Unlike fresh food which must be eaten quickly, the frozen food is held in quarantine while samples from each batch are laboratory-tested for bacteria and allergens.
Once results come back clear, meals are stored for an average of two months in a giant cold storage hangar by the M5 near Bristol, before being delivered.
At hospitals, the meals are cooked directly from frozen in heated trolleys on each ward before being served.
Mr Freeston said: “There’s a myth that food would be much better if it’s made in a hospital by a chef. But in a big hospital, the kitchen can be half a mile from some of the wards.
“By the time the food has got to the ward it has deteriorated.
“Our system is about keeping it frozen until the last possible minute and reheating it at the ward so it can be served to patients when it is at its best.”
But food campaigners are sceptical about the use of reheated ready meals in hospitals.
Rob Percival, of the Soil Association’s Food for Life programme, said: “Health Secretary Matt Hancock recently said that the best hospitals are those serving freshly prepared meals – and we agree.
“Patients deserve real food, and we know that it’s possible.”