SINGAPORE - Using the food products prior to the expiration date does not guarantee the safety of a food if it is not stored properly, and a product is not necessarily unsafe after the expiration date if stored properly.
Associate Professor Lim Bee Gim, programme director of chemical engineering and food technology at Singapore Institute of Technology, tells Mind&Body editor Ng Wan Ching what to watch out for when it comes to food safety.
Most expiration dates on food products are used as guidelines based on normal and expected handling of the food and its exposure to temperature.
There are many factors that could affect the storage life and the quality of food product, said Assoc Prof Lim.
The most important factors will be the temperature and humidity at which the product is stored before and after purchase.
After the "use by" or "best before" date has passed, a gradual change may be noticed in the unopened product's texture, colour or flavour.
Assoc Prof Lim gives some practical tips on how to extend the shelf life of food and to ensure its safety and quality.
- Buy food from established and reliable stores.
- Choose food that has not passed its expiry date, as shown on the packaging.
- Do not buy chilled or frozen food that is displayed at room temperature.
- Pick up refrigerated and frozen foods just prior to check-out. Refrigerated foods should be cold and frozen foods should be solid with no evidence of thawing.
- Handle food items gently. Improperly packaged foods, dented cans and broken packages provide places for micro-organisms, air, light and creatures to enter. Gentle handling of food items will help maintain food quality and safety longer.
- Buy in reasonable quantities, especially when it comes to perishable foods, to help avoid long-term storage.
- Practise Fifo (first-in-first-out) to ensure food stored in the refrigerator, freezer or pantry is consumed within the expiration dates. When stocking food, place recently purchased items behind the existing food items.
- Air, moisture, heat and light hasten spoilage for most shelf stable products. So keep these foods in a cool dry cupboard away from sunlight.
- Cereals, nuts and biscuits may be stored at room temperature in tightly closed containers to keep out moisture and insects. They can also be stored in the fridge. This will slow microbial growth, the enzymatic and oxidation processes, for example, retard rancidity of the natural oils, which are the main culprits for food spoilage and deterioration.
- Before consuming a food item, ensure its smell, flavour or appearance are not "off".
When in doubt, throw it out.
In addition, she offers more tips applicable to selected categories of food.
These have high protein and water content and are very perishable. They are easily spoilt by bacteria. Refrigeration slows down the biological, chemical and physical reactions that shorten the shelf life of food.
Proper refrigeration - from the processing plant to the kitchen - is critical for maximising the lifespan of milk. Keeping milk on the table or in a refrigerator that's above 4 deg C promotes bacterial growth and accelerates deterioration.
Grab the milk just before you are ready to pay, and transfer the milk to a refrigerator or cooler as quickly as possible, especially here where the weather is hot.
Store milk on an interior refrigerator shelf rather than in the fridge door, where the temperature fluctuates with frequent opening.
Remove milk from fridge only long enough to take what is to be used and then quickly return to the fridge.
Milk should be kept in its original container. Keep the container closed to avoid rancidity and microbial contamination.
Never drink milk directly from the container because bacteria in the mouth can be washed back into the product.
Don't pour unused milk back into the container because bacteria contamination could have occurred from exposure to the air.
Some common traits of bad milk are discoloration, lumpy texture and a very distinctive unclean sour smell. These changes occur because of lactic acid producing bacteria.
People vary in their susceptibility to bacterial infections. If you happen to be particularly susceptible to bacterial infections or belong to the very young, old and immuno-compromised groups, you should perhaps cut a day or two from the expiry date and proceed with more caution.
This is because some bacteria don't produce noticeable smells so you can't assume the milk is safe even if it passes the sniff test.
Store tinned food in cool dry places with no direct sunlight. Do not allow sealed cans to freeze. Freezing changes food textures, and leads to rust, burst tins and broken seals that may let in harmful bacteria.
If tinned goods are stored in ideal conditions, you can significantly increase the product's shelf life. A better way to tell if tinned foods are still safe to eat, than any arbitrary date printed on the packaging, is to use the sight and smell test.
Is the tin dented, rusted or bulging? These are signs that the food item contained in the tin has become compromised. Tins that spurt liquid upon opening is another sign that the food has become compromised.
A bad odour is one of the biggest indicators for spoilt food. It will let you know in most cases if the contents in the tin are safe to be consumed.
All fresh fish and shellfish are highly perishable and precautions need to be taken to ensure freshness. Only buy fresh seafood from reputable dealers who keep their products refrigerated or properly iced.
Buy fresh or frozen seafood at the end of your shopping trip. After purchasing, refrigerate or freeze your seafood as soon as possible. If your trip is longer than 30 minutes, place your seafood in a cooler with plenty of ice.
Storing it properly is also important in ensuring its overall freshness and reducing the risk of food-borne illness.
Once cleaned, fish should be stored in the coldest portion of the refrigerator. It should also be tightly wrapped to prevent its odours from coming in contact with other foods.
Fresh fish is best consumed within a day or two of purchase.
The flesh of fish is much more perishable than animal tissue as most raw seafood carry some bacteria, which multiply rapidly at temperatures above 4 deg C.
Freezing greatly extends the shelf life of fish, for up to six months.
Do not refreeze the fish once it has been thawed.
Frozen fish should be kept in airtight packaging in order to prevent them from drying out.
