Flavour enhancers are used to bring out the flavour in a wide range of foods without adding a flavour of their own. The concept of flavour enhancement originated in Asia, where cooks added seaweed to soup stocks in order to provide a richer flavour to certain foods.
The flavour-enhancing components of seaweed was identified as the amino acid L-glutamate, and monosodium glutamate (MSG) became the first flavour enhancer to be used commercially. The rich flavour associated with L-glutamate was called umami.
In this article, we shall explore flavour enhancers and sweeteners; food occurrence, stability, nonnutritive sweeteners and nutritive sweeteners.
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Understanding Flavour Enhancers in Food Production
1. Definition of Flavour Enhancers
Flavour enhancers can be defined as a group of chemical substances that do not impart any flavor into the food product but enhance the existing flavor in the food. The most commonly used substances in this category are monosodium L-glutamate (MSG), disodium 5′-inosinate (IMP), and disodium 5′-guanylate (GMP).
2. Natural Occurrence in Agricultural Products
It is important to note that both compounds comprising umami, that is, glutamate (salts of glutamic acid) and nucleotides, are key components of living organisms. Glutamate is naturally present in virtually all foods, including meat, fish, poultry, milk (human milk), and many vegetables.
It occurs in bound form when linked with other amino acids to form protein, and also in free form when it is not protein bound or in peptides. Therefore, protein-rich foods such as human milk, cow‘s milk, cheese, and meat contain large amounts of bound glutamate, while most vegetables contain little.
Despite their low protein content, many vegetables, including mushrooms, tomatoes, and peas, have high levels of free glutamate.
Glutamate is an important element in the natural and traditional ripening processes that allow the fullness of taste in food to be achieved. Perhaps this is why foods naturally high in glutamate, such as tomatoes, cheese, and mushrooms, have become important to the popular cuisines of the world.
Nucleotides are specifically distributed. Disodium 5′-inosinate (IMP) is dominant in meat, poultry, and fish, whereas adenosine monophosphate (AMP) is dominant in crustaceans and mollusks; furthermore, almost all vegetables contain AMP.
The GMP content of mushrooms is particularly high, especially in the shiitake species, which is a traditional cooking ingredient in Japan and China.
3. Stability of Flavour Enhancers in Food Processing
i. Glutamate Stability: Glutamate is not hygroscopic and does not change in appearance or quality during storage. The characteristic taste of glutamate, umami, is a function of its stereochemical molecular structure.
ii. 5’-Nucleotides Stability: IMP and GMP are not hygroscopic. IMP and GMP are stable in aqueous solution, but in acidic solution at high temperature, decomposition of the nucleotides occurs.
Enzymatic activity can also have a significant influence on flavor enhancer breakdown and build-up.
The phosphomonoester linkage of 5′-nucleotides is easily split by phosphomonoesterases, which are readily found in plant and animal products.
From a practical standpoint, these enzymes should be inactivated before the addition of 5′-nucleotide flavor enhancers to foods. Heating or storage below 0°C is usually sufficient to cause inactivation.
4. Toxicity Considerations in Agricultural Applications
An ample supply of a suitable nitrogen source is essential for L-glutamic acid fermentation, since the molecule contains 9.5% nitrogen. Ammonium salts such as ammonium chloride or ammonium sulfate and urea are assimilable.
The ammonium ion is detrimental to both cell growth and product formation, and its concentration in the medium must be maintained at a low level.
Role of Sweeteners in Agricultural Food Systems
Sweetness is one of the most important taste sensations for humans and for many animal species as well. Sweet compounds almost universally induce a positive hedonic response in humans, and this response, which is found in the neonate, is often thought to be inborn. There is scarcely any area of food habits today that does not in some way involve the sweet taste.
Sucrose is one of the most commonly used sweetener. It is not consumed only for its sweetness. It also has many functional properties in foods that make it useful as a bulking agent, texture modifier, mouth-feel modifier, and preservative. Sucrose additionally offers an important energy source for many food fermentations.
The sweetness of individual sweeteners is usually measured in model systems and compared to that of sucrose. The sweetness of individual sweeteners and mixtures can therefore also be estimated and expressed as the concentration of the equisweet reference sugar (usually glucose or sucrose).
The increased sweetness obtained by mixing natural and synthetic sweeteners has been of economic and nutritional interest. The unpleasant aftertastes of many artificial or non-nutritive sweeteners have also stimulated the development of sweetener mixtures to reduce these aftertastes.
1. Sugar Substitutes in Agricultural Food Processing
For nutritional and health reasons, there has been a growing desire in most countries to utilize sweeteners including artificial sweeteners other than sucrose.
The use of artificial sweeteners gives rise to a variety of problems in food technology due to some basic differences between them and the carbohydrate sweeteners.
Nonnutritive sweeteners are usually not carbohydrate based and therefore have different chemical and physical properties. Often nonnutritive sweeteners also have flavor characteristics that differ from those of carbohydrate sweeteners and are intensely sweet compared to carbohydrate sweeteners.
These properties often influence the cost of food manufacturing because the resulting dietetic or special dietary foods are expected to be as acceptable as those with carbohydrate sweeteners.
