Food changes as enzymes and microbes act on it. Some changes, such as a cut apple browning, affect appearance rather than safety. Others, including mould on soft food, a bulging tin or an unpleasant odour from meat, are reasons to discard it.
Don't taste a doubtful food to decide whether it is safe; harmful bacteria and toxins may not announce themselves through smell or flavour. Follow use-by dates and storage instructions for perishable products.
People who are pregnant, immunocompromised, very young or frail may need extra caution. When official guidance says to throw something away, reducing waste does not require taking the risk.
Discard mouldy bread, cooked grains, soft fruit and other soft or porous food because growth may extend beyond the visible patch. Do not simply cut the spot away.
Some hard cheeses can be trimmed generously around surface mould under established guidance, but follow authoritative advice for the specific product and discard it when uncertain.
Fuzzy on nuts, grains, or nut butters - discard (risk of invisible toxins).
Soft cheese, pâté, cooked meats - mould means bin all of it.
Firm veg - cut wide; if the inside smells off, compost.
Browning on avocado, limp celery destined for soup and liquid separating from yoghurt can be quality changes rather than danger. Check the date, storage and whole condition of the food.
Green potatoes contain increased glycoalkaloids. Remove small green areas generously, and discard potatoes that are extensively green, bitter, damaged or heavily sprouted.
Trust your nose after trusting your eyes
An unexpected sour, rotten or ammonia-like smell is a reason to discard food, but a normal smell does not prove that perishable food is safe. Pathogens may be present without changing odour.
Use dates, temperature and storage history first, then appearance and smell as additional information.
Dates are not the whole story
Use-by dates relate to safety and should be followed. Best-before dates mainly indicate quality for longer-life foods.
A cold fridge, intact packaging and first-in-first-out storage prevent more spoilage than repeatedly opening food to sniff it.
See what you own before it spoils
Record open staples and move older packs forward so they are used first. A weekly use-it-up meal can give safe vegetables, grains and tins a clear destination.
Do not let a wish to avoid waste override a use-by date or uncertain storage history.