Enzymatic Browning and Caramelization (Bio-Chemistry Notes)
Bio Enzymatic Browning
Under some conditions, reducing sugars produces brown colors that are desirable and important in some foods.
At other times, brown colors obtained upon heating or \long-term storage of foods containing reducing sugars are undesirable.
Common browning of foods on heating or storage is usually due to a chemical reaction between reducing sugars, mainly D-glucose, and a primary amino group (a free amino acid or amino group on a side chain of a protein molecule.)
This reaction is called the Maillard reaction and the overall process is sometimes designated Maillard browning.
It is also called non-enzymic or non-enzymatic browning to differentiate it from the more rapid, enzyme-catalyzed browning commonly observed in freshly cut fruits and vegetables, such as apples and potatoes.
When aldoses or ketoses are heated with amines, a variety of reactions ensue, producing numerous compounds (some of which are flavors, aromas, and dark-colored polymeric materials); but both reactants disappear only slowly.
The flavors, aromas, and colors may be either desirable or undesirable.
They may be produced slowly during storage and much more rapidly at the high temperatures encountered during frying, roasting, or baking.
Different sugars undergo nonenzymic browning at different rates. For example, d-glucose undergoes the browning reaction faster than does d-fructose.
Secondary amines give different reaction products than primary amines. Because the reaction has a relatively high energy of activation, application of heat is generally required.
The rate of the Maillard reaction is also a function of the water activity (aw) of a food product, reaching a maximum at aw values in the range of 0.6 – 0.7.
Thus, for some foods, Maillard browning can be controlled by controlling water activity as well as by controlling reactant concentrations, time, temperature, and pH.
Sulfur dioxide and bisulfite ions react with aldehyde groups, forming additional compounds, and thus will inhibit Maillard browning by removing at least some of a reactant (reducing sugar, HMF, furfural, etc.).
Color, taste, and aroma are, in turn, determined by the product mixture.
Reaction variables that can be controlled to increase or decrease the Maillard browning reaction are the following:
(1) temperature (decreasing the temperature decreases the reaction rate) and time at the temperature;
(2) pH (decreasing the pH decreases the reaction rate);
(3) adjustment of the water content (maximum reaction rate occurs at water activity values of 0.6–0.7 [about 30% moisture]);
(4) the specific sugar; and
(5) the presence of transition metal ions that undergo one-electron oxidation under energetically favorable conditions, such as Fe(II) and Cu(I) ions (a free radical reaction may be involved near the end of the pigment-forming process).
SUMMARY
In summary, Maillard browning products, including soluble and insoluble polymers, are found where reducing sugars and amino acids, proteins, and/or other nitrogen-containing compounds are heated together, for example, in soy sauce and bread crusts.
Browning is desired in baking, for example, in bread crusts and cookies, and roasting of meats.
The volatile compounds produced by nonenzymic browning (the Maillard reaction) during baking, frying, or roasting often provide desirable aromas.
Maillard reaction products are also important contributors to the flavor of milk chocolate, caramels, toffees, and fudges, during which reducing sugars react with milk proteins.
The Maillard reaction also produces flavors, especially bitter substances, which may be desired, for example, in coffee. On the other hand, the Maillard reaction can result in off-flavors and off-aromas.
Off-flavors and -aromas are most likely to be produced during pasteurization, storage of dehydrated foods, and grilling of meat or fish.
Application of heat to intermediate moisture foods is generally required for nonenzymic browning.
CARAMELIZATION
Heating carbohydrates, especially sucrose and reducing sugars, without any nitrogen-containing compounds, leads to a complex series of reactions known as caramelization.
The reaction is facilitated by small amounts of acids and certain salts.
It does not involve amino acids or proteins, caramelization is similar to nonenzymic browning.
To make caramel, a carbohydrate is heated alone or in the presence of an acid, a base, or a salt.
The carbohydrate most often used is sucrose, but D-fructose, D-glucose (dextrose), inverted sugar, glucose syrups, HFSs, malt syrups, and molasses may also be used.
In Maillard browning, flavor, and aroma compounds are also found.
Heating causes dehydration of the sugar molecule with the introduction of double bonds or the formation of anhydrous rings.
