Glasses often attract residue when cleaned in the dishwasher due to their smooth surface. This can sometimes contain food particles if the filter hasn't been cleaned recently. Jennifer Shaw, expert at Plumbworld, said: "It's frustrating when a dishwasher has been on for a two-hour cycle and items are still slightly dirty.
"It's even worse when glasses and mugs are covered in a gritty residue, which is difficult to remove. Glass and ceramic materials don't repel food particles the same way plastic and metal do.

"They also dry quicker, so stuff can stick onto the surfaces instead of washing away.
"These materials can also become slightly static in the drying stage and attract any loose particles."
The expert said using 30p white vinegar is the saving grace item to prevent this from happening. Not only will the white vinegar help the glasses, but it can also clean your dishwasher too.
She said: "The best approach is to place a small cup or bowl filled with white vinegar on the top rack of your dishwasher. The vinegar acts as a natural rinse aid, helping to break down any remaining detergent or hard water minerals that can leave a gritty residue.
"If you notice a lot of grit in your dishwasher, do this every cycle. But this hack can work wonders even just doing it once a week or less."
Keeping your machine clean also helps the efficiency of the wash cycles, so make sure to check the filter, spray arms and surfaces for debris.
Wiping down all areas in the appliance can help to maintain good hygiene, too.
The expert added: "You should also keep up with refilling dishwasher salts. Some machines have indicators when this is running low, but depending on the frequency of use and water hardness, this should be done every couple of weeks or monthly.
"You should also avoid overloading your dishwasher and allow items enough space for water and detergent to circulate freely around them."
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