Food chemicals changes

Overview

The food may undergo degradation reactions throughout the various stages involved in their production from field to consumer. These impairments, which are physical, chemical, enzymatic and / or microbiological, depend on several factors:

• The nature and condition of the food (fresh or processed).
• The conditions of harvesting, handling, processing, packaging, storage and marketing of food.

The main chemical reactions of food degradation are: enzymatic browning, non-enzymatic browning, lipid oxidation and enzymatic hydrolysis of food constituents such as lipids and carbohydrates.

The reactions of food degradation are generally undesirable as they lead to spoilage of food by modifying its organoleptic and nutritional characteristics. In other cases, some of these reactions are sought to give the food color and taste desired, for instance, the Maillard reaction.

Enzymatic browning

Enzymatic browning is the conversion of phenolic compounds in colorful polymer, usually brown or black that are designated melanins. This also leads to browning degradation of vitamin C.

Mechanism of enzymatic browning

Plant cells contain many phenolic substrates such as tyrosine, acid chlorogénioque, the pyrocatechol, and so on. Under the action of enzymes (polyphénoloxydase, peroxidases) and in the presence of oxygen, these phenolic compounds oxidize easily quinones . quinones formed in turn, oxidize, without using specific enzymes, and polymerize giving brown compounds that are responsible for superficial or deep brown that appears in various circumstances (peeling, cutting, grinding, etc.)..

Plant organs not brown if their tissues are injured or if their metabolism is profoundly disturbed. In healthy cells, the phenolic compounds are located in the vacuole while the oxidation enzymes are localized in the cytoplasm. The membrane separating the vacuole from the cytoplasm prevents any contact between enzymes and their substrates: the oxidation of these substrates did not occur. However, when cells are injured, all their constituents are mixed. The reaction of oxidation of phenolic compounds occurs then, provided that oxygen is present: browns appear. In addition, any malfunction or cellular physiological disorder leading to a change in membrane permeability may also result in browning.

Control or prevention of enzymatic browning

Control or prevention of enzymatic browning can be achieved in three ways. The first is based on the inhibition of enzymes polyphénoloxydases, the second is based on trapping the quinones and the third is based on limiting the availability of oxygen.

Inhibition of polyphénoloxydases

The polyphénoloxydases are metalloenzymes containing about 0.2% copper which acts as a coenzyme. They are active between pH 5 to 7. Their inhibition is achieved by making an acidification of the medium, to heat treatment or by the use of additives.

The techniques most often used to prevent enzymatic browning is the acidification and bleaching. A decrease in pH to a value close to 3 or a short exposure to temperatures of 70 to 90 ° C (money) is usually sufficient to obtain a partial or total inactivation of the enzymes.

It is also possible to use additives in order to limit the activity of polyphénoloxydases. These additives are mainly:

• compounds that destabilize the Cu₂⁺ ions associated with polyphénoloxydase (NaCl, CaCl₂),
• competitive inhibitors: acidic organic aromatic ring (benzoic and cinnamic acid),
• sulphites which are potent inhibitors of enzymatic browning. Inhibition of activity by sulphites polyphénoloxydases is complex inhibitor partially denatures the enzyme in complex with the protein resulting in structural changes. Although effectively limiting browning and possessing antifungal and antioxidant properties, the use of sulphites is very regulated.

Reduction and trapping quinones

Other reagents can also be used to inhibit browning. These are compounds that react with quinones such as: ascorbic acid, cysteine, the thiol and sulphite. These compounds reduce quinones and phenols retard the browning reaction.

Reduced pressure of oxygen

Enzymatic browning requires oxygen. Thus the reduction of browning can be achieved by maintaining the food or atmosphere devoid of oxygen greatly impoverished. Therefore, the coating or the dumping of food is sometimes useful to slow enzymatic browning.

Maillard Reaction

The Maillard reaction is also known as non-enzymatic browning, although it includes other reactions such as caramelization browning.

The discovery of the Maillard reaction, dates back to 1912 by Louis-Camille Maillard. While working on the synthesis of proteins by heating, by chance he got flavoring and colored it identified as melanoidins, brown polymers responsible for the color and flavor of many foods (bread crust, coffee and roasted chocolate, beer, etc.)..

