Introduction to the Chemistry of Foods and Forages János Csapó Introduction to the Chemistry of Foods and Forages

Odor threshold is the lowest quantity of a component that is just enough for the recognition of its odor. The concentration at which the compound is detectable but the aroma quality still cannot be unambiguously established is called detection threshold. Both values can be determined by smelling (orthonasal value) or tasting (retronasal value) the samples and these values can be quite different. The threshold values depend on the olfactory properties and the structures of the molecules but also on the food matrix in which they are present. The medium and the temperature exert an effect on the vapor pressure of the compounds and therefore on the presence or absence of sensation, moreover other odor-producing substances can also interact and can cause strong changes in the odor thresholds.

In the presence of more aroma substances the additive effects prevail. When components with a similar aroma are present, the total intensity of the mixture is usually lower than the sum of the individual intensities. When the aroma bearing substances have different aroma notes there are two cases. In the case of the components have approximately equal odor intensities the odor profile is composed of the odor profiles of the components. When the odor intensity of one component predominates, this component then largely or completely determines the odor profile. In consequence of the additive effects the aroma profiles of foods containing the same aroma substances can be dissimilar owing to quantitative differences. This situation can be arisen if the parameters of the production process are altered and as the result of that the balance of the aroma substances are changed causing unusual olfactory characteristics.

The undesirable off-flavors can be originated from the raw material (plant food or that of animal origin). They can be developed during food processing via thermal treatments or fermentation defects. When the food is stored for a prolonged time period volatile products of chemical reactions (e.g. oxidation, Maillard reaction) or that of microbial deterioration can cause flavor defects. For example skatole is an off-flavor with faecal-like aroma notes and 2-methylisoborneol has earthy-muddy aroma notes. Both of them are microbial metabolites and they are involved in the development of pigsty-like and earthymuddy off-flavors.

Undesirable flavor not always can be attributed to the appearance of foreign aroma substances which are normally not present in the food. The organoleptic properties also can change if the concentration of key odorants decreased or the concentration ratio of the important individual aroma substances altered.

The important odorants can be listed according to their formation (nonenzymatic or enzymatic reactions or both) or grouped according to classes of compounds.

The nonenzymatic reactions (e.g. lipid peroxidation, Maillard reaction, Strecker degradation) can cause considerable changes in the aroma profile even at room temperature if the food is stored for a long time. The intensity of these processes are accelerated if thermal treatments (e.g. roasting, frying) are applied. When the food surface is dried out the pyrolysis of carbohydrates, proteins, lipids and other constituents (e.g., phenolic acids) occurs. In the case of nonenzymatic reactions usually large number of volatiles is formed by the degradation of few constituents. The level of the developed aroma active constituents often does not reach the odor thresholds, even under harsh conditions. Therefore the fraction of aroma active compounds in heated foods is small related to the number of formed volatiles.

The most important reactions of the carbonyl compounds are lipid peroxidation, caramelization and amino acid decomposition by the Strecker degradation mechanism.

Pyranones derived from carbohydrates and have a caramel-like odor. They are able to mask the bitter flavor of hops and cola and give sweet taste of food.

Furanones are secondary products of the Maillard reaction (e.g. norfuraneol present in meat broth and possess roasted, chicory-like, caramel aroma notes).

The precursors of thiols, thioethers, di- and trisulfides are the sulfur containing amino acids, monosaccharides and thiamine. They can be involved in the formation of both delightful and unpleasant aromas. Thiols have intensive odor and they are also reactants for the formation of additional volatiles (e.g. they can form mercaptoalkanes that are important contributors for the aroma of meet from diketones originated from the Maillard reaction, Fig. 20.).

Thiazoles giving characteristic aroma compounds of boiled meat, heated milk, beer and coffee are formed from cysteamine and 2-oxopropanal.

Pyrroles and pyridines are N-heterocyclic compounds. A special part of the molecules required for a roasted odor has been identified.

