Craig Hassall KC, prosecuting, said the pleas were on the basis that the “breaches were causative of the deaths of Mr Moon and Mr Collins”.
He said that at the point of sentence he would invite the judge, Mrs Justice Judith Farbey, to formally record verdicts of not guilty to the corporate manslaughter charges.
The companies are due to be sentenced at a two-day hearing from 31 March.
Barrister Shauna Ritchie, representing Banham Poultry Limited, entered guilty pleas on behalf of the company to two counts of failing to discharge a duty under the Health and Safety at Work Act 1974.
Barrister Simon Antrobus KC, for Air Products plc, entered a guilty plea on behalf of the company to one count of the same offence.
The legal representatives for the companies each entered pleas of not guilty to two charges of corporate manslaughter in respect of Moon and Collins.
Hassall said the Crown Prosecution Service’s decision to accept the pleas was made “following consultation with the family” and Norfolk Police.
No details about the case were given at Wednesday’s hearing.