P1: SFK/UKS BLBS102-c04 P2: SFK BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come 59 Browning Reactions OH O CH2–CH2–NH2 OH O CH HO C OH H C O O OH OH OH HO OH HO C Caffeic acid OH Dopamine CH OH C OH OH D-Catechin CH OH OH OH O Quinic acid O OH O OCH3 HO Ferulic acid OH O C CH CH OH C OH O OH C HO OH OH OH OH OH Chlorogenic acid OH OH Shikimic acid OH Protocatechuic acid Catechol Phenolic substrates that serve as substrates for phenolase Figure 4.3 Structure of common phenols present in foods Table 4.1 Phenolic Substrates of PPO in Foods Source Phenolic Substrates Apple Chlorogenic acid (flesh), catechol, catechin (peel), caffeic acid, 3,4-dihydroxyphenylalanine (DOPA), 3,4-dihydroxy benzoic acid, p-cresol, 4-methyl catechol, leucocyanidin, p-coumaric acid, flavonol glycosides Isochlorogenic acid, caffeic acid, 4-methyl catechol, chlorogenic acid, catechin, epicatechin, pyrogallol, catechol, flavonols, p-coumaric acid derivatives 4-Methyl catechol, dopamine, pyrogallol, catechol, chlorogenic acid, caffeic acid, DOPA 3,4-Dihydroxyphenylethylamine (Dopamine), leucodelphinidin, leucocyanidin Catechins, leucoanthocyanidins, anthocyanins, complex tannins Chlorogenic acid, caffeic acid Chlorogenic acid, caffeic acid, coumaric acid, cinnamic acid derivatives Catechin, chlorogenic acid, catechol, caffeic acid, DOPA, tannins, flavonols, protocatechuic acid, resorcinol, hydroquinone, phenol Tyrosine, caffeic acid, chlorogenic acid derivatives Tyrosine Dopamine-HCl, 4-methyl catechol, caffeic acid, catechol, catechin, chlorogenic acid, tyrosine, DOPA, p-cresol Tyrosine, catechol, DOPA, dopamine, adrenaline, noradrenaline Chlorogenic acid, pyrogallol, 4-methyl catechol, catechol, caffeic acid, gallic acid, catechin, dopamine Chlorogenic acid, catechol, catechin, caffeic acid, DOPA, 3,4-dihydroxy benzoic acid, p-cresol Chlorogenic acid, catechin, caffeic acid, catechol, DOPA Chlorogenic acid, caffeic acid, catechol, DOPA, p-cresol, p-hydroxyphenyl propionic acid, p-hydroxyphenyl pyruvic acid, m-cresol Tyrosine Chlorogenic acid, caffeic acid, caffeylamide Flavanols, catechins, tannins, cinnamic acid derivatives Apricot Avocado Banana Cacao Coffee beans Eggplant Grape Lettuce Lobster Mango Mushroom Peach Pear Plum Potato Shrimp Sweet potato Tea Source: Reproduced from Marshall et al 2000 P1: SFK/UKS BLBS102-c04 P2: SFK BLBS102-Simpson March 21, 2012 60 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come Part 1: Principles/Food Analysis Control of Browning Enzymatic browning may cause a decrease in the market value of food products originating from plants and crustaceans (Kubo et al 2000, Perez-Gilabert and Garc´ıa-Carmona 2000, Subaric et al 2001, Yoruk and Marshall 2003, Queiroz et al 2008) Processing such as cutting, peeling, and bruising is enough to cause enzymatic browning The rate of enzymatic browning is governed by the active PPO content of the tissues, the phenolic content of the tissue, and the pH, temperature, and oxygen availability within the tissue Table 4.2 gives a list of procedures and inhibitors that may be employed for controlling enzymatic browning in foods A review on conventional and alternative methods to inactivate PPO has been recently published by Queiroz et al (2008) The inhibition of enzymatic browning generally proceeds via direct inhibition of the PPO, nonenzymatic reduction of o-quinones, and chemical modification or removal of phenolic substrates of PPO Among all these methods, inhibition of PPO is preferable According to Marshall et al (2000), there are six categories of PPO inhibitors applicable to control enzymatic browning: reducing agents, acidulants, chelating agents, complexing agents, enzyme inhibitors, and enzyme treatments Tyrosinase inhibitors from natural and synthetic sources have been reported by Chang (2009) Sulfites are the most efficient multifunctional agents in the control of enzymatic browning of foods The use of sulfites has become increasingly restricted because they can produce adverse reaction in some consumers