FURFURAL, HYDROXYMETHYLFURFURAL AND FUROSINE AS MAILLARD REACTION MARKERS IN FRUIT BASED FOODS INCLUDING JAMS AND BABY FOOD

The Maillard reaction occurs between amino acids and reducing sugars and goes through a series of reactions to form various products (MRPs – Maillard reaction products). The reaction is well-known for browning it causes in thermally treated foodstuffs. Many factors influence the type of MRPs formed: pH, sugar content, and processing conditions (temperature, pressure, duration). The compounds formed exhibit different biological properties, some are harmful to organisms and some are considered to be beneficial.
MRPs may also occur in jams and fruit-based baby foods at the level of mg/100 g. As these products are popular among consumers at every age including babies, many studies focus on the evaluation of Maillard reaction on their quality. In this review we have discussed the occurrence, factors affecting the process of formation of chosen MRPs, furosine and hydroxymethylfufural, in fruit jams as well as their influence on the organism.

Studies proved that MRPs also occur in processed fruit products, especially those rich in sugar such as jams or dried fruit (Murkovic and Pichler, 2006;Vorlova et al., 2006). There are various forms of fruit preserves available on the market, as fruits are perishable and some of them are obtained only seasonally. Fruit are present in baby foods too, which are similar to jams in their properties The Maillard reaction can be described as a series of reactions with many possible pathways and final products. Thus many sophisticated schemes and explanations of Maillard reaction have been elaborated through years of research (Henning and Glomb, 2016; Martins et al., 2000). In Figure 1 there is a simplified scheme of the Maillard reaction. The reaction can be briefly characterized in a few steps: 1. Reaction of reducing sugars' carbonyl group (glucose, fructose, disaccharides hydrolysis products) with a free amino group (present in amino acids or proteins, mainly the ε-amino group of lysine, but also the α-amino groups of terminal amino acids in proteins). As a result a Schiff base is formed which is a tautomeric form of N-substituted glycosylamine. 2. N-substituted glycosylamine undergoes rearrangement and gives ketosamine or aldosamine. Amadori rearrangement product (1-amino-1-deoxy-2-ketose) is formed if aldoses are react or Heyns rearrangement product is formed (2amino-2-deoxyaldose) if ketoses are used. One of the Amadori rearrangement products is furosine ( Figure 2) formed when the ε-amino group of lysine react with the carbonyl group of glucose (Erbersdobler and Somoza, 2007;Martins et al., 2000;Nahid and Niaz, 2015;Parker 2012). 3. Amadori rearrangement products degrade, following pathway which depend on pH. It is accompanied by dehydration. a. If pH is 7 or below then furfural or hydroxymethylfurfural ( Figure 2) among others are formed. b. If pH is higher than 7 then 4-hydroxy-5-methyl-2,3-dihydrofuran-3-one and highly reactive acetol, furaneol, maltol, pyruvaldehyde and diacetyl among other are formed. 4. All these compounds mentioned above are highly reactive. Carbonyl groups condense with free amino groups and these products undergo further transformations. Dicarbonyl compounds will react with amino acids with the formation of aldehydes and a-aminoketones. 5. The most distinctive products formed are melanoidins, which are polymeric compounds of high molecular weight, characteristic brown colour and rich in various chemical groups (e.g. carboxyl, ester, amine, hydroxyl and many more). Structure of melanoidins has still not fully been determined (  The Products of Maillard Reaction have a wide range of impact on the functions of organisms. As the reaction does not follow a simple pathway and it is not easily controlled during thermal treatment of food, pigments are not the only products. The complex network of interdependent reactions affect nutritional value, digestibility andcruciallythe toxicity of the food product. The safety of the food is affected in two different waysin formation of MRPs with antioxidant properties (e.g. melanoidins) and, the formation of toxic substances (

OCCURRENCE OF CHOSEN MRPs IN FRUIT PRODUCTS
Jam production consists of long periods of heating at high temperature and the plant material is usually rich in sugar (Murkovic and Pichler, 2006). Some MRPs require up to 3 hours to be formed and reach plateau. Jams prepared in traditional ways can be thermally treated even longer than 3 hours (Cendrowski and Mitek, 2012; Parker, 2012; "Powidła śliwkowe z węgierek," 2018; "Powidła śliwkowe łowickie," 2018). As fruit jams have acidic pH, the Maillard Reaction follows the 1,2-enolisation pathway leading to the formation of significant amounts of HMF ( Figure 1) (Murkovic and Pichler, 2006). Steber et al. (1987) proposed that less than 5 mg of HMF in 100 g of jam indicates the proper jam preparation procedure (Steber and Klostermeyer, 1987). Based on reference data it can be stated that HMF concentration in jams depends on type of sample and varies from less than 0.1 to almost 120 mg/100 g (Cendrowski, 2012;Murkovic and Pichler, 2006

