10 things you should never put in a dishwasher

From sandwich boxes to baby bottles, we use a plethora of plastic items in our kitchens. However, while some are designed to use in a dishwasher, others can’t stand the heat.  

While reusable plastics are usually fine in a dishwasher, disposable plastics, like the food containers that contained your last takeaway, or the bottle that contained your ‘meal deal’ fizzy drink or water, are unsafe. This is because they contain the industrial chemical BPA (bisphenol A), which cannot withstand high temperatures, breaking down and becoming warped in the heat. It can also harm your health, as once broken down it can leach into food.   

There’s also a word of warning about washing baby bottles in a dishwasher. Although it’s safe to wash them in a dishwasher, make sure you position them securely, and upside down to prevent them from tipping over during the wash cycle). The NHS advises that they will still need to be sterilised. 

If the item contains a dishwasher-safe label, pop it on the top shelf and avoid the hottest cycles. If you can’t find a label, be cautious and wash the item by hand. 

Water and wood are not a happy mix. When you add heat into the equation, the outlook is not bright. Your prized chopping board or pristine wooden salad servers will likely warp and splinter when exposed to high temperatures and moisture.  

Whether you slice or dice, knives are a chief tool in every kitchen. And while it can be tempting to pop them in the dishwasher for a thorough clean, especially if you’ve been chopping fish or meat and want to banish bacteria, it’s not the best method to keep them in tip-top condition.  

The high temperature and moisture can damage both the steel and the handle, particularly if your blade is carbon steel or the handle is wooden. There’s also the rough and tumble of the dishwasher, which can cause the knife blades to crash against other items, causing damage to the knife’s blade while scratching glass and ceramics.  

You may never ‘chink’ your crystal glasses again if you put them in the dishwasher. Crystal and hand-blown glass are extra sensitive to heat and may crack if they’re put through a dishwasher cycle.  

However, Dartington Crystal advises that many of their products can be washed in a dishwasher if handled with care, by stacking them carefully and using a low-temperature wash cycle. It’s worth checking to see if your dishwasher has a gentle cycle for crystal and china.  

If you opt to wash delicate glassware in the dishwasher, it will lose its sparkle over time, so it’s best to only use the dishwasher once in a while. 

It’s best to wash any vintage and antique items by hand to avoid any pattern getting discoloured by the high temperatures and harsh detergent used in a dishwasher. And any items with golden embellishments, such as gold-rimmed glasses, plates, vintage bowls and dinnerware, should always be washed by hand to maintain their beautiful finish. The combination of hot water and detergent can cause the rich gold colour to lose its sparkle, creating something altogether lacklustre.  

While your travel mug can keep your drink nicely chilled or hot for a length of time, its technology means it’s not great in the dishwasher. Water is able to seep between the inner and outer layers of insulation, damaging its effect and possibly forming mould. If your travel mug has a rubber ring that seals on the lid, this also has the potential to trap water, promoting the growth of mould.  

Non-stick pans can be a godsend in the kitchen – I love them for cooking risotto – but they must be washed carefully to preserve their special coating. While some brands can be placed in a dishwasher, such as Calphalon, Tefal and GreenPan, others are best washed by hand – it all depends on the coating.  

We recommend checking the manufacturer’s instructions before putting your new non–stick pan in the dishwasher. Any non-stick pan with a PTFE coating should be washed by hand, as a dishwasher’s harsh detergents can cause the coating to break down and deteriorate.  

There is some good news. With a non-stick pan, you won’t be slaving over a sink for hours scrubbing burnt risotto off the bottom! 

Cast iron can last a lifetime if cared for correctly. To prevent cast iron from rusting, it’s seasoned with a layer of oil that penetrates the metal – this is what gives the cookware its classic black patina. If you put cast iron in a dishwasher, this layer of seasoning will be broken down by the harsh detergent and will cause it to rust, which is accelerated by a dishwasher’s high temperature. To avoid rust, your best bet is to clean your cast–iron cookware in warm soapy water. 

If you’re like me, cheese goes with everything. So my much-loved grater is forever being washed in readiness for its next use. But should it go in the dishwasher? A grater’s sharp metal edges can damage the plastic coatings on the dishwasher racks and cause them to rust, so it’s best to stick to washing it up by hand. You may also find that the tiny grooves get missed by the water jets, and you’re stuck with a grater that’s still full of yesterday’s cheese. 

To keep your silver cutlery looking pristine for when you have guests for dinner, Whirlpool advises washing your flatware by hand. And if you do choose to clean your silver-plated and sterling silver cutlery in the dishwasher, don’t put them in the same load. If they are washed together a chemical reaction can occur, which could leave stains and rust marks on both sets.  

You’ll also need to avoid detergents containing citric acid, as it can wear down the silver plating and cause unsightly tarnishing. 

What exactly does it mean for an item to be dishwasher-safe? It must have been produced in a certain way to cope with the high temperatures and pressure from the pounding water during a dishwasher cycle. 

Still not sure if an item can be washed in the dishwasher? It’s worth checking whether the item has a dishwasher-safe symbol. According to the dishwasher detergent brand, Finish, there are two categories: 

• Dishwasher safe, top rack only  
• Dishwasher safe  

Top rack only 

With the heating element in the bottom part of the dishwasher, the top rack is cooler than the lower rack. In essence, items less resilient to high temperatures are likely to warp or become discoloured and should be placed on the top deck, where life is always more exciting!  

You’ll usually find a dishwasher-safe label on the bottom side of dishes or as an engraving on the handle of a pan.