Biological Properties of Melanoidins: A Review

Abstract

Melanoidins are brown, high molecular weight products of Maillard reaction, which takes place during thermal processing of food. They are formed in a multistage reaction between reducing sugars and compounds possessing free amino groups and found in roasted coffee, bakery products, cooked meat, beer, honey, sweet wine, processed tomatoes, and fiber. Nowadays, melanoidins have attracted a lot of attention, not only as a functional food ingredient, but also as a potential pro-healthy dietary supplement. In this field, their antioxidant, antimicrobial, anticancer, and detoxifying activity have been described. Based on recent research developments, an overview of the biological properties of melanoidins with implications for human health is presented.

Keywords:

• Melanoidins
• Maillard reaction
• Antioxidant
• Antimicrobial
• Anticancer

INTRODUCTION

Melanoidins are the final products of the Maillard reaction (MR), which takes place during heat processing (cooking, baking, and roasting) of foods containing reducing sugars and compounds possessing free amino groups.[1, 2] Maillard reaction leads to specific color and aroma formations, which greatly improve the taste of bakery products, roasted coffee, or cooked meat. To date, the chemical structure of melanoidins has not been completely established, although it is generally agreed that they comprise anionic, brown-colored, high molecular weight, nitrogen containing compounds.[3, 4] They exhibit the ability to absorb light at 405 nm, which enables them to be quantified in, for example, coffee brews or other foodstuffs.[5] To attain the requirements of consumers, it is of great importance to produce tasty, but also nutritious and safe food. In this field, Maillard reaction products (MRPs), including melanoidins, have attracted a lot of attention recently. They are considered as functional food ingredients due to their potential pro-healthy properties. On the other hand, melanoidins are referred to as the lowered nutritive value of foodstuffs.[1, 6, 7] Their capacity to chelate metal ions and reducing properties are also broadly studied.[1, 5, 8, 9] Considering broad distribution of melanoidins in the daily diet and consumption of approximately 10 g per day for the general population,[10] studying their biological activity and impact on human health seems to be of high importance. Therefore, the objective of the present review was to comprise and discuss mostly studied, health-related aspects of melanoidins.

KINETICS OF MAILLARD REACTION

Maillard reaction, which is the main origin of thermally generated flavors, was reported for the first time in 1912 by the French chemist, Louis Camille Maillard. It refers to a very complex reaction where the carbonyl group of a reducing sugar condenses with the amino group of an amino acid or protein, which leads to N-substituted glycosylamine formation. As a result of glycosilamine rearrangements, Amadori compounds are developed. The intermediate steps of the Maillard reaction comprise the degradation of the Amadori compound in order to generate different compounds that can react among them, even with additional amino groups. The composition of the end products and reaction pace depend on reaction conditions. Time and temperature of heating, pH, water content, and activity and complexity of reactants (type of amino acid or sugar) are the main parameters that determine the final compound's mixture. It was found that reactivity between carbohydrates and amino acids increases with the reaction temperature increase.[7] At a pH of <7, furfurals or hydroxymethylfurfural (HMF) are formed, whereas melanoidins are formed both at acidic and alkaline pH as a result of Amadori compounds degradation, followed by various chemical reactions, including dehydrations, cyclizations, and condensations.[1, 11]

MELANOIDINS AND FOOD

As mentioned above, the Maillard reaction is of high significance for the food industry, playing a crucial role during thermal processing of foods.[12– 14] Maillard reaction products are found abundantly in roasted coffee and bakery products, but also in other foodstuffs (Table 1).

Table 1  Melanoidins occurrence in foods

Food melanoidins	References
Coffee	[5,8,15,16,43]
Cocoa	[62,63]
Bakery products
Bread	[23,58]
Muffins	[21,41]
Cereal	[45]
Cooked meat	[47,64]
Malt	[65]
Honey	[66]
Milk	[46,67]
Dark beer	[19,20]
Wine	[68]
Balsamic vinegar	[69,70]
Processed tomatoes	[71]
Roasted potatoes	[72]
Soy sauces	[54,73,74]

Coffee

Coffee, a common beverage widely consumed all over the world, is a significant source of melanoidins. They constitute around 25% of the coffee brew dry matter, and are responsible for part of the specific organoleptic properties of coffee, as well as viscosity of this beverage.[15– 17] There is agreement about the participation of a diverse group of chemical components present in coffee beans in MRPs formation. The composition of melanoidins depends on polysaccharides (galactomannans and arabinogalactans), amino acids, proteins, and phenolic compounds (chlorogenic, caffeic, or ferulic acids) found in coffee. Some of the phenolic compounds are linked to melanoidins’ core via covalent interaction, others are linked through non-covalent bonds[8, 15] and can be released during intestinal digestion.[18] Low molecular weight (LMW) melanoidins constitute up to 41% of all coffee melanoidins, and the rest (59%) are high molecular weight (HMW) melanoidins.[16] The time it takes to roast coffee beans plays a crucial role in their formation. Prolonged coffee roasting leads to increased HMW melanoidins formation, whereas the level of LMW melanoidins seems to be unchanged.[5]