The Maillard reaction is the set of interactions resulting from the initial reaction between a reducing sugar and an amino group (amino acids, peptide, protein). It takes place during storage of food or more frequently when exposed to heat treatments. This reaction has an enormous importance in the chemistry of food. It is the main responsible for the production of fragrances, flavors and pigments characteristics of cooked foods. The emergence of a distinct brown color and flavor of foods associated with roasts, grilled or baked is a feature of this reaction. It has the result that food tastes with little or appetizing flavor when raw can be turned into desirable products after they have undergone heat treatment.

The Maillard reaction can also give rise to carcinogenic compounds and also reduce the nutritional value of foods by degrading the essential amino acids and vitamin C. In vivo, it is active in the process of degradation of collagen.

Chemistry of the Maillard reaction

We can subdivide the Maillard reaction in three main steps. The first leads to the formation of reversible glycosylamines which rearrange by rearrangements of the Amadori or Heyns. The second stage corresponds to the degradation products of Amadori rearrangements and Heyns. It leads in particular to the formation of heterocyclic compounds responsible for odors. The third step is the polymerization reaction of intermediate products during the second stage, and leads to the formation of melanoidins.

Temperature, reaction time, water content and the concentration and nature of precursors influence the Maillard reaction.

Step 1: rearrangements of Amadori and Heyns

The first stage of the Maillard reaction, is part of a broad spectrum of reactions called carbonyl-amine reactions. These reactions are involved in a number of enzymatic and biological processes such as vision, aging and deterioration of fabrics.

The Maillard reaction is initiated by the reaction between the open form of a reducing sugar (glucose, ribose, fructose, xylose, etc.). And an amino group (amino acids, peptide, protein). It leads to the formation of a Schiff base that exists in equilibrium with an acid glycosylamine (Figure 1).

The reactions of formation of Schiff base are reversible, in strongly acidic, sugar and amino acid can regenerate completely. However, the Schiff base undergoes slow rearrangement to produce a stable derivative. The nature of this derivative is variable and depends on the reducing sugar town. The aldose after Amadori rearrangement to produce for cétosamines (Figure 2), whereas ketosis suffer Heyns rearrangement and produce aldosamines (Figure 3).

The products of rearrangement of Amadori and Heyns compounds are relatively stable in some foods such as milk, and under conditions of mild heating, may represent the final stage of the Maillard reaction. These products, although not contributing to the formation of pigments and flavors in the food, reducing the availability of essential amino acids.

Step 2: Degradation products of rearrangements of Amadori and Heyns

This step is more complex than the first because it is composed of several reactions. However, three tracks stand out: two by the pH, and a third division of aldosamines or cétosamines.

The first track is called moderate dehydration (Figure 4). It is favored at pH neutral and slightly alkaline. It is an irreversible énolisation between carbons 2 and 3 of cétosamine (or aldosamine) and a loss of the amino residue. The intermediate compound, 1-methyl-2 ,3-dicarbonyl, is a réductone (diketones) responsible for the autocatalytic nature of the Maillard reaction, via the Strecker degradation (Figure 6). This réductone decomposes in a complex manner in a wide variety of compounds according to mono-and di-carbonyl: furanones, CYCLOPENTANONES, isomaltols.

The second is called dehydration strong (Figure 5), favored by acid pH. It is the main route of degradation cétosamines. It begins with the formation of an ene-diol between carbons 1 and 2 of glycosylamine. Rearrangement then led to a double bond 2, 3 and the deamination of carbon 1. This intermediate product is also a participant réductone to the Strecker degradation. The loss of a water molecule gives an unsaturated dicarbonyl compound, which by cyclisation, yielded the furfuraldéhydes, which the hydroxymethyl-furfural (HMF).

These first two ways are followed by the Strecker degradation (Fig. 6). The latter involves a deamination and oxidative decarboxylation of an α-amino acid in the presence of a réductone. It leads to the formation of an aldehyde (the Strecker aldehyde), which corresponds to the amino acid starting with a carbon and at least α-aminocétone. This reaction causes a release of CO₂, which can form foams in products with long shelf life.

Of Strecker aldehydes are important intermediates involved in the third step in the formation of melanoidins. Aminocétones While giving condensation of pyrazines (Figure 7), aromatic substances that are found in a large number of cooked foods such as roast meat and roasted coffee.

The third way is the division. A cut of cétosamines and aldosamines obtained may lead to the formation of small molecules or carbonylées acid by aldol condensation, give polymer products and other features of the odorants in Maillard reaction.

