The beetle cocoon that was manna for Moses

Molecule of the month: John Emsley looks at trehalose

Passover, the religious festival celebrated by Jews this month, commemorates the night when the Angel of Death spared the Israelites but killed the first-born of their Egyptian masters. The disaster persuaded the Egyptians to release their slaves, and so began their 40 years in the wilderness. Within a few weeks the Israelites were starving, so Moses appealed to God, who promised: "I will rain down bread from Heaven for you", [Exodus 16] and delivered the mysterious, but nutritious, manna which was "white like coriander seed and tasted like a wafer made with honey".

This Heaven-sent sweetness might again be saving lives - thanks to a British company.

Manna was almost certainly trehalose, a white crystalline carbohydrate made of two glucose molecules joined together. It is one of very few naturally occurring molecules that taste sweet, although it is only half as sweet as sugar. What the Israelites were gathering was the cocoon of the parasitic beetle Trehala manna from which trehalose gets its name, and which explains Moses' warning not to hoard it: "Some, however, did not listen ... and it became full of maggots and stank." The cocoons, found on thorn bushes in the Middle East, are highly nutritious, consisting of 30 per cent trehalose plus protein.

Trehalose occurs in honey, bread, beer, wine and vinegar, while Japanese shiitake mushrooms and baker's yeast contain as much as 20 per cent.

Trehalose has remarkable preserving power and is produced by creatures that lie dormant under drought conditions. Some plants can loose over 95 per cent of their water content and still survive, thanks to the trehalose in their cells.

John Crowe, of the University of California at Davis, suggested that trehalose has the right shape to replace water molecules around vital cell proteins and prevent them from collapsing. Michael Burke, of Oregon State University, believes that trehalose forms a supportive "glass" like that of boiled sweets within tissues.

Steve Ring, of the Institute of Food Research at Norwich, has shown that small amounts of protein are needed to keep trehalose glass stable at 37C and prevent it becoming opaque and microcrystalline.

Trehalose is now being used as a preservative for antibodies, vaccines, enzymes and blood coagulation factors. In 1985, Bruce Roser discovered that if trehalose was added to solutions of proteins like these, which were then dehydrated, the products could be stored at temperatures above 40C and when rehydrated were still active. This offers an alternative way of preserving medical supplies in Third World countries, where 90 per cent of vaccines are wasted through lack of refrigeration facilities. Mr Roser has set up his own company, Quadrant, at Cambridge, to exploit his discovery, and employs 30 people.

"After years of storage at room temperature, trehalose-dried antibodies worked well. Even notoriously unstable enzymes, such as DNA-modifying and restriction enzymes, worked after being stored for a month at 70C," says Mr Roser. Another use could be to store blood. "Fresh blood has a shelf life of 42 days, after which it must be disposed of. Trehalose-dried blood could mean an end to the critical blood shortages that are suffered by the health service."

Quadrant imports its trehalose from Japan. Production is set to increase to 50 tons a month. Scientists there have shown that trehalose preserves the quality and flavour of dried foods. Many people find that powdered egg has an unpleasant taste, but if it is dehydrated with trehalose it takes on the taste of fresh egg. Other foods, such as powdered puree of bananas, mangoes, apples and avocados, also rehydrate with the taste of the fresh fruit. The method used to dry trehalose-treated foods is much cheaper than the normal methods of vacuum-drying and freeze-drying.

John Emsley is science writer in residence at Imperial College, London.