Pyrazines are formed on heating food and they are present in several food items (e.g. coffee, bread crust, fried meat and cocoa liquor). The number of compounds is enormous. In the group of alkyl pyrazines molecules having only three elements: C, H and N, but there are 70 compounds in this group, having mostly earthy and roasted notes.

Amines are formed both in Strecker reaction and enzymatic decarboxylation of amino acids (e.g. during the production of cocoa both process is developed with the predominance of Strecker reaction). Their odor thresholds are pH dependent. Trimethylamine is especially odor intensive and formed in the degradation of choline.

Phenols can be formed from phenolic acids and lignin by thermal decomposition or microbial contribution. They are present in smoked meat and fish products moreover some alcoholic beverages (whiskey) and butter have also phenols in low amounts.

The enzymatic reactions contributing to the formation of flavor of food on the one hand can occur as part of the normal metabolism of living organisms. On the other hand these reactions can be triggered by tissue disruption, when the raw material is disintegrated e.g. during slicing of fruits and vegetables. Moreover, the product of enzymatic decomposition processes can be the precursors for nonenzymatic aroma forming reactions e.g. the Maillard reaction can be initiated by releasing amino acids from proteins and sugars from polysaccharides. These preliminary processes are important in the aroma enhancement of bread, meat, beer, tea and cacao.

There are great numbers of alcohols and carbonyl aroma compounds formed by enzymatic reactions. The oxidative degradation of unsaturated fatty acids by lipoxygenase or/and hydroperoxide lyase results in the formation of oxo-acids, aldehydes and allyl alcohols in fruits and vegetables. Fatty acids and amino acids are important precursors of a great number of volatile aldehydes (e.g. ethanal has a great importance for the fresh note, e.g., in orange and grapefruit juice).

Aroma possessing hydrocarbons occur in some fruits and vegetables (e.g., pineapple, apple, pear, peach). They are mostly unsaturated and having 11 carbon atoms. They are synthesized from unsaturated fatty acids with the contribution of lipoxygenases.

Esters are formed in intact cells of fruits in the presence of acetyl coenzyme A (originated from the β-oxidation of fatty acids or from the metabolism of some amino acids). In the case of plant tissue disruption the esters are rapidly hydrolyzed by hydrolases and the fruit aroma flattens.

The regio- and stereospecific oxidation of oleic acid and linoleic acid results in the formation of hydroxy acids. These compounds can form lactones with cyclization. Lactones can provide very pleasant sensations and they contribute to the typical aroma of butter, coconut oil, and fruits. These substances are also of interest for commercial aromatization of food.

Terpenes occur in vegetables, fruits, wine, herbs and spices. The aroma spectrum of mono- and sesquiterpenes is wide and perceived as very pleasant. They are present in part as glycosides that can be hydrolyzed enzymatically (by β-glucosidase) or due to the low pH of juices. In the latter case glycoside hydrolysis is accelerated by a heat treatment. Terpenes mostly have one or more chiral centers. The different optical isomers usually occur in different plants and the odor characteristics of enantiomers and diastereoisomers are usually not the same.

Volatile sulfur compounds can be formed from the enzymatic decomposition of glucosinolates or S-alkyl-cysteine-sulfoxides. They are participated at the development of the aroma of many vegetables (e.g. Brassicacea, Liliaceae). For instance, 2-isobutylthiazole is a secondary metabolism product of leucine and cysteine and contributes to the aroma of tomato.

Pyrazines can also be formed enzymatically from amino acids in vegetables (e.g. 2-isobutyl-3-methoxypyrazine from leucine in chillies). Microbial synthesis is also important but must be avoided because their presence can cause musty/earthy off-flavors, e.g. 2-isopropyl-3-methoxypyrazine in eggs, dairy products and fish.