l-ascorbic and its stereoisomer erythorbic acid have been considered as the best alternative to sulfite in controlling browning 4-Hexylresorcinol is also a good substitute to sulfite Sulphydryl amino acids such as cysteine and reduced glutathione, and inorganic salts like sodium chloride, kojic acid, and oxalic acid (Pilizota and Subaric 1998, Son et al 2000, Burdock et al 2001, Yoruk and Marshall 2009) cause an effective decrease in undesirable enzymatic browning in foods An equivalent of hypotaurine can be an extract of a foodstuff that has a high hypotaurine level such clam, oyster, mussels, squid, octopus, or any combination thereof (Marshall and Schulbach 2009) The application of this naturally occurring compound provides a longer-lasting protection from enzymatic browning in comparison with ascorbic acid or citric acid On the other hand, taurine addition may provide nutritional benefits to the supplemented foods because of its health-promoting properties A decrease in enzymatic browning is achieved by the use of various chelating agents, which either directly form complexes with PPO or react with its substrates For example, pcyclodextrin is an inhibitor that reacts with the copper-containing prosthetic group of PPO (Pilizota and Subaric 1998) Crown compounds have potential to reduce the enzymatic browning caused by catechol oxidase because of their ability to complex with copper present in its prosthetic group The inhibition effect of crown compounds, macrocyclic esters, benzo-18-crown6 with sorbic acid, and benzo-18-crown-6 with potassium sorbate have been proved for fresh-cut apples (Subaric et al 2001) Agro-chemical processes may also be employed for achieving an effective control of enzymatic browning In vitro studies based on the evaluation of the effect of a range of commonly used pesticides on the activity of purified quince (Cydonia oblonga Miller) PPO have indicated that PPO enzyme is competitively inhibited by pesticides such as benomyl, carbaryl, deltamethrine, and parathion methyl (Fattouch et al 2010) Salinity also affects PPO activity of the fresh-cut vegetables Increasing salinity conditions may allow fresh-cut vegetables possessing low PPO activity, high phenol content and high antioxidant capacity (Chisari et al 2010) Thiol compounds like 2-mercaptoethanol may act as an inhibitor of the polymerization of o-quinone and as a reductant involved in the conversion of o-quinone to o-dihydroxyphenol (Negishi and Ozawa 2000) Because of safety regulations and the consumer’s demand for natural food additives, much research has been devoted to the search for natural and safe anti-browning agents Honey (Chen et al 2000, Gacche et al 2009); papaya latex extract (De Rigal et al 2001); banana leaf extract either alone or in combination with ascorbic acid and 4-hexylresorcinol (Kaur and Kapoor 2000); onion juice (Hosoda and Iwahashi 2002); onion oil (Hosoda et al 2003); onion extracts (Kim et al 2005); onion by-products (residues and surpluses; Roldan et al 2008); rice bran extract (Boonsiripiphat and Theerakulkait 2009); solutions containing citric acid, calcium chloride, and garlic extract (Ihl et al 2003); Maillard reaction products (MRPs) obtained by heating of hexoses in presence of cystein or glutathione (Billaud et al 2003, 2004); N-acetylcysteine and glutathione (Rojas-Grau et al 2008); resveratrol, a natural ingredient of red wine possessing several biological activities, and other hydroxystilbene compounds, including its analogous oxyresveratrol (Kim et al 2002); and hexanal (Corbo et al 2000), Brassicacaea processing water (Zocca et al 2010); and chitosan (Martin-Diana et al 2009) are some examples of natural inhibitors of PPO In most cases, the inhibiting activity of plant extract is due to more than one component Moreover, a good control of enzymatic browning may involve endogenous antioxidants (Mdluli and OwusuApenten 2003) Regulation of the biosynthesis of polyphenols (Hisaminato et al 2001) and the use of commercial glucose oxidase–catalase enzyme system