NON-ENZYMATIC BROWNING

Mr. OMAR ALAJIL ( العجيل. عمر (أM.Sc Food Technology

TABLE OF CONTENT

INTRODUCTION Changes during non-enzymic browning Types of Non-enzymatic Browning Maillard Reaction mechanisms and products How to measure browning Kinetic of non enzymatic browning Controlling Factors of the Maillard Reaction

Products Prevention of non enzymatic browning CONCLUSION

Introduction Foods may develop a variety of

brown colors, from yellow-brown to red-brown to black-brown, during handling, processing, and storage.

These colors are desirable in certain foods (e.g., coffee, beer, bread, maple syrup).

In other foods, such as most dehydrated fruits and vegetables, dried eggs, and canned or dried milk, browning is detrimental.

Even when desirable, browning should not be excessive, as in potato chips, French fries, and apple juice.

Numerous reactions lead to browning in foods. Some of these may also generate flavors and/or alter the nutritional properties of foods.

enzymatic and nonenzymatic browning Enzymatic browning

occurs when fresh food or beverage products are exposed to air and there are active enzymes present within the food or beverage product. When an apple slice or banana turns brown, it is usually due to polyphenol oxidase enzyme activity.

Non-enzymatic browning can produce brown, fluorescent, highly cross-linked pigments, such as melanoidin chromophores.

NONENZYMATIC BROWNING

Types of Nonenzymatic Browning

A number of chemical processes not involving enzymes may result in food browning.

1. Maillard reaction2. Caramelization3. ascorbic acid browning4. metalpolyphenol browning.

Changes during non-enzymic browning this changes some times are

desirable and sometimes undesirableProduces flavorProduces colorProduces antioxidant productsProduces toxic productsloss and Destroys nutrients (lysine)formation of undesirable products such

as HMF (5-hydroxymethylfurfural)

1. Caramelization This reaction leads to brown products

when sugars are heated dry or in solution.

The large quantities of industrial caramel color that are added to beverages (cola drinks), baked goods, and confections are made by heating high-conversion corn syrups in the presence of catalysts (acids, alkalis, salts).

The chemical transformations involved in caramelization are complex and poorly understood.

They include dehydration, fragmentation, and polymerization.

On the heating of pentoses, furfural is formed which polymerizes to brown products.

Heating hexoses results in hydroxymethylfur-fural, which polymerizes similarly.

2. Ascorbic Acid Browning When ascorbic acid is heated in the

presence of acids, furfural is formed. The latter, either by itself or after

reacting with amino compounds, polymerizes to brown products.

Citrus juices, especially their concentrates, develop browning, which has been attributed to ascorbic acid degradation.

3. Metal-Polyphenol Browning Polyphenolic compounds form complexes

with certain metals. The polyphenols of fruits and vegetables most commonly chelate iron. The resulting iron complexes are bluish black pigments.

Cutting apples with a non-stainless-steel knife results in darkening of both the blade and the surface of the apple.

This darkening is independent of the enzymatic browning that might develop as a result of cutting.

The iron of the tissue must first be oxidized to the ferric state for the blackish complex to appear.

Canned or pickled cauliflower may turn dark due to the interaction of polyphenols in the tissue with iron from external sources

4. The Maillard Browning

Maillard reaction is caused by the condensation of an amino group and a reducing compound, resulting complex changes in biological and food system.

This reaction was described for the first time by Louis Maillard in 1912.

John deMan States that Maillard reaction is: “The sequence of events that begins

with reaction of the amino group of amino acids with a glycosidic hydroxyl group of sugars; the sequence terminates with the formation of brown nitrogenous polymers or melanoidins”

MAILLARD REACTION MECHANISMS AND

PRODUCTS

Maillard Reaction mechanisms and products This reaction is actually a series of

reactions occurring from the first encounter of a carbonyl compound with an amine compound to the formation of brown pigments.

It is also known as the carbonyl-amine reaction, and its brown products are often called melanoidins, indicating their visual similarity to the melanins of enzymatic browning.

The most common carbonyl compounds of foods involved in the Maillard reaction are reducing sugars, and the most common amine compounds are amino acids.

Among sugars, pentoses are more reactive than hexoses, and hexoses are more reactive than reducing disaccharides.

When free amino acids react with sugars, lysine appears to be the most active among them.

Maillard reaction occurs when virtually all foods are heated, and also occurs during storage.

Maillard reaction form products that are desirable or undesirable

desirable - caramel aromas - and golden brown colors,

undesirable - foods darkness - and off-flavor development- loss of nutrition component (Lysine)

the Maillard reaction can seriously lower the nutritive value of the food.

Toasting, for example, may reduce to one-half the protein efficiency ratio of bread.