Beer

Melanoidins constitute an important ingredient of different kinds of beer affecting flavor, color, and viscosity of this drink. They are formed during malting and brewing processes and act both with polyphenols as antioxidants. Melanoidins present in beers were shown to protect against DNA damage caused by reactive oxygen species. A more intense effect was observed for dark than for blond beers, which was due to higher content of these compounds.[19, 20]

Bakery Products

Melanoidins have also been detected in bakery products, e.g., bread crust, biscuits, or muffins, influencing their color and texture properties. They prolong the shelf-life of such foods with respect of their antioxidant activity. Moreover, melanoidins from bread crust have been shown to improve Bifidobacteria growth, which utilized MRPs as a carbon source, and the effect was melanoidin types dependent.[21, 22]

BIOLOGICAL ACTIVITY OF MELANOIDINS

Taking into consideration the fact that MRPs are present in a variety of foods, it is important to determine their safety and influence on nutritional value. Despite their ability to affect the color, flavor, and texture of foods, melanoidins can exert a certain effect on the human body. Difficulties in ascribing definite properties to individual melanoidins are caused by their diversity, complexity, problems with purification and identification, and poor solubility in water and organic solvents. Additionally, melanoidins are usually linked to other low molecular weight compounds that may influence their properties. Further, the degree of digestibility and bioavailability in organisms is generally poor.[6, 22, 23]

The positive and negative effects of melanoidins in foodstuffs are under consideration. In the past, melanoidins were mainly perceived as a cause of decreased nutritional value of food. This is mostly due to the inactivation or destruction of amino acids or proteins, reduced digestibility of nitrogen, and impaired absorption of brown-colored compounds in the bowel following enzymes inactivation.[1, 6, 7, 24] It was shown in vitro and in vivo that melanoidins lower the activity of some proteolytic enzymes in the alimentary tract, including trypsin.[25– 27] Besides the undesirable influence of Maillard reaction end-products on food quality, melanoidins show a variety of beneficial effects. They exert pro-healthy activities, acting as antioxidant, antimicrobial, antihypertensive, antiallergenic, and prebiotic agents.[6, 28, 29] Melanoidins also demonstrate the ability to bind metal ions[30] and are considered as antimutagenic and tumor growth-inhibiting compounds.[28, 31– 33] Some of these features of melanoidins can improve the shelf-life of melanoidin-containing foodstuffs.

Antioxidant Activity

Reactive oxygen species can lead to the destruction of cells in respect of protein and DNA damage. Normally, cells possess a group of enzymes engaged in maintaining the regular level of oxidants. Nevertheless, it is also of high importance to provide exogenous antioxidants with nourishment.[34] Besides the health-promoting effects, antioxidants formed during processing and storage are also considered to increase its quality and edibility.[35]

Melanoidins can play as secondary antioxidants formed during the thermal processing of food and this ability has been broadly studied with use of synthetic model melanoidins, as well as obtained from foods like coffee or beer. Different methods have been used to define the antioxidant activity of malanoidins in vitro, including the measurement of lipid peroxidation (TBARS), protein oxidation (DNPH), ferric reducing ability (FRAP), free radical scavenging activity (DPPH assay), and antioxidant activity (ABTS⁺ assay). It has been confirmed that MRPs exert an antioxidant effect, which emerge from their ability to scavenge free radicals (hydroxyl, superoxide, and peroxyl radicals) and chelate metal cations, especially iron.[28, 36, 37] The structure of MRPs is still largely unknown, thus making it difficult to state which fraction, high or low molecular weight, is responsible for this activity. Wagner et al. and Yoshimura et al., in two independent studies, demonstrated that HMW products are mostly responsible for antioxidant activity.[35, 38] Similar observations were carried out by Del Castillo et al.[39] In contrast, Morales and Babbel indicate higher antiradical activity of low and intermediate molecular weight products.[37] These findings are in agreement with data obtained by Delgado-Andrade et al.,[40] who demonstrated the significance of LMW compounds linked to the melanoidin skeleton and which was confirmed by Rufián-Henares and Morales.[18]

Additionally, melanoidins exert some protective effect on cells. Valls-Bellés et al.[36] reported preventive activity of high molecular weight products of a glycine-glucose system against adriamycin-induced cytotoxicity in rat's hepatocytes in vitro. Adriamycin, an antibiotic with antitumor activity, often causes a variety of side effects due to its mechanism of action. It generates a large amount of reactive oxygen species (ROS), which destroy cancer cells but also normal cells. The addition of melanoidins to culture medium decreased the toxicity induced by adriamycin, which was measured by, for example, LDH release or protein damage. Protective effect of HMW compounds was attributed to its free radical scavenging ability.[36] Moreover, melanoidins isolated from foods were shown to protect cells in vitro against oxidative challenge. Soluble fraction of melanoidins from biscuits[41] and from coffee,[42] digested by simulating gastric plus pancreatic digestive conditions, were revealed to protect cultured human hepatoma HepG2 cells against oxidative stress. Antioxidant activity of coffee brew melanoidins has been broadly studied.[5, 28, 37, 40, 43, 44] This effect and its strength depend on the compounds (polisaccharides, amino acids, or proteins and phenols) involved in MRPs origin, their complexity, and solubility.[15, 37, 40]