for oxygen removal (Parpinello et al 2002) have been described as essential and effective ways of controlling enzymatic browning Commonly, an effective control of enzymatic browning can be achieved by a combination of anti-browning agents (Zocca et al 2010) A typical combination might consist of a chemical reducing agent such as ascorbic acid, an acidulant such as citric acid, and a chelating agent like EDTA (Marshall et al 2000) A great emphasis is put on research to develop methods for preventing enzymatic browning especially in fresh-cut (minimally processed) fruits and vegetables The most efficient way to control this problem is the combination of physical and chemical methods, by avoiding the use of more severe individual treatments, which could harm the appearance and texture of vegetables Technological processing, including microwave blanching either alone or combined with chemical anti-browning agents (Severini et al 2001, Premakumar and Khurduya 2002, Yadav et al 2008, Guan and Fan 2010); CO2 treatments (Rocha and P1: SFK/UKS BLBS102-c04 P2: SFK BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come 61 Browning Reactions Table 4.2 Inhibitors and Processes Employed in the Prevention of Enzymic Browning Inhibition Targeted Toward the Enzyme Inhibition Targeted Toward the Substrate Processing Enzymes Inhibitors Removal of Oxygen Removal of Phenols Inhibition Targeted Toward the Products Heating Chelating agents Processing Complexing agents Reducing agents Steam and water blanching (70–105◦ C) Pasteurization (60–85◦ C) 10 11 12 Cyclodextrins Sodium azide Vaccum treatment Cyanide Immersion in water, Sulphate polysaccharides Carbon monoxide syrup, brine Chitosan Halide salts (CaCl2 , NaCl) Tropolone Ascorbic acid Sorbic acid Polycarboxylic acids (citric, malic, tartaric, oxalic, and succinic acids) Polyphosphates (ATP and pyrophosphate) Macromolecules (porphyrins, proteins, polysaccharides) EDTA Kojic acid Cooling Aromatic carboxylic acids Reducing agents Refrigeration Freezing (−18◦ C) Benzoic acids Cinnamic acids Dehydration Aliphatic alcohol Physical methods Freeze drying Spray drying Radiative drying Solar drying Microwave drying Peptides and amino acids Ascorbic acid Erythorbic acid Butylated hydroxyanisole (BHA) Butylated hydroxytoluene (BTH) Tertiarybutyl hydroxyquinone Propyl gallate Sulphites(SO2 , SO3 2− , HSO3 − , S2 O5 2− ) Ascorbic acid and analogs Cysteine and other thiol compounds Enzymatic modification Amino acids, peptides, and proteins O-methyltransferase Protocatechuate 3, 4-dioxigenase Chitosan Maltol Chemical methods Sodium chloride and other salts Sucrose and other sugars Glycerol Propylene glycol Modified corn syrup Irradiation Substituted resorcinols Gamma rays up to kGy (Cobalt 60 or Cesium 137) X-rays Electron beams Combined treatments using irradiation and heat (Continued) P1: SFK/UKS BLBS102-c04 P2: SFK BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm 62 Printer Name: Yet to Come Part 1: Principles/Food Analysis Table 4.2 (Continued) Inhibition Targeted Toward the Enzyme Processing Enzymes Inhibitors High pressure (600–900 Mpa) Supercritical carbon dioxide (58 atm, 43◦ C) Ultrafiltration Honey (peptide ∼600 Da and antioxidants) Proteases Ultrasonication Employment of edible coating Inhibition Targeted Toward the Substrate Removal of Oxygen Removal of Phenols Inhibition Targeted Toward the Products Acidulants Citric acid (0.5–2% w/v) Malic acid Phosphoric acid Chitosan Source: Adapted from Marshall et al 2000 Morais 2001, Kaaber et al 2002, Valverde et al 2010); pretreatments employing sodium or calcium chloride and lactic acid followed by conventional blanching (Severini et al 2003); combination of sodium chlorite and calcium propionate (Guan and Fan 2010); high-pressure treatments combined with thermal treatments and chemical anti-browning agents such as ascorbic acid (Prestamo et al 2000, Ballestra et al 2002) or natural anti-browning agents like pineapple juice (Perera et al 2010); and UV-C light treatment of fresh-cut