Flavors and Aromatic Compounds in Foods

Flavors and Aromatic Compounds in Foods

Stability of Flavours During Processing

• Flavor is the most important parameter for food quality and competitiveness.
• Increasing attention is given to flavor stability.
• Traditional flavoring raw materials are produced under harsh conditions (heat, distillation, concentration, extraction), which destroys sensitive substances.
• 100% stability is not attainable with traditional methods.
• Modern flavoring raw materials are produced under controlled conditions (low pressure, low temperature distillation, extraction with low boiling solvents or CO_2).
• Sensitive substances survive modern production processes, influencing product quality.

Important Criteria for Understanding Flavour Stability

• Composition of flavorings.
• Raw materials used.
• Characterization of the flavoring.
• How to estimate flavor changes.
• Factors responsible for flavor changes.
• How to prevent flavor changes.
• Quality attribute of food aroma is influenced by:
  1. Chemical reactivity of food flavor.
  2. Environment of food (light, atmospheric oxygen).
  3. Food matrix system and constituents (protein, fat, carbohydrate, transition metals, radicals, polymers like melanoidins).

Factors Affecting the Stability of Flavours

• Heat treatment (evaporation of volatiles, formation of new flavor components).
• Oxidation (of terpenes, lipids); high oxygen concentration makes fat-containing products rancid.
• Enzyme activity (degradation and formation of flavoring components).
• Low pH; acid-catalyzed reactions occur, such as hydrolysis of esters or rearrangements like citral into p-cymene.
  C{10}H{16}O
  rightarrow C{10}H{14}
• Fat absorption of liposoluble components.
• Protein reactions.

Various Aspects of Flavour Stability

• Physical Stability:
  • Evaporation of volatile components.
  • Crystallization of non-soluble material (mainly in liquid flavorings).
  • Phase separation (in emulsions).
  • Solubility (in fat-containing food).
  • Absorption and adsorption effects in complex food systems.
• Chemical Stability:
  • Reactions with food components.
  • Reactions of Flavoring components through degradation, rearrangement, oxidation.
• Sensory Stability:
  • What is the standard sample to compare with (how is it stored?).
  • What does the customer expect and remember.
  • How does the customer evaluate the samples.

Various Processing Methods

• Thermal Processing:
  • In-Container Sterilization
  • Aseptic processing and Packaging.
  • Rapid heating and Cooling
  • Pasteurization (LTLT, HTST, UHT)
  • Sterilization
  • Various forms of Cooking
• Non-Thermal Processing:
  • High Pressure Processing
  • Pulse Electric Field
  • Ohmic Heating, Radio Frequency, Microwave
  • Ultra Filtration
  • Irradiation e.t.c

Effect of Thermal Processing: Key Notes

• Increase in reaction kinetics accelerates loss of flavor compounds.
• Cooked/Heated/Burnt and stale flavor of milk is due to ketones formation.
• Buttery, milky, coconut like flavors in milk are due to lactones formation from thermal breakdown of hydroxyacids.
• Furan derivatives formed when casein undergoes browning reaction with fructose at T>90ºC.
• Acetol and Acetonin gives off flavor to milk which has been heated above 90ºC.
• Chemical and rancid flavor increases in milk because of increased amount of Butyric and hexanoic acids when milk is treated above 100ºC.
• Hydrogen sulfide gives cooked flavor to milk and the intensity linearly corresponds to the intensity of heating.

Effect of Non-Thermal Processing: Key Notes

• Garlic Irradiation:
  • Diallyl disulfide reduced significantly when treated with gamma radiation (Wu et al., 2006).
• Ginger:
  • No major changes in volatile concentration in gamma irradiated ginger.
  • After 3 months decrease in a-zingiberene, B-bergamotene, neral, geraneal and a-curcumene were significant (Wu and Yang, 2004).
• High Intensity Pulsed Electric Field:
  • Study shows that PEF-processed tomato juice retained more flavor compounds of trans-2-hexenal, 2-isobutylthiazole, cis-3-hexanol than thermally processed or unprocessed control tomato juice.
  • PEF-processed juice had significantly lower non-enzymatic browning and higher redness than thermally processed or control juice.
  • Sensory evaluations indicated that the flavor of PEF-processed juice was preferred to that of thermally processed juice (Jia et al., 2006).