On the contrary, melanoidins have also been reported as components exerting the ability to absorb UV-A light and act as photosensitizers, thus leading to reactive oxygen species formation as a result of chemical reactions enabled by absorbed energy. Taking this property into consideration, melanoidin-content in foodstuffs may be harmful for its safety and nutritional value, as well as decreasing the shelf-life of stored products.[17] However, the ability of melanoidins to increase the storage life and quality of foodstuffs was also reported.[18, 40, 45– 47]

Antimicrobial Activity

Gut microflora play a crucial role in maintaining the homeostasis of a whole organism, therefore keeping proper balance between beneficial and harmful microorganisms is highly important. Bacteriostatic potential of synthetic melanoidins, as well as those isolated from foods, has been demonstrated.[48, 49] Low molecular weight products linked to melanoidins are responsible for this bacteria growth-inhibiting activity with respect to synthesized melanoidins. The opposite effect is observed for food-derived compounds where the core of melanoidins creates the antimicrobial activity of the derivatives.[28, 39] Three mechanisms have been postulated to affect microbial growth: chelating iron, the ability to inactivate siderophores-Fe3 ⁺ complexes, and binding magnesium ions, thus disrupting bacteria's membrane.[4, 9]

Importantly, melanoidins may inhibit growth of pathogenic bacteria, such as Helicobacter pylori.[50, 51] Escherichia coli, Bacillus cereus, Staphylococcus aureus, Proteus mirabilis, Pseudomonas aeruginosa, Salmonella typhimurium,[9] and, simultaneously, their beneficial effect on lactobacilli and bifidobacteria has also been proved; their prebiotic activity may, therefore, be stated.[24, 52] Despite the bacterial growth-inhibiting activity in human organisms, melanoidins may function directly on foods as antimicrobial agents, thereby improving their shelf-life. This effect can be mediated both by bacteriostatic and bactericidal action.[4]

Tumor Growth-Inhibiting Properties

There are several reports showing the ability of melanoidins to suppress cancer cell growth in vitro. Different melanoidins produced in foods under commonly used heating conditions were investigated, involving human cancer cells of the gastrointestinal tract. The Maillard reaction products decreased cells proliferation, which was probably due to inhibition of mitogen-activated protein kinase (MAPK) cascade. Tested substances affected the cell cycle, microtubule organization, and induced apoptosis.[33, 53] Another study carried out by Kamei et al.[54] demonstrated the growth inhibition of colon and gastric cancer cells in vitro, incubated with melanoidins obtained from miso and soy sauce. The inhibitory effect in tumor cells was due to S and G₂/M-phase cessation. Recently, we have described that water extract from heated potato fiber (Potex) elicits a prominent anticancer effect.[55] It was demonstrated that the extract inhibited proliferation of a variety of tumor cells, including rabdomyosarcoma-medulloblastoma, glioma, breast, colon, and lung carcinoma. Furthermore, it was revealed that the tested extract decreased cancer cell motility, produced remarkable morphological changes, induced apoptotic cell death, and reversed the stimulatory effect of insulin-like growth factor. Interestingly, a significant neurotrophic effect was observed in neuronal cultures. Subsequently, in a series of experiments, we have shown that melanoidins isolated from heated potato fiber (Potex) extract were responsible for altered glioma cell proliferation via specific molecular targets.[56] Observed cancer cell growth-inhibiting effect was linked to disregulated MAPK and Akt signaling pathways, as well as to cell cycle cessation. Further study in a wide range of in vivo cancer models is needed to verify anticancer potential of melanoidins and the possibility of their future application as functional food ingredients and chemopreventive agents.

Detoxifying Activity

Toxic compounds present in food or drugs acting as xenobiotics are considerably linked to several diseases, including cancer. Thus, an efficient detoxifying system within an organism is essential to provide systematic disposal of harmful substances from the body and protect against their detrimental effects. Detoxification enzymes, which enable xenobiotics biotransformation (i.e., phase I and phase II enzymes), are highly significant. Several studies reported the ability of melanoidins to modulate the activity of these proteins.[14] MRPs from heat-treated casein, bread crusts, and coffee were responsible for increased activity of phase II enzymes in vivo.[57– 59] N-methylpyridinium, identified as coffee key compounds responsible for regulation of glutathione S transferase (GST),[59] was shown to affect the Nrf2 transcription factor, inducing its transcription and translocation.[60] Similar conclusions were stated by Cavin et al.,[61] who linked chemopreventive activity of coffee in rats with the induction of Nrf2-mediated cellular defenses and alteration of phase I enzymes activities in the liver.

CONCLUSION

Regarding various aspects of melanoidins’ biological activity, they have been proven to have a beneficial effect on human health. Additionally, taking into consideration an estimated daily intake of melanoidins close to 10 g per day, further effort should be made in order to elucidate the function of these widely distributed in thermally processed food polymeric compounds. Based on many studies outlined in this review, it is believed that in the future, food supplementation with melanoidins may become a common dietary habit and/or potential chemopreventive strategy.