Step 3: Polymerization of intermediate reaction

The melanoidins which are the food brown pigments are produced in the third and final stage of the Maillard reaction. They are brown polymers, high molecular weight containing furans, and nitrogen and may contain carbonyl groups, carboxyl, amine, amide, pyrrole, indole, ester, anhydride, ether, methyl and / or hydroxyl . Their training is the result of the polymerization of highly reactive compounds produced during the second stage and especially unsaturated carbonyl compounds and furfural. Polymerization in the presence of amines gives brown pigments are insoluble in water. In addition to their contribution to the brown color of food, these polymerisation reactions involved in curing foods cooked and stored. The chemistry of these reactions is still poorly understood.

Applications of the Maillard reaction

The Maillard reaction has been used for many years to produce foods that seem attractive to the consumer, and this both in appearance and in terms of flavor.

The food industry uses the Maillard reaction in many processes of food processing in order to provide consumers with the flavors and colors he wants. To do this, it is essential to control this reaction. Under the conditions employed, the Maillard reaction may lead to the formation of color or discoloration, and may promote the formation of pleasant flavors or rancid, the production of antioxidant compounds or toxic compounds, or it may reduce the nutritional value food and possibly lead to the formation of carcinogens.

Factors influence the Maillard reaction

The effects of the Maillard reaction, are usually sought in the operations of cooking. In other cases, such as drying of milk, this reaction is undesirable because it is responsible for changing the color, taste and nutritional value of milk powder. Knowing what factors influence the reaction Maillard is essential for speeding up or slowing down depending on the objective.

Temperature and reaction time, pH and moisture of the medium, the presence of metals, oxygen and inhibitors and the nature and concentration of various reagents affect the speed of the Maillard reaction.

The temperature is certainly the most influential factor. Indeed, the speed of reaction is doubled when the average temperature increases of 10 °C (Z = 33 °C). Yet it should be noted that the reaction occurs even at 4 ° C and must take into account the torque-time duration. The storage of food at temperatures below 0 °C can slow the Maillard reaction.

The reaction is also influenced by the pH of the environment and its water content. She is in optimal pH of 6 to 10 and in environments with a relative humidity of 30 to 70%. Beyond these values, the reaction is slowed.

The use of inhibitors is also important to slow the Maillard reaction. Sulphites, for example, reacting the carbonyl compounds from various stages of the Maillard reaction, forming stable sulfonates and delay non-enzymatic browning.

Caramelization reaction

The caramelization, as the Maillard reaction is a reaction of non-enzymatic browning. It occurs when heating a sugar beyond its melting point (about 200 °C for sucrose) in the absence of nitrogen compounds. The reaction can be catalyzed by the addition of an acid such as citric acid and acetic acid. The products formed during the reaction give the caramel color, aroma and flavor of the product.

Chemistry of the reaction of caramelization

The caramelization reaction can be divided into two main steps. The first step for the degradation reactions of sugars leads to the formation of aldehydes and related compounds dicarbonylés, then there is emergence of non-colored or yellow, which absorb strongly in UV The second step is a condensation polymerization and after the first step and leads to the formation of dark brown high molecular weight:

• Step 1: The reactions of degradation when you are starting with sucrose hydrolysis catalyzed by the water that acts as an acid catalyst. Both obtained hexoses (glucose and fructose) are reactions of nonspecific degradation that depend on the pH. Alkaline degradation dare leads to the formation of pyruvaldéhyde and lactic acid. In a first acidic énolisation stage was soon followed by elimination of water molecules and cyclisation leading to three important intermediate 5- hydroxymethyl-furfural (5-HMF), 2(-2-hydroxyacétyl)-furans and maltol (Figure 8). These molecules are part of the volatile fraction responsible for the characteristic aroma of caramel.

• Step 2: The reactions of condensation and polymerization are more specific. The loss of two water molecules with cyclisation gives from sucrose, the fructose dianhydrides (DAF) (Figure 9), major constituents of the volatile fraction of caramel (70%).

Application of the caramelization reaction

The caramelization reaction is mainly used for the preparation of caramel. Preparing caramel uses simple materials (sugars food, water and sometimes a drop of citric acid (lemon) or acetic acid (vinegar). It can be prepared by the housewife to decorate desserts and pastries, but also made on an industrial scale according to recent thermal processes such as induction cooking or heating by microwave. There are two kinds of caramel: caramel aroma and caramel coloring, classified respectively as an ingredient or food additive.