Pathways for Chemical Changes in Flavours During Processing

• Maillard Reactions:

  • When aldoses or ketoses are heated in solution with amines, a variety of reactions ensue, producing numerous compounds, some of which are flavors, aromas, and dark-colored polymeric materials.
  • Reducing Sugars and α-amino acids \rightarrow N-glycosylamine or N-fructosylamine \rightarrow 1-Amino-1-deoxy-2-ketose (Amadori intermediate) or 2-Amino-2-deoxy-1-aldose (Heynes intermediate)
  • \rightarrow Reductones and dehydroreductones \rightarrow Retroaldol condensation
  • Amino acids Strecker degradation \rightarrow Aldehydes + α-aminoketone (Methional, NH3, H₂S)
  • \rightarrow Heterocyclizaion \rightarrow Pyrazines, Thiazoles, Pyridines, Pyrroles, Oxazoles
  • The flavors, aromas, and colors may be either desirable or undesirable. They may be produced by frying, roasting, baking, e.t.c.
  • Products include Furans, Hydroxyacetone, Thiophenes, Hydroxyacetylaldehyde, Pyrroles, Acetoin, Acetylaldehyde, Glyoxal, Pyruvaldehyde, Glycerolaldehyde

• During Maillard reaction, certain changes occur at high temperature due to flavour degradation as observed during drying. Losses of volatile components and oxidation of sensitive substances both result in a changing flavour profile.

• Flavours rich in proteins or amino acids, that are not fully reacted or flavouring compounds which contain especially yeast extract, are sensitive to the Maillard reaction.

• Due to the Maillard reaction an unpleasant roasted note might be developed within the product.

Lipid Oxidation

• The mechanism of flavor development in heated oils is essentially that of lipid oxidation. Thermally induced oxidation involves hydrogen radical abstraction, the addition of molecular oxygen to form the peroxide radical, formation of the hydroperoxide and then decomposition to form volatile flavor compounds.
• The products of thermally induced oxidations differ from typical lipid oxidation products formed at room temperature.
• The quantitative effects of heating time and introduction of moisture during deep fat frying were also reported generally, the production of individual volatile components increased with heating time up to 48 h of heating
• The introduction of moisture during heating of the oil resulted in a large reduction in volatiles in the oil (Newar et al., 2008).

Effect of Packaging on Flavour Stability

• The flavour stability will also be affected by packaging. A classic example here is sun-struck flavour, 3-methyl-2-butene-1-thiol (3-MBT or prenyl mercaptan) in beer (Blocksman et al., 2001).
• In the proposed pathway for 3-MBT formation, hop derived isohumulones are decomposed to 3-methyl-2-butenyl radicals due to sunlight exposure. Sulfur-containing amino acids and proteins decompose to SH radicals through riboflavin-photosensitized reactions. These two radical types then combine and form 3-MBT.
• Glass bottles are usually impermeable for oxygen. Plastic bottles have the advantage of being light and unbreakable; however, the oxygen permeability is usually much higher.

Physical Changes in Flavour During Processing

• Flavour products manufactured by spray or vacuum oven drying are often amorphous solids or they contain amorphous particles. The viscosity of such amorphous solids strongly depends on temperature and their moisture content.
• At a specific temperature called the glass transition temperature, the viscosity drops by 3-4 orders of magnitude. The solids texture changes from a glassy into a rubbery state.
• In this rubbery state, the powder particles can sinter together depending on the viscosity and the time available for sintering (Palzer et al., 2004). Such sinter processes might happen very fast during mixing processes in which moisture is added to the powder.
• Powdered flavours, intermediate powder masses stored before packaging and packed dehydrated convenience foods can show caking and lumping during storage. Increasing the moisture content or the temperature further, the amorphous solid can liquefy.
• If the amount of amorphous flavour components is high enough, liquefying can even result in significant texture changes of the final product.
• Bouillon tablets might undergo a posthardening and later they can even be transformed into a pastymass..

Reactivity and Stability of Some Selected Flavoring Compounds

1. Citral

• Citral, 3,7-dimethyl-2,6-octadienal, is the most important flavor compounds in citrus oils. Because citral is an unsaturated aldehyde, it is highly susceptible to acid-catalyzed cyclization and oxidative degradation, particularly in the presence of light and heat, leading to off-flavor formation, especially in lime and citrus juice products (Liang et al., 2004).
• The degradation process of citral under acidic conditions is accelerated by high temperature, light, and availability of oxygen.
• Both unstable monoterpene alcohols can be deteriorated with disproportionation and redox reactions under acidic condition, and more stable aromatic compounds (p-cymene, p-cymene-8-ols, and a-p-dimethylstyrene) can be obtained later in the presence or absence of oxygen (Kimura et al., 2003).
• Ueno et al., 2004 found that 4-(2-hydroxy-2-propyl) benzaldehyde is one of the oxidation products of citral, leading to off-flavor, in addition to a,p- dimethylstyrene, p-cymene, p- methylacetophenone, and p-cresol.
• Using plant extracts including grape seed, pomegranate seed, green tea, and black tea, Liang et al revealed their inhibitory effects on citral off-odor formation.

2. Allyl Isothiocyanate

• AITC, 3-isothiocyanato-1-propene, is the major pungent flavor compound naturally found in plants of the Brassicaceae family such as horseradish, mustard, wasabi, and cruciferous vegetables.
• AITC is an unstable compound and has been reported to gradually decompose to compounds with a garlic-like odor in water at 37 oC and even at room temperature (Kawakishi, 2009).
• The chemical reactivity can be generated through various chemical reactions such as hydrolysis, oxidation, thermal degradation, and reaction with proteins including cabbage and cauliflower.
• However, the decomposition through hydrolysis and oxidation of AITC readily occurs under an alkaline condition and at higher temperatures (Ina et al., 2001).