The intensity and quality of food aroma depends also on the presence and ratio of the other food constituents e.g. lipids, proteins and carbohydrates. They have an influence on aroma retention and therefore on the levels of volatiles in the gaseous phase. The odor profile of emulsions with fat content from 1% up to 20% aromatized with the same mixture of aroma differed significantly (Fig 21.). At higher oil content the perception of apolar compounds decreased while that of the polar constituents increased.
Chapter 9. Enzymes in the Food Industry

Enzymes used in the food producing technologies since ancient times. They can be original constituents of the food raw materials or originated from the microorganisms used in the fermentation of food. Nowadays purified enzyme preparations are often used. Enzymes can be used repeatedly when they are fixed to a carrier (immobilized enzymes).

The most important group of enzymes in the food industry is hydrolases. Chymosin or rennin has been used in the dairy industry for the formation of casein curd. Nowadays these enzymes are produced with genetically engineered microorganisms. Proteinases used in baking industry in order to modify the rheological properties of dough from wheat flour. The gluten can be softening with partial hydrolization. Proteinases of microorganisms and plant origin are also used in meat industry for meat ripening and tenderizing. Cold turbidity in beer can be eliminated with the hydrolysis of proteins that can form sedimentation in beer.

Lipases can be applied in bakery products in order to release mono- and diacylglycerols and therefore retard the staling. Partial hydrolysis of milk fat enhance the ’milky character’ of the flavor in chocolate.

Invertase (β-D-fructofuranosidase) is used in the candy industry to produce invert sugar (glucose: fructose 1:1) from saccharose. The advantages of invert sugar to sucrose are the increased solubility and the higher sweet taste intensity owing to the free fructose content. In milk products for individuals suffering from lactose malabsorption lactose is hydrolyzed by β-D-galactosidase (lactase).

High temperature-resistant bacterial amylases (α- and β-amylases) promote the hydrolysis of corn starch. Amylases are also used for the acceleration of the starch degradation in wort.

Lysozyme together with nisin is used for the preservation of cheese and meat products in order to prevent undesired puffing by Clostridia.

The shelf life of rye bread can be improved with the partial hydrolysis of rye pentosanes with pentosanase .

Pectin methylesterases deliberate pectinic acid from pectine in juices. Pectinic acid form precipitate with calcium and an undesired flocculation occur in the juice. Flocculation in the product can be avoided with the preliminary clarification of juices with partial pectinolysis.

Oxidoreductases are used in order to improve the organoleptic properties of the products. Glucose oxidase helps to preserve the original fragrance and color. With the elimination of glucose prevents the discoloration of the product via Maillard reaction, while with the removal of oxygen in sealed package systems lipid oxidation and decomposition of natural pigments and aromas are reduced.

Aldehyde-type off-flavors can be formed during soya processing. They can be oxidized to carboxylic acids with aldehyde dehydrogenase to elucidate ’bean-like’ off-aromas.

Lipoxygenase are used to oxidize the carotenoid pigments of wheat flour (’bleaching’). Besides of color effect the volume of bread also increases and the rheolological properties are improved.
Chapter 10. Food Technological Additives

The addition of different substances to a certain food item can be achieved through diverse reasons. Additives can be used as preservatives with the suppression of microbial spoilage or retarding undesired chemical and physical changes. The organoleptic properties of food often changes in unfavorable direction during the units of processing, therefore readjustment of the wanted quality is required with additives. Flavor and color is improved with aroma enhancers and pigments, respectively, and the consistency can also be stabilized (e.g. with polysaccharides). Nowadays the consumers’ need often require the fortification of food with beneficial ingredients (e.g. vitamins, minerals, amino acids). This product group is called functional food or nutraceuticals and they can contribute to the preservation of health.

The most important requirements of the antimicrobial agents are wide biological activity, negligible toxicity for consumers and acceptable cost. Weak acids are useful as preservatives in the undissociated form because they have to penetrate into the inside of the microbial cell therefore they are suitable only for the preservation of acidic foods. In some cases anion is also active against microbial spoilage. Propionic acid and acetic acid are formed during fermentation process and benzoic acid is also present in nature in glycoside form. Propionic acid is very active against molds while acetic acid is primarily against yeasts and bacteria. Benzoic acid gives antimicrobial activity against yeasts and molds. Sorbic acid and parabens (p-hydroxybenzoic acid alkyl esters, PHB) are used against fungi and yeasts.

