The naturally occurring furanones: formation and function
from pheromone to food.
Colin Slaughter J.
Department of Biological Sciences, Heriot-Watt University, Edinburgh, UK.
J.C.Slaughter@hw.ac.uk
Three closely related 4-hydroxy-3(2H)-furanones have been found in a range of
highly cooked foodstuffs where they are important flavour compounds with aroma
threshold values as low as 20 micrograms kg-1 water (approximately 0.14 mumol
l-1). The compounds are formed mainly as a result of the operation of the
Maillard reactions between sugars and amino acids during heating but one
compound, 5-(or 2)-ethyl-2-(or 5)-methyl-4-hydroxy-3(2H)-furanone, appears in
practice to be produced by yeast, probably from a Maillard intermediate, during
the fermentation stages in the production of soy sauce and beer. The compounds
are also important in the flavour of strawberry, raspberry, pineapple and tomato
but the route of biosynthesis is unknown. Two 3-hydroxy-2(5H)-furanones,
emoxyfuranone and sotolon, which are produced spontaneously from amino acids
such as threonine and 4-hydroxy-L-leucine are major contributors to meaty and
spicy/nutty flavours in foods. The biosynthesis of 5-(1,2-dihydroxyethyl)-3,4-dihydroxy-2(5H)-furanone
(ascorbic acid, vitamin C) and 5-hydroxymethyl-3,4-dihydroxy-2(5H)-furanone (erythroascorbic
acid) from sugars in plants and yeast, respectively, has been characterized to
the enzymic level. After treatment with chlorine, humic waters contain a range
of chloro-furanones, some of which, particularly 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone
(MX), are powerful mutagens. The furanones which occur in foods are also
mutagenic to bacteria and cause DNA damage in laboratory tests. However, these
compounds are, in practice, very effective anti-carcinogenic agents in the diets
of animals which are being treated with known cancer-inducing compounds such as
benzo[alpha]pyrene or azoxymethane. Two of the food-derived furanones have
antioxidant activity comparable to that of ascorbic acid. A biological function
has been discovered for some of the furanones besides vitamin C. 5-Methyl-4-hydroxy-3(2H)-furanone
is a male pheromone in the cockroach Eurycolis florionda (Walker) and the 2,5-dimethyl
derivative deters fungal growth on strawberries and is an important component of
the attractive aroma of the fruit. The red seaweed Delisea pulchra (Greville)
Montagne produces a range of brominated furanones which prevent colonisation of
the plant by bacteria by interfering with the acylated homoserine lactone (AHL)
signalling system used by the bacteria for quorum sensing. In addition, these
compounds can deter grazing by marine herbivores. It is proposed here that the
evolved biological function of a number of furanones is to act as inter-organism
signal molecules in several different systems. This has resulted in two
coincidental effects which are important for humans. Firstly, the easily
oxidized nature of the furanones in general, which is likely to be an important
property in their functioning as signal molecules, results in both mutagenic and
anti-carcinogenic activity. The balance of these two effects from compounds in
the diet has yet to be fully established. Secondly, and more specifically, the
4-hydroxy-3(2H)-furanones associated with fruit aromas act to attract animals to
the fruit, which ensures seed dispersal. In the case of humans, the coincidental
synthesis of some of these compounds in foods during preparation results in
these foods appearing particularly attractive through the transferred operation
of the original signalling mechanisms.