HOW TO MEASURE BROWNING

 How to measure browning As the thermal treatment resulted in the

increase in colored substances content, which is measured as an increase in absorbance at 420 and 560 nm by reactance colorimetry A420.

KINETICS OF MAILLARD BROWNING

Kinetic of non enzymatic browning First-order and zero-order kinetic

models have been used to evaluate the development of non-enzymaticbrowning. These kinetic models are expressed by the equations

It is not always possible to apply kinetics as simple as first-order or zero-order to describe the colour changes produced in fruit purees,

since these changes can be due not only to the Maillard reaction but also to the thermal destruction of pigments present in the samples.

First-order kinetics has been suggested for the destruction of natural fruit pigments.

Based on data for the deterioration produced by thermal treatments, a two stage mechanism is proposed.

The first stage, color formation, is zero-order and the second stage, pigment destruction, is first-order.

This can be written as- sugars. Amino acids k0 colored polymers - pigments k1 uncolored products

This two-stage kinetic mechanism can be expressed mathematically.

If C is the variable used to measure the color changes caused by non-enzymatic browning reactions, then Where the left-hand side represents the fractional conversion at time t.

Eqn (5) is the final expression for the kinetic model of the two-stage mechanism proposed for the evolution of color changes caused by non-enzymatic browning.

The value of K expresses the maximum color difference (DE?) obtained for long treatment times.

It was observed that K increased with treatment temperature.

Consequently, an increase in temperature implies a greater increase in colour.

According to the proposed kinetic model, K represents the ratio of kinetic constants k0 (colour appearance) and k1 (pigment destruction).

It was observed that the kinetic constant values for the colour formation stage increased with treatment temperature, while the kinetic constant values for the pigment destruction stage showed no definite tendency.

However, for higher temperatures the values of this constant were similar.

The kinetic constants for the colour formation stage increased in value with increasing temperature.

An analogous behavior was observed to describe the increase in color.

The value of the Kinetic constant increased with treatment temperature, which indicates that the yellowish hues decreased more rapidly with increasing treatment temperature.

The zero order model (eqn [1]) was the best model to describe this variation.

Controlling Factors of the Maillard Reaction Products factors that influence the browning

reaction are temperature, pH, moisture level (aw), oxygen, metals, phosphates, sulfur dioxide, and other inhibitors

Raising the temperature and/or the pH accelerates the Maillard reaction. Intermediate water activity appears to maximize this reaction.

1- Water activity (aw) Water is produced during Maillard

reaction, thus the reaction occurs less readily in foods with a high aw values while, at low aw, the mobility of reactants is limited, despite their presence at increased concentrations.

As the figure shown the Maillard reaction occurs most rapidly at intermediate aw values (0.5-0.8),

and aw is of most significance to the reaction in dried and intermediate- moisture foods (IMFs), which have aw values in this range.

humectants, such as glycerol, can lower the aw value for maximum browning.

2- pH Since the reaction itself has a strong

influence on pH it is hard to evaluate the pH influence.

Ph can have a little affect on final products and aroma

pH had a less dramatic effect on aroma than did temperature, time or water content.

The figure is "effect of pH on Maillard browning of L-lysine, L-alanine, and L-arginine heated with D-glucose at 121C for 10 min".

Under weakly alkaline conditions, and with a strongly basic secondary amino components, the 2,3-enolization pathway is favored.

3- Temperature The temperature dependence of chemical

reaction is often expressed as the activation energy (Ea).

The activation energy is highly dependent on pH.

The temperature dependence of the Maillard reaction is also influenced by the participating reactants.

So it is difficult to isolate the effect of temperature as a single variable.

PREVENTION OF BROWNING

 Prevention of non enzymatic browning

As already indicated, nonenzymatic browning is desirable in certain instances and undesirable in others.

The availability of reactants and the type of conditions (temperature, pH, moisture) will determine the extent of browning.

A chemical preservative often used to inhibit nonen-zymatic (and enzymatic) browning is sulfur dioxide.

An obvious way to prevent metal-polyphenol browning is to eliminate contact between susceptible tissues and reactive metals and use inoffensive equipment (stainless steel, glass-lined tanks, etc.)

CONCLUSION

Conclusion Non enzymatic browning is desirable in some products like

bread crust and meat and undesirable in other products like apple puri, there is many types of browning like maillard browning, Caramelization, ascorbic acid browning and metalpolyphenol browning.

The maillard reaction is actually known as the carbonyl-amine reaction, and its brown products are often called melanoidins. The most common carbonyl compounds of foods involved in the Maillard reaction are reducing sugars, and the most common amine compounds are amino acids.

factors that influence the browning reaction are temperature, pH, moisture level (aw), oxygen, metals, phosphates, sulfur dioxide, and other inhibitors.

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