the effects of processing.

Microorganisms in food manifest their presence in one of several ways:

1. they can cause spoilage
2. they can cause foodborne illness
3. they can beneficially transform a food's properties via food fermentation (3)

For most people (healthy adults), food poisoning might cause a mild illness. However, some foodborne infections by microorganisms in food can be serious or even fatal. People at higher risk, including children, older adults, pregnant women, and those with weakened immune systems, are more likely to experience severe illness and may require hospitalization. In some cases, food poisoning can lead to long-term health problems or even death. 

We can avoid food poisoning by understanding food safety and applying good food hygiene practices in our homes. Proper storage and handling of food are crucial to avoiding food poisoning.
Relevant microorganisms and microflora of different food categories:

Natural Microflora of Different Food Categories

Milk

Milk has a high water activity, moderate pH, and plenty of nutrients, which make it a great medium for
microorganisms to grow. As a result, milk requires high hygiene standards in its production and
processing. The microorganisms found in milk are naturally present in the cow, especially in the udder and its surroundings. Additionally, milking equipment and milk handling practices also contribute to milk microflora.

After the lactation, the milk in most developed countries is chilled almost immediately and kept at a low
temperature from that point onwards. The milk is kept below 7 oC after that, and the only microorganisms that can grow, including pathogens, are Psychotrophs, which need to be absent from the milk to ensure food safety. There are many psychotropic species; however, those most commonly found in raw milk include Gram-negative rods of the genera Pseudomonas, Acinetobracter, Alcaligenes, Flavobacterium, psychotropic coliforms, predominantly Aerobacter spp., and Gram-positive Bacillus spp.
(3).

Meat

Raw meat is a perishable product due to many of its characteristics, such as abundant nutrients, high moisture content corresponding to aw around 0,99, and pH around 5-7. In big pieces of meat, anaerobic microorganisms can only grow in the internal part. In contrast, on the surface, both aerobic and facultative anaerobic microorganisms can grow (2). Though many microorganisms that grow on meat are proteolytic, they grow initially at the expense of the most readily utilized substrates- the water-soluble pool of carbohydrates and non-protein nitrogen (3). The microflora of raw meat produced under hygienic circumstances consists mainly of Gram-negative bacteria of the genus Acinetobacter, Aeromonas, Alcaligenes, Flavobacterium, Moraxella, Pseudomonas and species of the family Enterobacteriaceae (mainly Escherichia coli, Serratia liquefaciens, Pantoea agglomerans, Citrobacter freundii, Klebsiella pneumoniae, Enterobacter cloacae και E.hafniae) but also species of genus Enterococcus, Bacillus και Clostridium, lactic acid bacteria, the genus Brochothrix thermosphacta but also yeasts and fungi. Dominant microflora of raw meat when stored at temperatures around 0oC and at a high relative moisture (85-95%), are bacteria of the genus Psychrobacter, Pseudomonas, Acinetobacter και Morella. Raw meat can contain pathogens resulting from contamination of the animal's gut. These pathogens typically include Salmonella, Staphylococcus aureus, Clostridium perfringens, and Escherichia coli (2).

In comparison with individual pieces of meat, minced meat carries a higher microorganism population for the following reasons:

• It has a bigger surface for the growth of aerobic bacteria, which are the most important microorganisms that contribute to the spoilage/degradation of the product at low temperatures.
• The cutting equipment is a significant source of contamination with coliforms, enterococcus, staphylococcus, and micrococcus. 
• The microorganism population of minced meat increases as the fat content increases. In general, minced meat carries a 10-100 times bigger microorganism population than individual pieces of the carcass it comes from. In freshly cut minced meat, the microflora is composed mainly of Micrococcus, Lactobacilli, Pseudomonas, and bacteria of the family Enterobacteriaceae. Salmonellas are usually isolated occasionally from pork minced meat. The most important pathogens that can be found in minced meat are Clostridium perfringens (2).

Fish

The kind and number of microorganisms found in fish are greatly influenced by the water quality where the fish live and how the fish are handled, processed, and stored. Usually, the microflora of fish consists of
bacteria of the genus Moraxella, Pseudomonas, Acinetobacter, Flavobacterium, and Shewanella. Clostridium perfringens is found in a small number of fish, while Listeria monocytogenes has been found in soft shells. Additionally, depending on the water quality from which they are caught, Salmonella can be found, especially S.typhimurium or Vibrio parahaemolyticus in soft fish. Fish skin, gills, and the gut are the main locations where the fish microflora can be found. The fish’s flesh is considered to be sterile. The bacteria that cause the spoilage of fish belong to the genera Pseudomonas, Acinetobacter, Shewanella, Aeromonas, Enterobacter, Enterococcus, Escherichia, Flavobacterium, Moraxella, Psychrobacter, and Vibrio. The fungi that cause fish spoilage belong to the genera Aureobasidium, Aspergillus, and Penicillium. The yeasts that cause fish spoilage belong to the genera Candida, Cryptopoccus, and Rhodotorulla. Pseudomonas, Acinetobacter, and Moraxella, which grow at temperatures 0-1°C, are the main bacteria that cause spoilage to fresh fish. Many Pseudomonas species can cause fish spoilage at a slow pace, even at a temperature of -3°C. Meanwhile, yeasts and molds spoil the salted and dehydrated fish due to low water activity. Microorganisms like Photobacterium phosphoreum and Shewanella putrefaciens usually cause spoilage of fish that are packaged in modified packaging or in vacuum packaging (2).

