Workers at a Pot Noodle factory in Caerphilly county are striking over changes to their pensions.
Members of the Unite union were taking part in the industrial action at the site in Crumlin on Thursday - part of 11 days of walkouts from Unilever sites across the UK.
They believe the new pension scheme will see some people losing a fifth of their pension.
But Unilever said it would still be "exceptionally competitive".
The company said it had been given time to prepare for the action, which it described as "disproportionate".
But unions claimed the walkouts will hit production of leading brands including Pot Noodle, Persil, PG Tips, Marmite, Lynx, Flora and Wall's ice cream.
Union officials said workers were "furious" at the plans to scrap the final salary scheme, which they said would lead to pension cuts of between 20% and 40%.
Unite national officer Jennie Formby said: "The hard working workforce made Unilever into the global giant it is today but consumers will soon know the company better for its greedy pensions snatch than its products.
Allan Black, national officer of the GMB, which is also taking part in the strikes, said Unilever could afford to sustain the current pension scheme unchanged due to the profitability of the company.
"The unions are able to put forward constructive alternative proposals on pensions, but these have never been considered by Unilever," he added.
'Rising life expectancy'
Unilever maintained that almost 90% of its affected employees will retain 80% or more of their pension.
The closure of the current final-salary scheme to existing staff - which has 5,000 active members - was first proposed last April with the aim of shutting it this January.
The company said it needed to change to a career average scheme to curb the rising cost of funding staff pensions, which has been driven up by rising longevity and volatile investment returns.
After consultation with staff, the closure date was put back to 1 July 2012, but the alternative, of staff contributing to a career average scheme instead, is still in place.
As of 31 March 2010 the scheme, which was closed to new joiners in 2008, had a deficit of £680m.
In a statement, Unilever said: "The reality is that the union representatives had multiple opportunities to help shape the greatly improved final outcome of consultation we reached in October, but unfortunately they decided to walk away from talks.
"Making these changes was a tough but necessary choice which reflects the realities of rising life expectancy and increased market volatility."
The factory in Crumlin, near Blackwood, has been making Pot Noodle for more than 30 years.
On its website, Unilever says it typically produces 155m pots of the hot snack every year, which is a 77% market share.