3. Vanillin

• Vanillin, 4-hydroxy-3-methoxybenzaldehyde, commercially called p-vanillin, is a major constituent of vanilla flavor, and is a well-known flavoring agent used in various food industries such as bakery, confectionary, ice cream, fragrance, cosmetics etc.
• Oxidation of vanillin can occur both under alkaline condition and by enzymes including milk enzymes such as xanthine oxidase and peroxidase.
• Fargues et al., 2006 found that heating birch syrup at 100 oC decreased the aroma intensity of vanillin. Vanillin is highly oxidized when reacted with oxygen in an alkaline solution through various pathways, and the reaction is favored at higher temperatures, > 100 oC, as well as under elevated alkaline conditions.
• Oxygen concentration has an impact on the rate of vanillin oxidation at a higher, but not at a lower pH (Noryhery, 2007).
• Vanillin also reacts with amino groups of proteins, thus influencing flavor perception, decrease the intensity of vanillin flavor, and are likely to influence the release of flavor compounds during consumption.
• Different types of proteins, protein conformation, pH, temperatures, and concentration of proteins have impacts on vanillin protein binding or the types of chemical interactions (Li et al., 2002).
• Mikheeva et al., 1998 reported that vanillin interacted with proteins including blactoglobulin, bovine serum albumin, and ovalbumin mainly through electrostatic interactions, since certain physical factors such as temperature, pH, and ionic strength had effects on the binding.

4. MFT and Furfurylthiol

• While MFT was first identified in 1988 in heated canned tuna fish, 2-furfurylthiol (FFT), also called 2-furfuryl mercaptan, was known for the first time as a food component in roasted coffee.
• FFT is also an important odorant in freshly popped corn and roasted white sesame. FFT can be formed by heating glucose with hydrogen sulfide and ammonia.
• Although, MFT represents the pleasant characteristic odor of certain foods, in some products such as orange juice, it is an offflavor compound generated during storage (Hoffman et al., 2006).
• Mottram et al., 2006 found that heating a flavor compound containing thiol and disulfide groups in an aqueous solution at 100 oC with egg albumin causes a decrease in the concentration of flavor.
• Apart from food proteins, FFT is capable of binding to other polymers such as brown macromolecules that are formed during thermal processing of food. Of particular concern is the covalent binding of FFT to melanoidins which are generated when carbohydrates react with amino compounds at higher temperatures during roasting of coffee bean.
• This type of binding may also have an impact on flavors containing thiols and brown colors in many food items including meat, bread crust, or roast sesame seeds, besides coffee.

5. Methional

• Methional, 3-(methylthio)propanal, having a cooked potato-like flavor, is formed by the Strecker degradation reaction between a-dicarbonyl compounds, the key intermediate products of the Maillard reaction, and methionine (Met) (Di et al., 2006). In orange juice, it causes the off-flavor problem.
• Potato processing causes the loss of a large amount of methional because it is heat labile and readily decomposes to methanethiol, which oxidizes to dimethyl disulfide (Di et al., 2006).
• Besides the thermal instability, methional is also unstable to light and can be converted to many sulfur compounds, particularly in light-exposed milk. Methional is decomposed to methanethiol and dimethyl sulfide by exposure to light.
• A report showed that the broth and potato flavors of methional changed to methanethiol-like flavors on additional light exposure ( Jung et al., 1998).

Enhancing the Stability of Flavour

1. Microencapsulation:

• The affinity of the flavour compounds with the food matrix is extremely important because it will affect the flavour delivery process.
• As a result of encapsulation, the rate of flavour release is reduced and it is possible to control flavour intensity and quality of foods.
• The encapsulation process should be done prior to use in foods or beverages in order to protect food flavourings limiting aroma degradation or loss during processing and storage.
• Moreover, it will influence the overall acceptance by consumers (Naknean andMeenume, 2010; Madene et al., 2006).

Advantages of flavour encapsulation:

• Increases shelf-life of flavour
• Minimize flavour-flavour interactions
• Allows a controlled release
• Guard against light-induced reactions and/or oxidation
• Protection from evaporation and chemical deterioration
• Protects from undesirable interactions with food

2. Antioxidant:

• Gallotannin is used to improve the flavour stability of beer. Together with metabisulfite of potassium and ascorbic acid, developed as Antioxin®SBT, which is most effective for improving flavor in beer.

Conclusion

• Many chemical reactions and numerous factors, including temperature, pH, storage period, enzymes, and oxygen influenced the stability of flavor compounds.
• An understanding of flavor stability and the knowledge of an effective approach or technique to retard flavor degradation and improve the stability of flavor compounds are important to obtain desirable flavor for maximizing food product qualities.