Oxidation of lipids

The main factors determining the life of the lipid oxidation reactions. Substrates of these reactions are mainly unsaturated fatty acids. They oxidize faster in general when they are free and more unsaturated. Saturated fats can oxidize at temperatures above 60 ° C, whereas the same oxidize polyunsaturated acids during storage foods in a frozen state.

In terms of living tissues, there are natural mechanisms for controlling the oxidation in order to prevent oxidative destruction of membrane lipids, proteins and nucleic acids. Thus, there is a regulatory system pro-oxidant and anti-oxidants which keep the balance between the factors involved in the oxidation reactions. The factors regulating pro-and antioxidants is disturbed at the death of the animal or plant cells and during processing and storage of food, which promotes the development of oxidation reactions.

The factors that influence or initiate, the oxidation of lipids are many. These are intrinsic factors such as the fatty acid composition of lipids (number and position of unsaturation), the presence of pro-oxidants (metal ions, enzymes, etc.) Or natural antioxidants (tocopherols, carotenoids, etc. .) and external factors such as temperature, light, the partial pressure of oxygen, water activity, storage conditions and processing.

Depending on the agents initiators class on the oxidation of lipids in 3 types:

• the auto-oxidation catalyzed by the temperature, metal ions and free radicals;
• the photo-oxidation, initiated by light in the presence of photosensitizers and
• the enzymatic oxidation initiated by the presence of oxidative enzymes.

The main problems posed by the oxidation of lipids residing in the degradation of biochemical properties, organoleptic (formation of volatile compounds smell unpleasant: rancidity) and nutrition (by interaction of oxidation products with amino acids) of the food . The lipid oxidation also leads to the formation of peroxides which are carcinogenic molecules.

Auto-oxidation

The oxidation of lipids is an auto-catalytic reaction. This is a series of radical reactions taking place in three stages. The first reaction produces a free radical by elimination of a hydrogen from fatty acid (initiation). Then follow the reactions to produce more free radicals (propagation) that combine to form compounds not radical (termination).

• Initiation (Fig. 10): In the presence of an initiator (I), unsaturated lipids (RH) lose a proton (H °) to form a free radical lipid (R °). The pull of a proton is facilitated by heat (molecular agitation) and by radiation or catalysts (metals such as Cu, Fe, Co, Mn, Ni ...).

• Propagation (Figure 11): Free radicals formed (R °) determine molecular oxygen and form free radicals unstable peroxides (ROO °) which can react with another molecule of fatty acid (RH) to form hydroperoxides (ROOH ).

• Termination (Figure 12): The radicals formed react with each other to lead to a product that is not a free radical.

The hydroperoxides, the first products of oxidation of lipids are unstable. They will be returned to a series of reactions that will lead to a myriad of compounds having varying molecular weights. At this stage, the taste of fat is impaired and there is talk of rancidity.

Photo-oxidation

The photo-oxidation is an important route for the production of hydroperoxides in the presence of oxygen, light energy and photosensitizers such as hemoproteins, chlorophyll and riboflavin.

The photosensitizers (Sens) absorb light energy and pass to the triplet excited state (Sens³). They are involved in the oxidation of lipids in two types of mechanisms:

• The type I photosensitizers such as riboflavin, act as free radical initiators. In their triplet state, they pull a hydrogen atom or an electron to the lipid molecules to form a radical capable of reacting with oxygen (1).

Sens³+ RH -> H + R Sens ° (1)

• The second mechanism, the molecules of type II, such as chlorophyll and erythrosine, respond in their excited state(Sense³) with triplet oxygen to which they transfer their energy to give singlet oxygen (¹O₂) (2).

Sens³ + ³O₂ -> ¹O₂+ Sense (2)

The singlet oxygen thus formed is very electrophilic and can react directly to an unsaturated fatty acid (RH) forming a hydroperoxide (ROOH) (3).

¹O₂ + RH -> ROOH (3)

Thereafter the reactions involved in radical chain auto-oxidation. The hydroperoxides formed are different from those formed by autooxydation.

Enzymatic oxidation

The phenomenon of oxidation of unsaturated fatty acids may be the original enzyme. Both enzymes are mainly involved lipoxygenase and cyclooxygenase.