FACTORS INFLUENCING THE FORMATION OF MRPs IN FRUIT PRODUCTS
Jams contain fruits (fresh or already processed), fruit juice, sweetening agents, sometimes also texture modifiers (pectins), acids (e.g. citric acid) and additional antioxidants ( In jam manufacturing, a few major elements of the process influence the pathway of Maillard Reaction: sugar content, pressure along with temperature, processing time, pH and storage conditions.

Sugar and fruit content
Sugar content influences moisture and water activity in food. Lower sugar content means not only less reactant available, but also higher moisture and water activity, which affect intensity of the Maillard Reaction In the Table 2 based on the reference data HMF and furosine concentrations in jams and fruit-based baby food were shown and compared according to the sugar content. It can be noticed that jams of high sugar content (60% or more) contained more HMF (0.45 -15.96 mg/100 g of product) than other samples (0.08 -7.5 mg/100 g of product). In almost all products of low sugar content (40% or less) HMF concentration was lower than 0.7 mg/100 g. HMF content in fruit-based baby food that usually do not have additional sugar (Rada-Mendoza et al., 2002a) is low and stays in range of undetectable amount to 1.0 mg/100 g. It is possible to avoid excessive formation of HMF even in jams rich in sugar through manipulation of preparation process parameters. It was proven by Cendrowski et al. (2012) in a study where two jams of same sugar content (68%) were prepared. The sample without addition of citric acid had significantly lower HMF concentration, therefore in jams rich in sugar addition of citric acid would be unfavorable (Cendrowski, 2012). There is little data on the impact of fruit share on HMF and furosine concentration.

Rada-Mendoza et al. (2002b; 2002a)
carried out a comparative study of a several samples of commercially available jams from various types of fruit in terms of the influence of fruit share, sugar content, pH and moisture on the HMF and furosine concentrations. The research showed that the formation of MRPs in jams depends on different factors -not necessarily on the fruit share. To eliminate the possible variability due to the type of fruit, a study on jams made only from peach was carried out and it showed that reduced-sugar jam samples had the lowest levels of furosine. The amount of HMF in these samples varied regardless of the pH, sugar or fruit content. According to the Rada Mendoza et al. (2002b) low HMF contents in fruit-based infant foods, compared with those found in jams, may in part be due to the lower fruit concentration in fruit-based infant foods.

Temperature, pressure and processing time
Higher temperature levels and long processing periods correspond with an increase in the abundance of Maillard Reaction markers in jams. It is caused by an increase in reactivity between sugars and amino acids. Studies on peach puree proven that darkening of this product is intensified with an increase of temperature and time, with thermal treatment being more influential than the length of heating period (Garza et al., 1999). Moreover, it was noted that both HMF and furosine are formed in higher amount in jams that undergo more severe heat treatment (Rada -Mendoza et al., 2004). The pressure is also the factor on which the course of Maillard Reaction depends. There are two major pressure condition types used in the jam production: atmospheric pressure and under the vacuum (Rada-Mendoza et al., 2002a; Korus et al., 2015) conducted an experiment where cooking jam in an open pan and cooking in a vacuum evaporator were compared. The second method allowed the application of lower temperatures but took longer than cooking under the normal pressure. Open pan cooking resulted in 5 times higher HMF concentrations than cooking under vacuum. While storage caused an increase in HMF only in vacuum cooked samples, these concentrations were still notably lower than in open pan cooked jams. It is important to note that in the vacuum cooked samples there was also no loss of polyphenols, lesser loss of flavonoids and anthocyanins (

pH
The pH affects reactivity of sugars and amino acidskey compounds in the earliest stage of Maillard Reaction (Martins et al., 2000). The value of pH in jams depends mostly on the fruit species used in production and on addition of pH modifiers into the product (Rada-Mendoza et al., 2002a). In fruit jams and similar products relatively low pH can be measured: about 2.1-3.9 in jam samples and about 3.7-4.2 in fruit-based baby food (

Storage conditions
Prolonged storage can alter compounds formed in the Maillard Reaction too. During 12 months of storage browning in jams still occurs, more noticeably at higher temperature. Increases in amount of HMF and furosine were observed by Rada -Mendoza et al. (2004), but HMF quantity seemed to be more dependent on temperature than furosine quantity. During storage of commercial jams HMF amount significantly changed: − from 0.6 mg/100 g of product to 2.5 mg/100 g of product in 20°C; − from 0.6 mg/100 g of product to 35.2 mg/100 g of product in 35°C (Rada-Mendoza et al., 2004).