The efficacy of sulfite and sulfur dioxide covers a wide spectrum (against yeasts, molds and bacteria) but the hazard of mutagenic activity has arisen. The toxicity is negligible at the applied levels. Sulfite reacts with a number of food constituents. Besides antimicrobial activity sulfur dioxide hampers Maillard reaction (blocking of carbonyl group) and also enzymatic browning (inhibition of polyphenol oxidases).

Ethylene oxide and propylene oxide practically can be used against all microorganisms and viruses. Owing to their severe toxicity they must be removed from food before consumption. These additives are prone to react with several food constituents and their application can contribute to the decrease in the level of essential components (e.g. pyridoxine, folic acid and methionine). They are mostly used as gaseous sterilants for low water content foods.

Nitrate is reduced to nitrite with the nitrate reductases formed by bacteria present in meat products. Nitrite is effective against microbial activity moreover it is important for aroma preservation (via hampering the oxidation of flavors) and also for color formation.

In the presence of nitrite, myoglobin (and hemoglobin also) can form both nitroso-myoglobin and metmyoglobin (nitroso-hemoglobin and methemoglobin, respectively). The acute toxicity of nitrite via methemoglobin formation is not hazardous at the applied levels of concentration. In contrast with this the development of nitrosamines having carcinogenic activity is a real problem. However, the application of nitrite as preservative is justified in meat products when the risk of Clostridium botulinum infection arise (e.g. milder heat impact than sterilization).

The antibiotics nisin is applied against Gram-positive microorganisms in cheeses. Natamycin is used against mold growth during the ripening of raw sausages.

Acids used as additives have a complex effect. Besides antimicrobial activity they can affect the taste . The sourness is correlated with the number of undissociated H⁺ ions.

Citric acid and its salts are often used in the industry in order to achieve aroma improvement (e.g. in dairy products, fruit juice) and diminishing of browning (in fruits and vegetables). It can amplify the influence of antioxidants as synergetic compound.

Phosphoric acid is also a widespread food additive. The primary field of use is the area of soft drinks (e.g. cola) but it is also added to processed cheese, fruit jellies, baking powder (as acid salts release CO₂ from NaHCO₃). In fermentation technology it can be the part of buffering agent for pH-adjustion.

Lactic acid hinders discoloration in fruit and vegetable based foods, improve the flavor of beverages and used in the form of calcium lactate as additive in milk powders. It forms intermolecular esters in concentrated solutions therefore it can be used as acid generator.

The release of gluconic acid from glucono-δ-lactone is slow thus it can be applied in processes when slow acidification is required (e.g. in raw sausage ripening, sour milk production).

Tartaric acid is susceptible to form metal complexes therefore acts as a synergist for antioxidants. It can also be used as an acidifier in drinks (wine, fruit juice) owing to its ’rough’ or ’hard’ sour taste.

Malic acid in the form of fatty alcohol monoesters is useful antispattering agents in frying oils and also can be present in canned fruits and vegetables (e.g. tomato).

Fumaric acid is often used together with other preservatives (e.g. with benzoic acid). It is used for pH reduction. Gel setting is also promoted.

The addition of adipic acid has a positive influence on the consistency of the products (e.g. cheeses, jellies and marmalades).

Bases are used in the development of light or dark brown surface of some bakery products (e.g. dough pieces are dipped in diluted NaOH solution prior to baking).

The preservation of unsaturated lipids in products with significant fat content is a challenge in food industry. Lipid oxidation affects not only the glicerydes but also other important food constituents (e.g. vitamins, aromas, colorants). Besides the loss of nutritional quality the formation of unwanted degradation products (e.g. off-flavors) is also an accompanying phenomenon of the oxidation of food lipids. In order to prevent this process lipid oxidation retardants are used.