Lipoxygenase catalyses the insertion of a molecule of oxygen on unsaturated fatty acid by a reaction stéréospécifique and leads to the formation of hydroperoxides. It acts specifically on non-esterified fatty acids. Its activity is often coupled with that of lipases and phospholipases.

Cyclooxygenase is a lipoxygenase that incorporates two molecules of oxygen to an unsaturated fatty acid hydroperoxides to form specific.

The enzymatic oxidation occurs even at low temperatures. During storage in frozen state enzymatic activity is slowed. However, once the thaw began, and temperatures from 0 ° C to 4 ° C reached, and resume the activity increases. A -40 ° C, the enzymatic oxidation of lipids is completely stopped.

Factors influence lipid oxidation

The main factors involved in lipid oxidation during processing and storage of foods are: temperature, pH, water activity and the partial pressure of oxygen.

A rise in temperature favors the oxidation of lipids. The oxidation of lipids is more rapid than the temperature is important. Thus, the cooking operations are well known to have a marked pro-oxidant. In contrast, freezing is a good way to increase the shelf life of foods, because the rate of lipid oxidation was significantly reduced at low temperature .

The pH influence the course of oxidation through several mechanisms. First, for the redox reactions involving reduction of protons (H⁺) redox potential decreases linearly with pH. An acidic pH favors the oxidation reaction, particularly when pro-oxidant species (ions of transition metals) or antioxidants (eg ascorbic acid) soluble in aqueous phase are present. The pH also plays a role in the solubility of the compounds involved in the initiation of the reaction. Thus, the higher the pH is low, the greater the solubility and redox potential of these metal ions and thus their reactivity towards oxidizable molecules are high. In the case of muscle tissue, a low pH promotes the distortion of hemoproteins and release of iron which is a pro-oxidant.

The water activity of a system affects the oxidation of lipids. Indeed, the water allows the mobilization of substances pro-oxidant or antioxidant. In general, a awcomprise between 0.2 and 0.3 corresponds to the oxidation rates lower. These values correspond to the formation of a monomolecular layer of water around the components. A a_w between 0.6 and 0.8 corresponds to speeds greater oxidation. A very low activity of water is also conducive to oxidation. Against by the enzymatic oxidation of lipids are slowed when water activity is less than 0.7-0.8.

The concentration of oxygen (oxygen partial pressure) in the space surrounding the product and the product itself affects the rate of oxidation of lipids. It is also in the nature of secondary products formed by decomposition of hydroperoxides. Its impact is both on the shelf life of the product and the nature of odors collected when the product is oxidized. The relationship between oxidation rate and oxygen partial pressure depends on several factors such as water activity, temperature, nature of the catalysts. When the oxygen concentration is high enough, the rate of oxidation is independent of the concentration. Conversely, when oxygen concentration is sufficiently low, the rate of oxidation is independent of substrate concentration and directly proportional to the concentration of oxygen. For intermediate concentrations, the rate of oxidation depends on both oxygen concentration and substrate.

Control or inhibit the oxidation of lipids is based on controlling parameters such as temperature, pH, a_w, oxygen concentration. Action on several parameters at the same time can increase or reduce the rate of oxidation.

The use of antioxidants (tocopherols, polyphenols, flavonoids, vitamin E, vitamin C, etc..) is often the most common method in food industry to inhibit the oxidation of lipids. Antioxidants are used either for prevention agents that block the initiation phase by reacting with the initiators of the reaction (O₂, light, metals, ...), or termination agents that block the continuation of the propagation phase reacting with free radicals and converting them into stable compounds.

Lipolysis

Lipolysis takes place in the plant and animal cells during post-harvest (or post-mortem) during processing and storage of food. The hydrolysis of lipids is mainly made of tissue lipolytic enzymes: lipase. Lipases hydrolyze ester linkages of glycerides and free from triglycerides of fatty acids, diglycerides and monoglycerides.

These enzymes remain active even at a storage temperature of -18 ° C although the kinetics of formation of free fatty acids is slowed. Therefore the enzymatic hydrolysis of lipids is the main weathering reaction of fresh foods during their storage in a frozen state.

Hydrolysis of carbohydrates

The hydrolases are problematic in the case of foods of plant origin are enzymes and PECTIQUES amylases.

Amylases hydrolyze the starch foods in reducing sugars. This is the case of potatoes stored at temperatures below 5 ° C. The potato does not lend itself well to frying.

The pectinases degrade the cell walls of fruits and vegetables and lead, therefore, softening of the walls.