vegetables under nonthermal conditions (Manzocco et al 2009) have been employed to prevent enzymatic browning in foods The use of edible coating from whey protein isolate-beeswax (Perez-Gago et al 2003); edible coatings enriched with natural plant extracts (Ponce et al 2008); oxygen-controlled atmospheres (Jacxsens et al 2001, Soliva-Fortuny et al 2001, Duan et al 2009), and the use of active films (Endo et al 2008) seem to improve the shelf life of foods by inhibition of PPO Biotechnological approaches may be also employed for the control of PPO (Rodov 2007) Transgenic fruits carrying an antisense PPO gene show a reduction in the amount and activity of PPO, and the browning potential of transgenic lines are reduced compared with the non-transgenic ones (Murata et al 2000, Murata et al 2001) These procedures may be used to prevent enzymatic browning in a wide variety of food crops without the application of various food additives (Coetzer et al 2001) NONENZYMATIC BROWNING The Maillard Reaction Nonenzymatic browning is the most complex reaction in food chemistry because a large number of food components are able to participate in the reaction through different pathways, giving rise to a complex mixture of products (Olano and Mart´ınez-Castro 2004) It is referred to as the Maillard reaction when it takes place between free amino groups from amino acids, peptides, or proteins and the carbonyl group of a reducing sugar The historical perspective showed by Finot (2005) shows the great importance of Maillard reaction on food science and nutrition The Maillard reaction is one of the main reactions causing deterioration of proteins during processing and storage of foods This reaction can promote nutritional changes such as loss of nutritional quality (attributed to the destruction of essential amino acids) or reduction of protein digestibility and amino acid availability (Malec et al 2002) The Maillard reaction covers a whole range of complex transformations (Figure 4.4) that produces a large number of the so-called Maillard reaction products (MRPs) such as aroma compounds, ultraviolet absorbing intermediates, and dark-brown polymeric compounds named melanoidins (Kim and Lee 2008a) It can be divided into three major phases: the early, intermediate, and advanced stages The early stage (Figure 4.5) consists of the condensation of primary amino groups of amino acids, peptides, or proteins with the carbonyl group of reducing sugars (aldose), with loss of a molecule of water, leading, via formation of a Schiff’s base and Amadori rearrangement, to the socalled Amadori product (1-amino-1-deoxi-2-ketose), a relatively stable intermediate (Feather et al 1995) The Heyns compound is the analogous compound when a ketose is the starting sugar In many foods, the ε-amino group of the lysine residues of proteins is the most important source of reactive amino groups, but because of blockage, these lysine residues are not available for digestion, and consequently the nutritive value decreases (Brands and van Boekel 2001, Machiels and Istasse 2002) Amadori compounds are precursors of numerous compounds that are important in the formation of characteristic flavors, aromas, and brown polymers They are formed before the occurrence of sensory changes; therefore, their determination provides a very sensitive indicator for early detection of quality changes caused by the Maillard reaction (Olano and Mart´ınez-Castro 2004) The intermediate stage leads to breakdown of Amadori compounds (or other products related to the Schiff’s base) and the formation of degradation products, reactive intermediates (3deoxyglucosone), and volatile compounds (formation of flavor) P1: SFK/UKS BLBS102-c04 P2: SFK BLBS102-Simpson March 21, 2012 11:59 Trim: 276mm X 219mm Printer Name: Yet to Come 63 Browning Reactions +Amino compound Aldose –H2O N-substituted glycosylamine A Amadori rearrangement B Amadori rearrangement product (ARP) (1-amino-1-deoxy-2-ketose) C –3H O Schiff’s base of hydroxymethylfurfural or furfural C –2H2O Sugars