Rice Krispies and Coco Pops are no longer being made with whole rice grains, with the tiny toasted puffs being made using corn flour and rice flour instead.
A Kellogg's spokesperson said retaining the iconic sound had been a "huge focus" when designing the new recipe, but that customers "might notice that the sound may be quieter and not as instant as before".
The firm was responding to complaints online that the taste of the breakfast staples had changed, with some fans adding they missed the "pop" sound.
Kellogg's said it had adapted the recipes due to "changing weather conditions" that have disrupted supplies of some grains and cereals.
The cereal giant said the new versions had been "carefully assessed for taste, appearance and texture", but some shoppers said their children would no longer eat them.
Some expressed dismay that the taste and texture were "totally different" and dubbed the change "grim" and "disgusting".
"There's no snap, crackle and pop in these," a parent named Dani wrote on TikTok. "My toddler won't eat them."
Lauren, also posting on TikTok, added that she had been eating Rice Krispies every day for breakfast "for years and years".
"I had a bowl of the new recipe and you can immediately tell the difference," she said. "They don't have the snapping sound when you put in the milk."
Jessicake meanwhile said they tasted "weird and stale".
The "Snap, Crackle and Pop" – made when milk is added to the puffed rice – has been a centrepiece of the brand's marketing since the 1950s.
Kellogg's insisted the snap, crackle and pop is "still there, but consumers might notice that the sound may be quieter and not as instant as before".
Recipe changes are notoriously difficult for big brands. Chocolate makers are constantly accused of reducing cocoa content in their products and debates rage online over whether ketchup ingredients have changed.
When Scottish fizzy drink Irn Bru cut its sugar content in 2018, fans famously stockpiled crates of the old version.
No changes are being made to Rice Krispies products in the US and Canada, which are owned by a separate company.
Kellogg's said the recipe change in the UK would help "maintain consistent quality" as extreme weather takes a toll on crops and pushes up ingredient prices.
Extreme weather in Spain in 2022 and 2023 in particular had reduced availability of the types of rice the company uses for its breakfast cereals, the company said
Rice Krispies, launched nearly a century ago, are sold around the world and come in variations such as vanilla and strawberry flavours in some markets alongside the widespread chocolate-flavoured Coco Pops version.
"We appreciate that even small changes to a much-loved product can feel noticeable and that's why we have approached this with a lot of care and consideration," Kellogg's said.
The firm said it appreciated the customer feedback and would "continue to look at ways to make [its] products even better in the future".
The cereals have already undergone some change, with reductions in salt content in the 1990s and lowered sugar in 2018. The company said Rice Krispies and Coco Pops will continue to meet UK food standards on levels of fat, salt and sugar in food.
A backlash against processed foods in recent years, especially those low in fibre and high in sugar, has put cereals generally under the spotlight from healthy eating advocates.
Kellogg's stressed the cereals contain no artificial colours, flavours or sweeteners and the nutritional benefits in each bowl, such as fibre, Vitamin D and iron are staying the same.
However, the new recipes mean packaging will now include a "contains wheat gluten" warning. It also continues to contain barley-malt extract.
Watch: How does El Niño affect world weather?
Kellogg's said it has not changed the recipe for any of its other cereals.
Many parts of the world have been affected by droughts, floods and heatwaves this year due to the El Niño weather phenomenon. The weather event is expected to severely affect crop production from South East Asia in particular. The region produces more than a quarter of the world's rice.
The World Meteorological Agency expects the record-breaking El Niño will last until at least 2027.
That will intensify pressure on food manufacturing companies, already looking at ways to source reliable and affordable ingredients.
The original Kellogg company separated into two distinct firms in 2023.
The largest is Kellanova, which focuses on snacks, cereals, noodles and other food products like Eggo, Pringles and Pop-Tarts sold globally. It is now owned by Mars, which makes confectionery and pet food.
The other business, WK Kellogg's, operates in North America and has since been taken over by Italian chocolate giant Ferrero.
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