It is thought refrigeration units at three Tesco stores in Cardiff and one in Pontypool could not cope with soaring temperatures on Tuesday.
Hundreds of fresh food products, including butter, milk, meat and yogurts had to be thrown out.
A Tesco spokesman said: "The vast majority of our fridges and freezers are working as normal."
"When our stores report a problem, we have been working quickly to put things right for customers."
Temperatures in some parts of Wales reached 32C (90F) on Tuesday.
The fridges at the Pontypool store are now working again and staff are re-stocking.
Supermarkets in south Wales have been forced to throw out large amounts of fresh food as their fridges failed.