Seasonal Food Safety

  12/12/2018 14:34:58

At this time of year there are many types of food that are especially popular such as: turkey, mince pies, Christmas pudding, roast potatoes with a variety of seasonal vegetables, soft and hard cheeses, crackers and biscuits and not forgetting cakes, chocolates, toffees and sweets. Ironically the foods that are the safest – microbiologically – include those that have the most sugar! The reason for this is generally due to low water activity.

Water activity (a_W) is the measurement of how tightly water is bound within a substance. This can be chemically or structurally. It is basically a way of measuring what amount of water is available in a food substance. The more free moisture available the more likely the possibility of microbiological growth.Food substances with lower water activity levels are often stable under ambient conditions and have longer shelf lives. Highly perishable foods tend to have high water activity levels such as meat, fish, milk and fruits and vegetables. This makes these types of foods more susceptible to food spoilage and allowing growth of pathogenic 
 

Food Type	aW
Distilled Water	1.00
Fresh meats	0.95-1.00
Fruits/Vegetables	0.97-1.00
Bread	0.95-1.00
Cheese	0.68-1.00
Sponge Cakes	0.87
High Sugar / Fruit Cakes	0.80
Dried Fruits & Nuts	0.60-0.65
Nougats, Fudge, Marshmallow	0.40-0.65
Caramels, Toffee, Honey	0.45-0.60
Cookies, Crackers	0.20-0.30

Generally there is no microbiological proliferation below 0.60 a_W; with the limit for bacterial growth around 0.86 a_W and for Yeasts and Moulds down to 0.70 a_W.

Food preservation techniques to lower the water activity include adding sugar (e.g. jams, sweets) or salt (e.g. meats and fish); drying (e.g. fruits, vegetables). Where these types of techniques are not suitable, alternatives such as sterilisation (e.g. canning), freezing or refrigeration can be used.
 
Chilling food properly helps stop harmful bacteria from growing; the coldest part of the fridge should be below 5°C with good air circulation to maintain the cold temperature throughout the fridge and around the items in it. Freezing food is an excellent way to maintain food safety – stopping bacterial growth – without the food deteriorating (providing it is additionally wrapped to prevent cold air from drying the food out!). The bacteria aren’t killed by freezing but will be prevented from growing and producing toxins. On defrosting the organisms present may revive and resume their growth cycle, therefore it is important to refrigerate defrosted foods as soon as possible or ideally defrost in a fridge.
 
Fresh meats like turkey tend to be bought fresh at Christmas but can often be bought frozen to defrost at home. Ideally the turkey should be fully defrosted in a refrigerator, although a cool room can be used instead. FSA guidance is 10-12 hrs/kg at 4°C or 3-4 hrs/kg in a cool room at <17.5°C. Partially defrosted turkey may not cook evenly under the normal cooking instructions – resulting in harmful bacteria surviving the cooking process. If cooking from frozen, follow the manufacturers recommended guidelines as these will have been validated to ensure the food is safe.

It is unnecessary to wash your turkey before cooking and doing so can actually increase the risks of cross-contamination by splashing bacteria onto worktops, hands and even clothes. Whether you cook your turkey from frozen or fresh, always make sure it’s steaming hot, there’s no pink meat visible when cutting into the thickest part and juices run clear. Leftovers should be refrigerated or frozen within 1-2hrs and if refrigerated, eaten within 2 days to ensure it remains safe to eat.
 
Lastly if you are throwing or attending a Christmas party or buffet – ensure any items requiring cooking are checked to be fully cooked before serving and avoid preparing too far in advance. Legally it is recommended that hot food should be served and maintained above 63°C; if the temperature is less than this the food should not remain out beyond 2 hours before refrigerating or discarding. For chilled foods, these can remain out up to 2 hours.
 
Therefore, if you race to the front of the buffet queue (like me) and tuck into the buffet as soon as it is out and open – you are actually getting to and consuming the buffet food at the safest point!!
 
So to finish – My top tips for food safety at this time of year:
 

• Keep food refrigerated but don’t overfill the fridge and consume within use by date.
• Best before dates on ambient foods are quality guidelines and food will still be microbiologically safe – so those mince pies and chocolates don’t have to be all consumed before New Year!
• Chocolates, sweets and toffees are good for you (microbiologically speaking!).
• Get to the buffet first at the party!

 
Kirsty Holmes, Microbiologist, QIB Extra.

QIB Extra provides microbiological expertise and contract research to the food industry and allied services. For more information see our main page.