D Fission products (acetol, diacetyl, pyruvaldehyde, etc.) +Amino acid –CO2 E Reductones Amino compound –2H E +2H +H2O F Hydroxymethylfurfural or furfural G Strecker degradation Dehydroreductones +Amino compound F Aldehydes F Aldols and nitrogen-free polymers F G G +Amino compound +Amino compound G +Amino compound G +Amino compound Melanoidins (brown nitrogenous polymers and copolymers) Figure 4.4 Scheme of different stages of Maillard reaction (Hodge 1953, Ames 1990) The 3-deoxyglucosone participates in cross-linking of proteins at much faster rates than glucose itself, and further degradation leads to two known advanced products: 5-hydroxymethyl2-furaldehyde and pyraline (Feather et al 1995) The final stage is characterized by the production of nitrogen-containing brown polymers and copolymers known as melanoidins (Badoud et al 1995) The structure of melanoidins is largely unknown, but in the last few years, more data became available In particular, it was shown that they can have a different structure according to the starting material Melanoidins have been described as low-molecular weight (LMW) colored substances that are able to cross-link proteins via ε-amino groups of lysine or arginine to produce high-molecular weight (HMW) colored melanoidins Also, it has been postulated that they are polymers consisting of repeating units of furans and/or pyrroles, formed during the advanced stages of the Maillard reaction and linked by polycondensation reactions (Martins and van Boekel 2003) Chemical structure of melanoidins can be mainly formed by a carbohydrate skeleton with few unsaturated rings and small nitrogen components; in other cases, they can have a protein structure linked to small chromophores (Borrelli and Fogliano 2005) In foods, predominantly glucose, fructose, maltose, lactose, and to some extent reducing pentoses are involved with amino acids and proteins in forming fructoselysine, lactuloselysine or maltuloselysine In general, primary amines are more impor- tant than secondary ones, because the concentration of primary amino acids in foods is usually higher than that of secondary amino acids (an exception is the high amount of proline in malt and corn products) (Ledl 1990) Factors Affecting Maillard Reaction The rate of the Maillard reaction and the nature of the products formed depend on the chemical environment of food, including water activity (aw ), pH, and chemical composition of the food system, temperature being the most important factor (CarabasaGiribert and Ibarz-Ribas 2000) In order to predict the extent of chemical reactions in processed foods, knowledge of kinetic reactions is necessary to optimize the processing conditions Since foods are complex matrices, these kinetic studies are often carried out using model systems in which sugars and amino acids react under simplified conditions Model system studies may provide guidance regarding the directions in which to modify the food process and to find out which reactants may produce specific effects of the Maillard reaction (Lingnert 1990) The reaction rate is significantly affected by the pH of the system; it generally increases with pH (Namiki et al 1993, Ajandouz and Puigserver 1999) Bell (1997) studied the effect of buffer type and concentration on initial degradation of amino acids and formation of brown pigments in model systems of glycine and glucose that are stored for long periods at 25◦ C The ... such as cysteine and reduced glutathione, and inorganic salts like sodium chloride, kojic acid, and oxalic acid (Pilizota and Subaric 19 98, Son et al 2000, Burdock et al 2001, Yoruk and Marshall... o-dihydroxyphenol (Negishi and Ozawa 2000) Because of safety regulations and the consumer’s demand for natural food additives, much research has been devoted to the search for natural and safe anti-browning... Ajandouz and Puigserver 1999) Bell (1997) studied the effect of buffer type and concentration on initial degradation of amino acids and formation of brown pigments in model systems of glycine and