The ground-floor dining area at the Kingsdale Care Home in Streatham, South London, is a dispiriting place. The room is clean and neat but its paintwork is scuffed and the place is in need of a few pictures or a bunch of flowers.

Twenty minutes before lunch, one Thursday a few weeks ago, I was standing in this room with several elderly residents who were already seated at the tables, and with a senior carer at the home. The residents, for want of anything better to do, were looking blankly at their knives and forks. The carer, a kindly late middle-aged woman who had been showing me around, was now looking at me with an expression I found hard to read. Was it discomfort, or just surprise that I should be asking these outlandish questions? Were the vegetables freshly cooked, I wanted to know? No, she said. They sometimes came out of the freezer. And the soup that was served up every single evening for supper? Was it home-made? No, she said. It came out of a packet. The bread, it transpired, was usually white and pre-sliced.

Standards of nutrition in care homes such as this one are a matter of pressing national concern. According to the National Institute for Health and Clinical Excellence, as many as four out of ten residents in care homes for the elderly could be suffering from malnutrition. The reasons are complex — a stroke or dementia can lead to a loss of appetite, for example. But elderly people need a good, balanced diet just as the rest of us do. It is doubly important, given this loss of appetite, that the food offered to them is enjoyable.

I have come across the problem before. I have worked undercover in a number of care homes in England and Scotland and, in every one I have seen residents who are frail and undernourished and for whom eating has lost its joy. The best homes devote staff time to sitting with them, coaxing them to eat. I decided to investigate the issue further. Posing as a daughter whose mother was in need of nursing care, I made appointments to view six homes in different parts of the country.

My experience at Kingsdale, then, was depressing but not surprising. Sitting in its reception area I picked up an impressive brochure which promised, in big silver letters on a purple background, that residents would receive “a wide variety of wholesome, home-cooked food”. But it was not just the quality of the food at this purpose-built, 93-bed home that concerned me.

The brochure also claimed that “at Kingsdale we respect the rights of our residents to make personal choices”, yet it was difficult to see how these people could exercise any real choice about their diet.

In the reception area, a menu was pinned up, declaring lunch would be sweet-and-sour chicken followed by tinned fruit. In the kitchen I saw a different menu promising beef stew. But the cook told me he had cooked lamb. When I asked whether residents were asked in advance what they wanted to eat — there was a sausage alternative that day — I was told they would have to read the menu and make their views known to a carer.

Before setting out I had turned for advice to Jane Bulmer, a qualified dietitian who has set up a company called Kealth Foods, which makes specialist meals for people with swallowing difficulties. She advised me to look out for varied menus with a good choice of ingredients. Homes should be prepared to make sure food was available to residents when they wanted it, she said, rather than when it was convenient. Bad signs include hot foot left to go cold or kept standing around in heated trolleys. If a resident was not eating, someone should be asking why.

“Fundamentally,” she said, “are the residents showing interest in the food and clearing their plates? Would you eat it?” In several of the homes I visited, I saw residents looking disconsolately at uneaten food. At Aspen Court, a BUPA home in Derby, the first impressions were good. The building was modern, clean and bright. I was told all the vegetables were fresh, and I even saw two residents eating bananas. But the tea being served that evening looked cheap and nasty — battered fish fingers with baked beans, followed by lurid lime-green jelly with squirty synthetic cream. Several residents were sitting in front of half-eaten food.

Later I asked Clive Bowman, the medical director of BUPA Care Services, why such unappetising fare was being served. He insisted that elderly people loved jelly, and said the bright green colour would help those with poor eyesight to identify what was on their plate. “You are imposing your values on our customers,” he said. “You have to be very careful in doing that. I have to say when I go to a care home on a Friday and fish is being served, the smell does not always appetise me. But very often that is what people expect.”

Yet despite his reassurances my programme of visits left me with a sense of gloom. Time and again, I arrived at homes that were nicely presented, clean and fresh-smelling, but left feeling a profound relief that, unlike the residents, I was free to eat somewhere else.

Austhorpe House Nursing Home in Norfolk, for example, was attractively decorated and clean. When I arrived residents were eating tea in a light, airy conservatory. Its literature boasted “only the best fresh produce” was served. There was a menu pinned up, offering lunch dishes such as fisherman’s pie and treacle pudding, and there was also a whiteboard in the kitchen on which residents’ likes and dislikes were noted.

But I found it hard to identify the tea the residents were eating that day. It was a sort of yellowy mush that could possibly have been scrambled egg — I was assured later that indeed it was — and some of the residents were pushing it around their plates. For pudding they were being given another yellowy mush — puréed apple. This was better, nutritionally, than some other places I visited but surely with a little bit more thought, something more varied could have been served — though to be fair, residents and relatives told inspectors on their latest visit that the food here was good.

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