Food safety tips

What to watch out for at the supermarket

• Frozen chicken that's slightly soft to touch and has begun to thaw. If it looks 'dented', it may have been thawed and refrozen.
• Frozen vegies (such as peas or corn) which are frozen into a solid mass. They’ve probably been semi-thawed and refrozen. (Spinach is the exception.)
• Frozen food cartons that are wet, damp or sag in your hand.
• Frozen foods that are stacked above a horizontal freezer’s 'load line' (usually a blue or black line with the words 'load limit' written above it). They may be at too high a temperature.
• Swollen cans, or dairy products (like yoghurt) with the foil cover bulging. They may be going off.
• Leaking cartons, cans, bottles or other containers.
• Products with broken or imperfect seals or badly dented cans.
• Vacuum-packed products that  aren't tightly packed, with the packet loose around the product.
• Mould on foods such as cheese and crumpets.
• Check use-by dates and dates of packing, particularly on meat and dairy products.
• If you find frozen or chilled foods left standing unattended in the aisle, go to another store.
• Shop last for chilled and frozen food and pack meat products separately. 
• If you find things wrong in your local store report it to the store manager. If they don't take action, shop elsewhere next time and let your local authority know.

A: Mould on cheese can be tricky – some, like blue cheeses and the rind on camembert, are obviously meant to be mouldy. But when it comes to small mould patches on normally-not-mouldy hard varieties, you need to be able to cut away at least a 1cm chunk under and around the mould because it may have penetrated further than you can see. 

Always throw away any food that's very mouldy. Cutting or scooping the mould off soft cheeses is very risky – its invisible threads can go well below the surface. It's safest to chuck them, as the moulds can produce toxins that can damage your liver, kidneys and immune system.

A: The same goes for all soft foods like these, other processed foods and soft fruit and vegies – invisible mould can go deep and any toxins produced can be dangerous: the safest place is the bin. Hard fruit and vegetables with small patches of mould are probably OK to use if you follow the 1cm rule. Many people say they've been cutting the mould off bread their whole lives with no ill effects. It's true that in the case of bread the risk is small because you'd have to eat a lot of bread mould to do much damage, but it's still better to throw it out. Toasting the bread may kill the mould, but won't do anything to toxins that have already been made.

A: If the can is still in good condition – no rust, corrosion, dents, holes or swelling – it's probably fine. Some canned foods can last for years, though planning to use them within 12 months will mean you'll eat them at their best (and baby foods should be used within 12 months). The older a can gets the more likely the contents will start to lose quality – flavour, texture, colour, aroma or nutritional value.

A: You have to use a bit of common sense with dents – small dints in otherwise new and undamaged cans may be fine. However, cans that are damaged enough to let in air are extremely dangerous, and even a small dent could be enough to damage the seams where the can is sealed. Since you can't tell for sure, it's safer not to eat from a dented can. Rusting can happen when cans aren't stored in dry enough conditions, and a small amount probably won't damage the can enough to affect the contents. If there's a lot of rust it's safest not to use it. Food from a bulging can should definitely not be eaten.

A: The whitish spots are freezer burn – dry patches caused by air getting inside the wrapping. It won't make you sick – you can just cut the spots off either before or after cooking. But as for the prehistoric nature of the chops, as a general rule only keep lamb chops for about six to nine months in the freezer (beef steak about six to 12 months, an on-the-bone roast for six months, pork chops four to six months, mince or diced meat two months, sausages only one to two months and bacon one month). Always date-mark things as they go into the freezer and try to check the dates regularly.

A: Apart from the yuck factor (which rules this out for most people, but clearly not for those who asked us this question), there are good reasons not to use flour or other dry goods that have been infested with weevils, moths and the like. Some insects can produce harmful chemicals, and a heavy infestation can also encourage mould growth, which in turn can produce dangerous toxins. So the bottom line is: if it moves, chuck it out.

A: It's not so much a question of how long you can leave it in the fridge (a couple of days is probably OK), but more about what happened to it before you put it there. Raw rice can contain bacteria called Bacillus cereus, which can survive cooking and can cause serious food poisoning. If cooked rice is left too long at room temperature or in a too warm fridge, the bacteria can multiply. B cereus produces toxins that aren't killed by reheating, so reheating dodgy rice won't help. So, if you're storing cooked rice, cool it quickly, keep it in the fridge – and make sure your fridge is cold enough (4°C or lower).

A: They're just suffering from dehydration (water loss) so they're safe to use. Revive them in cold water for an hour or so, and just watch out for any mouldy bits that may be a problem. You need to be able to cut away at least a 1cm chunk under and around the mould because it may have penetrated further than you can see. Old vegies will have lost some of their nutritional value, though.

A: As long as it's a large, unpeeled piece, and the slimy bit was only a minor part of the whole, you should be able to safely cut it off. Be generous when cutting away, as the rotting part could leave unpleasant flavours behind in parts that still look OK. And eat it promptly, then and there – don't turn it into fruit salad for later or take it for lunch, for example. However, there are exceptions to the cut-the-slime-off rule: pre-cut, pre-prepared foods, like fruit salad or bagged lettuce, and, in particular, slimy bean sprouts. The slime on these foods may contain dangerous bacteria.

A: The ham is slippery because there are bacteria multiplying on its surface. You've no idea what they are, so treat slime as a warning and bin it.

A: This is just from temperature changes – the cocoa butter and sometimes the sugar start to separate from the chocolate. It won't hurt you and you may not even notice it when you eat it, but once it's gone too far (you'll know when you try some) the chocolate will have a very strange and not-very-pleasant grainy texture.

A: Food in a doggy bag has likely had a tough time. It's been sitting around on your plate cooling down, it's been contaminated by you while you were eating it, and then it's been carried home before finally being put in the fridge, so it is a risk. Experts advise that if you are going to risk it, make sure it spends no more than four hours between the time it first landed on your plate and when you get it home into the fridge, which could be tricky – and it should always be reheated really well.

A: Bottled water is likely to be safe to drink a few days after opening, as long as the cap was on and you hadn't drunk directly from the bottle. Drinking directly from the bottle is likely to contaminate the water with bacteria from you mouth. So if you don't plan to drink the whole bottle within a few hours, it's better to pour what you want into a cup, then store the bottle in the fridge.

A: Leftovers should be eaten or frozen within two to three days of cooking, so it would be safest to throw it out. After about four days, potentially harmful micro-organisms may have reached risky levels. The two to three-day rule assumes your sauce was cooled quickly after cooking and then kept in a clean, covered container in the fridge (which was 4°C or less.). If you haven't eaten it within two or three days you can still freeze it and reheat it – thoroughly – later. But once it's been reheated, any leftovers should be binned.

A: It depends. Small pieces of meat, fish or poultry may have completely thawed, and should not be frozen again without cooking. If they feel chilled to touch, you could refrigerate them and use them quickly – that day or the next. If they don't feel chilled, it would be safest to throw them away. Larger items of these foods can be re-frozen, provided they still are mainly frozen and the defrosted bits still feel chilled. Some items may not be as nice on second chilling – ice cream, for example. If in doubt, throw away the food, especially if it's a hot day. A cool bag kept in your boot for frozen foods is a good idea – and the super-organised might like to use frozen ice bricks, too.