The probe by David Maguire, a successful restaurateur, provides an insight into the way patient meals are produced, reheated and served in NHS Greater Glasgow and Clyde (NHSGGC).
According to Mr Maguire, all produce, including frozen peas, is put into ovens for 90 minutes, drying some food out. He revealed mashed potato is made from bought-in frozen potato pellets that are meant to be reheated in microwaves but are put into the ovens for more than an hour.
Issues with the equipment and tight staffing levels mean simple steps that could make the food look and taste better are missing, he said.
Mr Maguire added: "What passes for food in the NHS bears no resemblance to anything that I have encountered. It was like a pastiche of food. On occasions it looked like food.
"It rarely had the texture of food. It reminded me of the worst components that I have encountered at school meals. I am astonished that anyone eats any of it."
The co-founder of reservation web service 5pm Ltd and a former sales director for a food supplier, Mr Maguire offered his services for free. NHSGGC said that "in the spirit of openness and transparency" it gave him an "access-all-areas pass" to its food production and service facilities.
The health board said feedback has been increasingly positive since a £10 million investment in its two super-kitchens in 2011 and 2012 and that it uses patients' comments to continually improve satisfaction levels.
However, Mr Maguire said he discovered "basic procedural things at every stage that compromise the food".
He said the white plastic trays used for reheating food in hospitals "lose their rigidity" at 145°C. Mr Maguire said hospital food is "anaemic" looking because it cannot be heated to 160°C.
Caramelisation, which causes roast potatoes and meat to brown and adds flavour, is associated with higher temperatures.
Ready-made slabs of frozen mash come with brown dots painted on them to make it look as though they have caramelised, according to Mr Maguire.
He said that when blast freezing, food should be chilled to -18°C rapidly, preventing the formation of large ice crystals that can make food watery on defrosting.
However, the super-kitchens used the blast freezer to reduce food temperature to -4°C, he said,with staff then transferring items to conventional freezers.
Mr Maguire blamed this on the closure of the kitchens at 5pm, cutting the time that could be spent waiting for the food to chill further.
NHSGGC said some of Mr Maguire's questions were valid, "if not wholly practical in a catering environment such as ours which demands the production of 1.2 million patient meals every month".
The health board added: "As a restaurateur we can understand his expectation that such items as carrots would be freshly peeled and diced in the kitchen where the meal is prepared. This manually intensive approach would simply not be cost-effective when catering on a scale such as ours."
NHSGGC said it had "recently recruited a suitably qualified mass-production catering expert to help us further drive up quality in our production processes and deliver even higher patient satisfaction levels".
It also said it had addressed the quality of mashed potato by changing supplier.