No one knows for sure, and consumer groups and, this week, Prince Charles, find that uncertainty worrying. At a conference on biodiversity the Prince criticised the "confidence bordering on arrogance" of the developers of genetically engineered plants and animals, and said he was "profoundly apprehensive" about their products' possible effects.
His words may hit a sensitive spot with a public already alerted to food issues by the row over the safety of beef. In the past fortnight concern has grown about whether eating beef carries the risks of developing the human equivalent of mad cow disease (an ailment which is unequivocally not caused by genetic engineering).
If the Prince wanted to start another food scare, his timing could hardly have been better. Next February, Safeway and Sainsbury's will start selling tomato puree made from tomatoes whose genes have been tweaked so that the fruit takes far longer to rot. Though they are not legally obliged to label the product any differently from other purees, both supermarkets say they will do so. But equally, they also intend to go ahead and put them on sale.
"There are obvious benefits from genetically engineered foods, such as taste and longer shelf life," says a spokesman for Sainsbury's. "They will be clearly labelled as such." Is the company worried about another food scare? No, it is not. "The Prince is entitled to his own opinion, as is the consumer."
Developed by the British biotechnology company Zeneca (formerly part of ICI), the products that make the puree have already been dubbed "Frankenstein tomatoes". In the Zeneca tomato, a gene that creates a plant hormone called ethylene is repressed. Ethylene is one of the key components in rotting. The effect of repressing it is that the fruit takes up to 40 per cent longer to rot - a key figure for food companies trying to distribute it to far-flung shops or processing centres. And the commercial benefits are self-evident: the world market for the fruit is worth pounds 2.3bn annually.
But the whole process of altering foods and animals by the most modern of techniques - to create what is known as a "genetically modified organism" (GMO) - has drawn fire from consumer organisations, and from Tim Lang of the Centre for Food Policy at Thames Valley University. "It's symbolic of what's happening in the food market. The consumer only gets any choice about what happens at the end."
Professor Lang is strongly critical of the enormous commercial bandwagon behind genetically altered food. "Eighty per cent of the patents taken out on GMOs have been taken by 14 companies," he says. "There is an important point about the politics of science here: these companies are the ones pushing this. But when have consumers ever asked for tomatoes that take longer to rot, or salmon that grow 10 times faster?"
The possibility of creating organisms that might not arise naturally has been around since the 1970s, when the technique of using "recombinant DNA" was developed. DNA is the genetic material for every living organism, consisting of a double helix of millions of pairs of four basic amino acids. Every few million such pairs comprises a gene. Recombinant techniques can splice a gene from one plant or animal into another. Using these methods, researchers have already produced a menagerie of GMOs. There are potatoes with extra genes from bacteria that live in the gut; when fried, the potatoes make crisper chips. There is wheat that is resistant to herbicides, so the fields where it grows can be sprayed with impunity. There are tomatoes made frost-resistant by including genes from cold-water fish.
Professor Lang says: "The big question is the 'What if?' What if a pest- resistant gene gets out from a crop and becomes incorporated into other plants? What if mutant insects get resistance to pesticides? That's an aspect which hasn't been brought out."
His questions are echoed by Julie Sheppard, a spokeswoman for the Genetic Forum, a consumer association which monitors the use of genetic engineering in modern life. "The implications of these foods are so colossal," she says. "What's different about genetic engineering is that we are able to move genes from one species to another, from animal to animal, or from an animal to a plant. We don't know what the risks associated with that are.
"After all, if someone had suggested to you 40 years ago that the use of under-arm deodorants and hairspray would lead to an increase in skin cancer because of ozone damage, people would have laughed. Now they take it as read. The issues with genetically engineered food are so complex that we don't even know what the questions should be."
However, Gavin Cree, who chairs the Bioindustry Association's regulatory affairs advisory committee, says: "No one anywhere has ever been harmed by a GMO - so we're doing pretty well. Judged by its record, genetic modification hardly needs regulation at all."
There is, however, regulation: in the UK, the release of GMOs is policed by a part of the Department of the Environment known as ACRE (Advisory Committee on Releases to the Environment). It carries out risk assessments and oversees the implementation of UK regulations in the use of GMOs.
Critics of companies involved in genetic engineering portray them as pushing ahead without knowing the full implications of their work. In fact they have a very strong motive for being cautious: if somebody did eat a GMO and some dire result followed, they would claim astronomical damages. A lot of the research now going on tries to determine what the risk is that spliced genes might jump between species. It happens with ease between bacteria; why not with higher organisms?
"It's a remarkably safe field," insists Dr Cree. "Genetic modification lets you be much more precise about what changes you make. It's quicker and easier, for example, to produce a new breed of wheat. If you want a higher plant, you work out which part of the genome [the entire sequence of the plant's DNA] affects its height and just alter that."
Dr Cree admits that the possibility of genes jumping from one species to another is a concern, "though that is not borne out by experience". Scientists' confidence about genetically engineered food stems from the fact that we have been eating other species' genes for centuries, and so far no one has grown wheat on their head or begun to moo.
It is commercial pressure that is driving genetically engineered foods towards the market: once the gene has been added to an animal or plant, ittakes less effort to get the finished result. And the companies can argue that shoppers always want fresher, brighter, tastier foods, in greater volumes.
But consumers may have the last laugh. There will be growing pressure on retailers and manufacturers to put clear labels on altered foods. That in turn makes it likely that people will turn away from such "Frankenfoods" to ordinary ones - even though there might be no risk and they could taste better.
Fried bread: made from wheat that is pest and herbicide resistant. Earlier this year two biotechnology companies failed to obtain patents for crops that had been modified to make them resistant to herbicides
Tomatoes: Zeneca, a British biotechnology company, has already produced tomatoes that have been designed not to rot. The new tomatoes will be sold in puree form from February
Bacon: from pigs that are genetically engineered to grow bigger, with bulkier muscles and less fat
Milk: from cows engineered to produce vitamin-enriched milk in large quantities
Eggs: from hens that have been genetically adapted to lay bigger eggs more frequently. Yolks modified for a yellower hue