Sequestrants (chelating agents) bind metal ions. Metals in complexes are unable to catalyze the oxidative deterioration processes therefore the color, aroma and texture of food are preserved, and moreover oxidation of ascorbic acid and fat-soluble vitamins are prevented. Many chelating agents are found in food itself (e.g., oxalic, succinic, lactic, malic, tartaric, citric and polyphosphoric acids, amino acids, peptides, proteins, porphyrins).

Antioxidants often have a phenolic structure. Natural antioxidants are e.g. tocopherols, ascorbic acid esters. Synthetic additives are e.g. gallic acid esters, tert-butylhydroxyanisole (BHA) and di-tert-butylhydroxytoluene (BHT). The use of mixed sequestrants/antioxidants preparations is often advantageous.

Flavor enhancers are compounds that possess aroma enhancement affect in the matrix of food, without being aroma active themselves in the applied concentrations.

Monosodium glutamate (MSG) enhances the sensory perception of meat-like aroma notes. Responsible for the sensation of the fifth quality of taste called umami. In canned, dehydrated or frozen meat or fish products these flavors are partially deteriorated therefore MSG has to be added to restore the initial aroma. It is responsible for the ’Chinese restaurant syndrome’ disorders (headache, stomach ache and drowsiness) at hypersensitive persons.

5-Nucleotides in a certain concentration range possess aroma enhancing properties for several foods (e.g. soups, canned meat, sauces).

Sweeteners are tools of calorie-reduced nutrition when fighting against obesity is necessary. Their consumption should not have health risks. Their sweet taste should be clean without side or post-flavor effects. Other requirements are adequate solubility in matrix and a good stability at a wide pH temperature range moreover an affordable price.

Cyclamate (Na- or Ca-salt of cyclohexane sulfamic acid) is a worldwide used sweetener (Fig 22.). It has lower taste intensity with not as pleasant taste character as saccharin but it bears no bitter after-taste.

Saccharin is usually used in the form of the Na or Ca salt (Fig 23.). The solubility of this form is higher but the sweetening ability is lower than that of saccharin. This sweetener has a metallic to bitter after-taste at higher concentrations.

Aspartame (’NutraSweet’, L-aspartyl-L-phenylalanine methyl ester) is also a widespread sweetener. Stability problems can occur at heating or at longer period of storage but no problem in use at drinks that are consumed immediately (tea, coffee).

Dipeptides containing L-Asp and D-Ala are sweet. Alitame is the N-3-(2,2,4,4-tetramethyl)- thietanylamide of L-Asp-D-Ala dipeptide (Fig 24.). The stability of alitame type dipeptide amides is significantly higher than that of aspartame type dipeptide esters therefore their application spectrum is wider and can be used in bread and confectionery industry.

The sweet taste of oxathiazinone dioxides (Fig 25.) appears quickly and this artificial sweetener is very stabile during the usually applied processes, and also throughout the storage of products. The acceptable daily intake (ADI) is 9 mg/kg BW for K-salt.

Another alternative to obtain sweet sensation is the use of sweet proteins . They are originated from the pulp of different fruits. Monellin is a protein with two not covalently bound peptide chains and possess sweet taste only in solutions that are present in its native conformation. If the peptide chains are separated the sweet character disappears therefore its stability is low and cannot be applied as a commercial sweetener. Thaumatins can be applied in milk products and chewing gum and has a synergistic effect when applied in combination with saccharin and acesulfame.

Food is often regarded as a dispersed multiphase system and the stabilization of these dispersions is particularly important in food processing. Dispersions (emulsions, suspensions, foams, aerosols) consist of two phases. One of them is dispersed and the other is the continuous outer phase. The types of emulsions can be oil in water (o/v e.g. milk) or water in oil (w/o e.g. mayonnaise).