It is a good practice to eat fresh shellfish the day they are bought.
If they must be kept, live shellfish should be refrigerated in containers covered with clean, damp cloths. These containers should not be airtight. It is best to discard shellfish, such as crabs, mussels, lobsters, clams and oysters, if they die during storage, or if their shells crack or do not respond when tapped.
Bread and cake
The shelf life of these products depends on a number of factors, including the ingredients used, packaging, storage container, and the temperature and humidity of the storage location. Microbial spoilage (typically mould growth) is often the limiting factor in the shelf life of bread.
Packaged bread may remain fresh for five to seven days at room temperature when stored in a cool, dry place with minimum exposure to the direct sunlight and humidity.
Improper handling and storage of bread may encourage mould growth before the end of expiry date.
Bread stored in the refrigerator will have a longer shelf life and can be kept for one to two weeks. Preservative-free bread such as organic and fresh-baked varieties usually won't last as long as packaged bread. It should generally be consumed or frozen within a few days of purchase.
Refrigerators are usually the most appropriate places to store cakes. Most kinds of cakes are safe to eat for up to four days if they are stored in the refrigerator.
However, if the cake is unfrosted, it should be safe stored in an airtight container or wrapped with plastic wrap to keep out air and other contaminants. Bakery items containing custards, meat or vegetables, and frostings made of cream cheese, whipped cream or eggs must be kept refrigerated to avoid the growth of bacteria which can result in food-borne illness if consumed.
No matter how long your bread or cake has been stored, if it develops mould or smells sour, do not eat it.
Cooked food such as a roast chicken needs to be handled in the same way a cake with frosting made of whipped cream or eggs.
It's advisable not to leave cooked food or other perishable food at room temperature for more than two hours because bacteria grow rapidly at temperatures between 5 deg C and 60 deg C.
If the food is not to be consumed within two hours, it should be stored in the refrigerator to prevent the growth of micro-organisms.
Cooked food such as a roast chicken can be kept in the fridge for two to four days, or up to six months if it's frozen.
The best way to tell whether the roast chicken is still good is by smelling and checking the appearance of the meat. If it has a sour smell and slimy texture, it shows that the meat has gone bad and should be discarded.
Fruit and vegetables
Generally, fresh fruit and vegetables need low temperatures and high relative humidity to reduce respiration and loss of moisture, slow down metabolic processes and thus maximise storage life and maintain quality.
Low temperatures also slow the growth of pathogenic fungi which cause spoilage of stored fruit and vegetables.
Therefore, refrigeration is critical for most fruit and vegetables. However, the ideal storage conditions can vary for each fruit and vegetable category. These conditions can generally be classified as below:
- Cold (4 deg C) and moist (95 per cent relative humidity) - grapes, berries, apricots, cherries, broccoli, cauliflower, lettuce, spinach.
- Cool (10 deg C) and moist (95 per cent relative humidity) - eggplants, tomatoes, cucumbers, apples, mangoes and citrus fruit. For these produce, cooler temperatures interfere with normal metabolic processes. The eggplant, for example, will develop surface scald if stored below its minimum safe temperature of 8 deg C.
- Cool, dry and dark place in a well-ventilated area of the pantry - garlic, onions, pumpkins and sweet potatoes. However, onions and potatoes should not be stored next to each other, because they shorten each other's shelf life.
As some fruit and vegetables ripen, they release ethylene, a colourless and odourless gas that induces rapid ripening and softening of a lot of fruits.
Ethylene also causes several kinds of vegetables, such as spinach, broccoli, cucumbers and celery, to yellow.
In a process known as abscission, cabbage and broccoli will drop leaves upon ethylene exposure. Furthermore, iceberg lettuce may begin developing small brown spots in the midribs. The gas also causes most fruit to have increased susceptibility to rot and moulds and shorten storage life.
To slow down these effects, it is important to keep ethylene-sensitive fruit and vegetables, such as broccoli, cabbage, cauliflowers, lettuce and other leafy green, separate from ethylene-producing varieties such as apples, apricots, kiwis, nectarines, peaches, plums, mangoes.
One possibility is to store the ethylene-producing varieties on the counter when they are unripe. Once they have ripened, refrigerate them. To prevent possible loss of moisture, store these fruit separately in paper bags or perforated plastic bags on the counter away from sunlight and heat. After ripening, store in the refrigerator and consume within one to three days.
Generally, fresh meat should not be stored in the refrigerator longer than three to five days. The cool temperature in the refrigerator (4 deg C) slows down bacterial growth but it will not stop the growth completely.
It is thus important to use food in a timely fashion to help maintain freshness and quality. If you would like to keep the meat longer, wrap the meat tightly in aluminium foil, heavy plastic bags or freezer paper and store in the freezer.
The shelf life of meat can be extended for up to six months in the freezer.
It is better to make more frequent purchases than to freeze meat for extended periods of time, which can affect its quality.
Freezer burn, caused by loss of moisture from the frozen food's surface, can result if meat is stored longer than the recommended storage time or when it is not wrapped properly. Vacuum packaging can also aid in reducing freezer burn.
Always shop for meat just prior to check-out. Use an insulated shopping bag that helps to keep meat cold. If you need to travel for more than 30 minutes before reaching home, place your chilled and frozen purchases in an insulated cooler.