2. Nonnutritive Sweeteners in Food Production
A. Saccharin in Agricultural Applications
i. Chemistry: Saccharin is a general name used for saccharin, sodium saccharin, and calcium saccharin. The molecular formula of saccharin is C7H5NO3S, and the structural formula is presented in Fig. 1. Chemically saccharin is 1,2-benzisothiazol-3(2H)-one-1,1-dioxide and its sodium or calcium salt.
ii. Intake: The major uses of saccharin include soft drinks, tabletop sweeteners, and dietetic foods. Saccharin and sodium saccharin have other uses in cosmetics and pharmaceuticals.
iii. Toxicology: The safety of saccharin has been assessed in many epidemiological studies involving both normal subjects and diabetics. Most studies in many population groups including diabetics have failed to demonstrate any statistical evidence of an association between human bladder cancer and saccharin consumption.
B. Cyclamates in Food Systems
i. Chemistry: Cyclamates is a group name used for the following compounds: cyclamic acid, sodium cyclamate, and calcium cyclamate. The molecular formula for calcium cyclamate is C12H24CaN2O6S2 ⋅ 2H2O. Cyclamates are chemically synthesized products that are not found in nature.
They are synthesized from cyclohexylamine by sulfonation of various chemicals (chlorosulfonic acid, sulfamic acid) followed by neutralization with hydroxides. Cyclamates are stable at both high and low temperatures.
They provide a sweet taste that is 30 times sweeter than sugar. Cyclamates are easily soluble in water and can be used as noncaloric sweeteners in most foods, including soft drinks, confections, desserts, and processed fruits and vegetables.
ii. Toxicology: Epidemiological studies in humans indicate that there is suggestive evidence that the use of cyclamate/saccharin mixtures may be associated with a small increase in the risk of bladder cancer.
In addition to carcinogenicity data there are some adverse effects observed in laboratory animals such as testicular atrophy in animals exposed to cyclohexylamine. The clarification of these adverse effects as well as some other questions give rise to the need for further studies.
C. Aspartame in Food Processing
i. History: Aspartame was discovered accidentially in the G. D. Searle laboratories by J. M. Schlatter in the early 1960s. Since the discovery safety studies on aspartame have been carefully conducted by Searle laboratories and by many other independent research laboratories. In the early 1980s aspartame was approved in many countries as an alternative sweetener to saccharin and cyclamate.
ii. Chemistry: Chemically, aspartame is the methyl ester of L-aspartyl-L-phenylalanine. Aspartame is produced from the amino acids phenylalanine and aspartic acid. Preliminary amino acids can be produced by fermentation.
iii. Toxicology: Available evidence suggests that normal consumption of aspartame is safe because consumption of aspartame from foods is far below any suspected toxic levels. Data has not provided evidence for serious adverse health effects, although certain individuals might have an unusual sensitivity to the product.
D. Acesulfame-K in Agricultural Products
i. History: Acesulfame-K is one of the most recently introduced nonnutritive sweeteners. It was developed by Hoechst Company in West Germany in 1967 and has only recently been recommended for use in foods in several countries.
ii. Chemistry: Acesulfame K is a name utilized for the potassium salt of 6-methyl-1,2,3-oxathiazine4(3)-one-2,2-dioxide. The composition of acesulfame K is C4H4NO4KS. The compound is freely soluble in water and forms a neutral solution.
Acesulfame K is not hygroscopic, and it decomposes during heating at temperatures over 235°C. The molecular weight of the compound is 201.2. At room temperature acesulfame K is an intensely sweet (150–200 times sweeter than sucrose), white, odorless, crystalline powder.
iii. Toxicology: Acesulfame K was first evaluated by JECFA in 1981, but some shortcomings were found in long-term carcinogenicity studies and therefore no acceptable daily intake value was allocated (WHO, 1981).
E. Thaumatin in Food Applications
i. Chemistry: Thaumatin (Thalin) is a macromolecular protein sweetener with a molecular weight of around 22,000. The major protein constituents of the sweetener consist of the normal amino acids except for histidine, which is absent.
The extensive disulfide cross-linking confers thermal stability and resistance to denaturation. The tertiary structure of the polypeptide chain gives thaumatin its sweet character. Cleavage of just one disulfide bridge results in a loss of sweet taste.
ii. Toxicology: It has been demonstrated that thaumatin is not allergenic, mutagenic, or teratogenic. Both short-term tests and clinical human exposure studies, some at exaggerated levels, showed no adverse effects.
However, long-term studies have not been conducted. It has been questioned whether sufficient data exist for the safety assessment of thaumatin. In any case thaumatin has a long history as a sweetening agent in West Africa and has been used for many years in Japan without any reported reactions.
F. Sucralose in Food Manufacturing
Sucralose is the generic name of a relatively new intense sweetener made from ordinary sugar. Sucralose was first discovered in 1976, and it is a unique sweetener as it is made from ordinary sugar. It is a thrichloro derivative of the C-4 epimer galactosucrose which is not broken down during its passage through the gastrointestinal tract and thus does not provide calories.