In Maillard browning, intermediates such as 3-deoxyosones and furans are formed.
The unsaturated rings may condense to form useful, conjugated double-bond-containing, brown-colored polymers.
Catalysts increase the reaction rate and are used to direct the reaction to specific types of caramel colors, solubilities, and acidities.
Acids that may be used are food-grade sulfuric, sulfurous, phosphoric, acetic, and citric acids.
Bases that may be used are ammonium, sodium, potassium, and calcium hydroxides.
Salts that may be used are ammonium, sodium, and potassium carbonates, bicarbonates, phosphates (both mono- and dibasic), sulfates, and bisulfites.
FOUR RECOGNIZED CLASSES OF CARAMEL
Class I caramel (also called plain caramel or caustic caramel)
It is prepared by heating a carbohydrate without a source of either ammonium or sulfite ions; an acid or a base may be employed
Class II caramel (also called caustic sulfite caramel)
It is prepared by heating a carbohydrate in the presence of a sulfite, but in the absence of any ammonium ions; an acid or a base may be employed.
This caramel, which is used to add color to beers and other alcoholic beverages, is reddish brown and contains colloidal particles with slightly negative charges.
It has a solution pH of 3–4.
Class III caramel (also called ammonium caramel)
It is prepared by heating a carbohydrate in the presence of a source of ammonium ions, but in the absence of sulfite ions; an acid or a base may be employed.
This caramel, which is used in bakery products, syrups, and puddings, is reddish brown and contains colloidal particles with positive charges.
It gives a solution pH of 4.2–4.8
Class IV caramel (also called sulfite ammonium caramel)
It is prepared by heating a carbohydrate in the presence of both sulfite and ammonium ions; an acid or a base may be employed.
This caramel, which is used in cola soft drinks, other acidic beverages, baked goods, syrups, candies, pet foods, and dry seasonings, is brown, contains colloidal particles with negative charges.
It gives a solution pH of 2–4.5.
Caramelization may also occur during cooking or baking, especially when sugar is present.
It occurs along with nonenzymic browning during the preparation of chocolate and fudge.
FORMATION OF ACRYLAMIDE IN FOOD
The Maillard reaction has been implicated in the formation of acrylamide in many foods that have been heated to high temperatures during processing or preparation.
Levels of acrylamide (typically < 1.5 ppm) have been reported in a wide range of products made by frying, baking, roasting, or elevated temperature processing schemes.
It is not detected in boiled food products e.g. boiled potatoes as temp. During boiling does not go above 100ᵒC
Acrylamide can be detected or undetected at very low levels
It is a neurotoxicant and a weak human carcinogen at exposure levels much higher than are obtained from food.
Acrylamide is derived primarily from the second-order reaction between reducing sugars (carbonyl moiety) and the α-amino group of free L-asparagine
The reaction requires the presence of both substrates.
Fried potato products, such as potato chips and French fries, are particularly susceptible to acrylamide formation because potatoes contain both free d-glucose and free L-asparagine
Acrylamide formation requires a minimum temperature of 120◦C, which means that it cannot occur in high-moisture foods, and is kinetically favored with increasing temperatures approaching 200◦C.
With extended heating at temperatures above 200◦C, acrylamide levels may decrease via thermal elimination/degradation reactions.
Food levels of acrylamide are also impacted by pH.
Acrylamide formation is favored as the pH is increased over the range of 4–8.
Furthermore, acrylamide appears to undergo increased rates of thermal degradation as the pH decreases.
Acrylamide levels increase rapidly in the latter stages of the prolonged heating process as the water at food surfaces is driven off to allow surface temperatures to increase above 120◦C.
Products with high amounts of surface area, such as potato chips, are among those high-temperature processed foods that exhibit the highest acrylamide levels.
Thus, the exposed surface area of a food can be an additional factor, provided that reaction substrates and processing temperatures are sufficient for acrylamide formation.
POLYSACCHARIDES
Polysaccharides are polymers of monosaccharides.
The number of monosaccharide units in a polysaccharide, which is termed its degree of polymerization (DP), varies.
Only a few polysaccharides have DPs less than 100; most have DPs in the range 200–3,000
Starch amylopectin is even larger, having an average molecular weight of at least 107 (DP > 60,000).