Fruits and Vegetables

The types of microorganisms found on vegetables depend on the cleanliness of the original plant and how the vegetables are handled from the farm to your table. Bacteria are the main microorganisms that cause vegetable spoilage due to the pH and the high humidity of vegetables. The main bacteria that spoil vegetables belong to the genus Erwinia. The bacteria that cause sepsis in vegetables are mainly Pectobacterium carotovorum and follow some Pseudomonas, like Pseudomonas marginalis. Bacteria like the genus Bacillus and Clostridium have a secondary role in the spoilage of vegetables.

Bacteria of genus Erwinia and Pectobacterium produce pectin enzyme, which causes the softening and sepsis of the vegetables. Those bacteria dominate the microflora of the vegetables because of their fast-paced growth, and they cause spoilage of the vegetables before fungi can even grow. Fungi are a significant factor in the spoilage of vegetables. The fungus Botrytis cinerea grows in vegetables with high moisture content and high temperature and produces a grey mycelium. Vegetables can also get contaminated with the fungus Geotrichum candidum from the ground/soil and the fungus grows in the spots of injury to the skin of fruits and vegetables. The fungi Rhizopus stolonifer is transferred to vegetables with insects and goes into the product from the injured parts of the vegetable skin. The fungi produce a cottony mycelium, with small black spots that cover the vegetable. Despite the high water activity of most fruits, the low pH leads to their spoilage being dominated by fungi, both yeasts and molds, but especially the latter. The main genus of fungi that are found in the fruits and cause spoilage in them are Penicillium, Aspergillus, Mucor, Alternaria, Cladosporium, and Botrytis. The yeasts that can be found in fruits belong to the genera Saccharomyces, Hanseniaspora, Pichia, Kloeckera, Candida, and Rhodotorula (2).

Recommendations to avoid spoilage of fruits from yeasts and molds:

• Avoid injury to the fruit surface. Many species of microbes come from contamination from the soil, the air, rainwater, and insects, which are located on the fruit surface. The yeast and mold populations are usually low.
• Thoroughly clean the surfaces that the fruits come in contact within the packaging factories, and also
  the boxes/crates they are being transported in.
• Thoroughly clean the fruits with drinking water. Just drinking water removes 99.9% of microorganisms from apples. Brushing and rinsing with chlorine water removes 95% of micros from oranges' surfaces. Immersion in water at 45-55 °C reduces many fungi spoilages in pears.
• Preserve fruits in chilled temperatures (2).

Ensuring Hygiene in Food Production Facilities and at Home

Here are a few simple but critical practices that can help reduce the likelihood of product cross-contamination and how to keep good hygiene standards in the factories and food production areas:

1. Having appropriate and sufficient hygiene measures (cleaning and disinfection) in place in the
   factory or food production area.
2. The personnel wearing the appropriate PPE (personnel protective equipment), such as dedicated production area work wear, gloves, etc.
3. Mandatory hand washing is required to enter and leave the production area after breaks.
4. Implementing and following a validated HACCP study. (3).

Here are a few good practices that you could follow to minimize the risk of cross-contamination in the
fridge and kitchen and thus reduce the risk of food poisoning at home:

1. Cleaning work surfaces properly.
2. Handle raw food carefully to avoid cross-contaminating other foods or surfaces.
3. By chilling food at the correct temperatures (0-5°C).
4. By cooking food thoroughly.
5. Use food and drinks by the use-by-date, even if they look and smell fine.
6. Wash your hands thoroughly before preparing, cooking, eating, and after using the toilet, changing nappies, or touching animals.
7. Cook food thoroughly and serve it immediately, or keep it hot (60 °C or hotter) before serving.
8. Cool cooked food quickly if it is going to be used later: put it in the fridge (or freezer) as soon as it stops steaming.
9. Divide large amounts of hot food into smaller containers to let them cool faster. Make sure your fridge is 5 °C or colder.
10. Keep leftovers in the fridge (or freezer) and dispose of refrigerated leftovers if not eaten within 3-4 days (or within 1 day for vulnerable people).
11. Wash your hands with soap and dry them before preparing or eating food.
12. Keep your kitchen and equipment clean.

References:

1. Food Standards Agency Website: https://www.food.gov.uk/
2. Book: Food Microbiology and Microbiological Analysis of Food, Kotzekidou P., 2009.
3. Book: Food Microbiology by Adams M. et al., 2016.

Further reading

Home Food Preservation