Sucralose tastes like sugar, but it is about 600 times sweeter. However, the taste profile is similar to sucrose and it can be used for almost all applications where sucrose is used.
The sweetness does not react with food components or other ingredients. It has good water solubility. Sucralose has excellent product stability even under high temperatures and it can be used in a broad range of food products.
i. Safety A large number of studies have proven that sucralose is safe for human consumption. Sucralose does not break down in the gastrointestinal tract or accumulate in fatty tissues. Sucralose is also non cariogenic. It is currently evaluated by several regulatory bodies.
G. Other Nonnutritive Sweeteners in Development
In addition to the traditional and extensively studied nonnutritive sweeteners, a growing number of new compounds have been suggested as sugar substitutes.
3. Nutritive Sweeteners from Agricultural Sources
A. Fructose in Food Production
Fructose is a hexose monosaccharide that is one of the most commonly occurring natural sugars. It is often called fruit sugar or levulose. Free fructose is found in almost all fruits and berries and in most vegetables.
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Properties and Uses
Commercially crystalline fructose is produced from sucrose by inversion or from glucose by enzymatic isomerization. One of the major advantages of the use of crystalline fructose is its relative sweetness, which is about 1.5 times that of sucrose.
Fructose is the only carbohydrate with a sweetness higher than that of sucrose on a weight basis. Usually smaller amounts of fructose are needed for the same sweetness in food products, and this often results in reduction of energy content.
Fructose is also more slowly absorbed than glucose or sucrose, and it does not stimulate insulin per se. Fructose does not increase serum triglycerides in normal subjects.
Therefore, fructose has been recommended as a sweetener for diabetics and is presently included in the diabetic diet recommendations in many countries. However, one must take into account the calories in fructose.
B. Xylitol in Agricultural Food Systems
Xylitol is a pentitol that can be found in most fruits and berries as well as vegetables. Commercially xylitol is produced from xylan-containing plant material by acid hydrolysis, hydrogenation, and purification.
Xylitol can also be produced by microbiological methods.
At room temperature xylitol is equisweet with sucrose and therefore twice as sweet as sorbitol and three times as sweet as mannitol.
The main application of xylitol appears to be in confectionery, especially in sugar-free products and noncariogenic chewing gum.
Xylitol can also be utilized in diabetic and dietetic foods provided its energy value is taken into account.
i. Toxicity: The toxicological studies include studies on carcinogenicity, mutagenicity, and teratogenicity. All these studies have indicated that xylitol is safe for food use. A comprehensive review is available from the World Health Organization (WHO, 1983).
C. Sorbitol in Food Applications
Sorbitol is a six-carbon sugar alcohol that was originally found in the berries of mountain ash. It occurs in many fruits and vegetables. Sorbitol has the same steric configuration as glucose, and it is chemically synthesized from glucose or dextrose for commercial use.
i. Toxicology and Safety: The toxicology of sorbitol has been reviewed recently by WHO. The Joint FAO/WHO Expert Group on Food Additives has given sorbitol an ADI of ‗‗not specified,‘‘ which means that no health hazards are foreseen (WHO, 1982).
D. Mannitol in Agricultural Products
Mannitol is a hexitol that is stereoisomeric to sorbitol. It is commonly found naturally in some plant foods, including beets, celery, olives, and seaweed. Mannitol has about 0.4–0.5 the sweetness of sucrose, and its properties are fairly similar to those of sorbitol.
i. Toxicology and Safety: A laxative effect is observed in humans after intakes of 20–30 g of mannitol. Toxicity studies have not indicated any adverse effects other than diarrhea. Therefore mannitol is considered safe for use in foods.
Mannitol is also on the U.S. FDA GRAS list. An evaluation of its health effects has been conducted. An acceptable daily intake of ‗‗not specified‘‘ has been allocated for mannitol.
E. Lactitol in Food Manufacturing
Lactitol is a disaccharide alcohol [(4-O-β-D-galactopyranosyl)-D-glucitol] produced by the hydrogenation of lactose or lactulose. Lactitol as well as most polyols can be applied to special dietary foods that can be consumed by diabetics provided that the calories are taken into account. Lactitol can be used as a sweetener in most foods, but due to its low sweetness it is not very attractive.
i. Toxicology and Safety – Studies indicate that apart from diarrhea after consumption of large lactitol doses no toxicologically significant adverse effects have been noted (WHO, 1983). The EEC Scientific Committee on Food has accepted lactitol and most other polyols for use in food. However, it was pointed out that laxation may occur at high intakes.
F. Maltitol in Agricultural Applications
The maltitol molecule, 4-O-β-D-glucopyranosyl-D-glucitol, consists of a glucose and a sorbitol unit linked 1,4 (1,4-glucosyl-glucitol). Maltitol is produced by enzymatic hydrolysis of starch (potato or corn) to obtain a high maltose syrup, which is hydrogenated to the corresponding high maltose syrup, from which crystalline maltitol is obtained.
i. Toxicity Maltitol has a low acute toxicity by oral administration (LD50 > 24 g/kg body weight). Maltitol is not mutagenic. Teratogenic studies have also been negative.
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