They found the metal in the jars of Heinz food, after Nigel Wright allegedly contaminated them in a £1.4m bitcoin plot, the Old Bailey was told.
Mr Wright, 45 and of Lincolnshire, claimed he was a dairy farmer underpaid by Tesco, the jury heard.
He denies two counts of contaminating goods and three counts of blackmail.
Mr Wright, from Market Rasen, told Tesco he would reveal which stores he had planted the contaminated jars in, between May 2018 and February 2020, if they paid him in bitcoin, prosecutors said.
He claimed to be one of a number of dairy farmers calling themselves “Guy Brush and the Dairy Pirates” who believed they had been underpaid, they said.
The court heard Morven Smith was feeding her 10-month-old son a jar of Heinz sweet and sour chicken baby food in December 2019 when she noticed the shards of a craft knife blade.
Tesco then issued a national recall of all jars of the product, prompting Harpeet Kaur Singh to say she too had discovered fragments of metal when she was feeding her nine-month-old daughter.
Prosecutor Julian Christopher QC told the jury: “The defendant hoped to make himself rich by means of blackmail.”
The two customers had found the slivers of metal in November and December 2019, in Rochdale and Lockerbie.
‘Forced by travellers’
There is no evidence any other products were actually contaminated, the court was told.
Mr Wright also claimed salmonella and chemicals had been injected into cans and threatened to continue poisoning Tesco products if payment was not made, Mr Christopher said.
In a separate charge of blackmail he is accused of demanding £150,000 worth of bitcoin from a driver with whom he had had a road rage altercation.
A draft of messages sent to Tesco was found on his laptop along with photos of food tins, jars of baby food and slivers of metal, the court heard.
Mr Wright admits various elements of the campaign but claims he was forced to do so by travellers who had demanded he give them £1m and he was acting in fear of his life.
The trial continues.