If all the glycosyl units are of the same sugar type, they are homogeneous as monomer units and are called homoglycans. Examples of homoglycans are cellulose.
When a polysaccharide is composed of two or more different monosaccharide units, it is a heteroglycan.
POLYSACCHARIDE SOLUBILITY
Most polysaccharides contain glycosyl units that, on average, have three hydroxyl groups.
Each of the hydroxyl groups has the possibility of hydrogen bonding to one or more water molecules.
The ring oxygen atom and the glycosidic oxygen atom connecting one sugar ring to another can form hydrogen bonds with water.
Modify and control the mobility of water in food systems
When a polysaccharide solution is frozen, a two-phase system of crystalline water (ice) and a glass consisting of perhaps 70% polysaccharide molecules and 30% non-freezable water is formed.
Water-soluble polysaccharides and modified polysaccharides used in food and other industrial applications are known as gums or hydrocolloids.
Food gums are sold as powders of varying particle sizes.
POLYSACCHARIDE HYDROLYSIS
Polysaccharides are relatively less stable to hydrolytic cleavage than are proteins.
They may undergo depolymerization deliberately or undeliberately during food processing and/or storage of foods.
One reason why food gums would be deliberately depolymerized is so that a relatively high concentration can be used to provide body (mouthfeel) without producing undesirable viscosity.
Hydrolysis of glycosidic bonds joining monosaccharide (glycosyl) units in oligo- and polysaccharides can be catalyzed by acids (H+) and/or enzymes.
The extent of depolymerization, which has the effect of reducing viscosity, is determined by the
pH (acid),
Temperature
Time at that temperature and pH, and
Structure of the polysaccharide
Hydrolysis occurs most readily during the thermal processing of acidic foods (as opposed to storage) because of the elevated temperature.
Polysaccharides, like any and all other carbohydrates, are subject to microbial attack because of their susceptibility to enzyme-catalyzed hydrolysis.
DE
Maltodextrins are usually described by their dextrose equivalency (DE).
The DE is related to the DP through the following equation DE = 100/DP
DE is inversely related to average molecular weight.
Maltodextrins of the lowest DE, that is, the highest average molecular weight, are non-hygroscopic.
While those with the highest DE tend to absorb moisture.
Maltodextrins are bland with virtually no sweetness and are excellent for contributing body or bulk to food systems.
Hydrolysis to DE values of 20–60 gives mixtures of molecules that, when dried, are called corn syrup solids.
One of the most common has a DE of 42.
These syrups are stable because crystallization does not occur easily in such complex mixtures.
MODIFIED FOOD STARCHES
Native starches produce weak-bodied, cohesive, rubbery pastes when cooked and undesirable gels when the pastes are cooled.
Modification is done to improve the characteristics of the pastes and gels.
Some modifications are done so that resultant pastes can withstand the conditions of heat, shear, and acid associated with particular processing conditions; others are done to introduce specific functionalities.
Modifications can be chemical or physical.
Chemical modifications have the greatest effects on functionalities, and the majority of modified food starch products have been derivatized with reagents that react with hydroxyl groups to form ethers or esters.
Modifications can be single modifications, but modified starches often are prepared by combinations of two, three, and sometimes four processes.
Starch, like all carbohydrates, can undergo reactions in its various hydroxyl groups.
In modified food starches, only a very few of the hydroxyl groups are modified.
Normally, ester or ether groups are attached at very low degrees of substitution (DS) values.
DS values are often <0.1 and generally in the range of 0.002-0.2, depending upon modification.
DIETARY FIBER AND CARBOHYDRATE DIGESTIBILITY
Plant cell wall materials, primarily cellulose, other non-starch polysaccharides, and lignin, are components of dietary fiber.
The only common feature of these polymers is that they are non-digestible
The definition of dietary fiber also includes substances other than polymers.
The key characteristic is that the substance not be digested in the human small intestine, so non-digestible oligosaccharides, for example, raffinose and stachyose, are included as dietary fiber substances.
Oligo- and polysaccharides may be
Digestible (most starch-based products),
Partially digestible (retrograded amylose, so-called resistant starch), or
Non-digestible (essentially all other polysaccharides).
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