INFLUENCE OF MR ON THE QUALITY OF FOOD
Heat treatment and/or prolonged storage conditions might partially decrease the amount of biochemical components of foodstuff such as vitamins and nutrients and unfortunately -at the same time enhance the content of MRPs (Kesić et al., 2014;  Shapla et al., 2018). HMF, which occurs at a very low concentration or is not detected in fresh products, can serve as the factor of quality and freshness of sugar rich foodstuffs. The Codex Alimentarius Standard Commission set the maximum concentration for HMF in honey at 4.0 mg/100 g (with a higher limit of 8.0 mg/100g for honeys originating from tropical countries) to ensure that the product has not undergone extensive heating during processing and is safe for the consumers (Codex Alimentarius Commission, 2001). HMF content in honey is an important parameter for the analysis of the quality of honey, its freshness, antioxidant activity, as well as its nutritional value. Some of the studues had also focsued on the influence of storage time. on the quality of honey. It was shown that samples of 4 year old honey contains on average 52.44% higher HMF amount compared to fresh honey samples (Kesić et al., 2014). It is worth noting that for jams no standard describing the acceptable content of HMF has been introduced yet though its concentrations can be higher in fruit products than in honey (Table1). During the Maillard Reaction some biological active ingredients of food undergo degradation, and among these are valuable vitamins, polyphenols and amino acids. Many studies note loss of vitamin C (ascorbic acid) during the preparation of jams It is important to underline the fact, that as phenolic content is unfortunately decreased in jams compared to fresh fruit, the final product still has a notable antioxidant capacity (Aksay et al., 2018). The Maillard Reaction uses amino acids contained in the food product which leads to the loss of protein content, thus lowering nutritive value of product. During the reaction amino acids can be destroyed and become biologically inactive (essential amino acids undergo these changes too). Depending on the conditions of the experiment, both decrease and increase of protein digestibility can be observed

INFLUENCE OF MRPs ON ORGANISM
The importance of a balanced diet has become one of the priorities of modern life.The general society's awareness and knowledge regarding the influence of many chemical compounds formed in food during processing (also under home conditions) on human health had lead to the need for further investigation ( Scientific data provides contradictory information on hydroxymethylfurfural's safety. In vitro studies suggest that it is not dangerous in low amounts, such as 0.5 -1 mg/kg body weight, though it poses a serious health risk in concentrations much higher than present in processed food. It was proven that HMF in high concentrations shows irritating properties ( This shows the immunosensitizing potential of these MRPs which can lead to autoimmune disorders. Alternation of food proteins immunogenicity by MRPs is considered (Teodorowicz et al., 2017).
The another MRPfurosineis widely present in many food products, including infant fruit food and formulas (  Saeed et al., 2017). While the damage does not lead to increased mutagenicity it is possible that furosine rather causes apoptotic cell death than mutations leading to carcinogenesis (Saeed et al., 2017). In mice models furosine was proven to be toxic towards the kidney and liver and affected count of leukocytes, lymphocytes, neutrophils, red blood cells and blood palateles (Li et al., 2018). These early findings might help us to better understand the risk assessment and provide the evidence for further food safety regulations regarding MRPs occurrence. Not all MRPs have negative effects on human health. Among MRPs some compounds are beneficial for health have been identified. These include melanoidins, which have antioxidative and antibiotic properties (since they contain catechol and reductone groups) (Delgado-Andrade, Seiquer, Haro, Castellano and Navarro, 2010; Šebeková and Brouder Šebeková, 2018). The properties of these compounds depend strongly on the product processing conditions (e.g. the higher temperature of food processing the higher amount of melanoidins will be generated). Furthermore it was proven that, the intake of melanoidins can also reduce fat accumulation and oxidative stress exposure (Šebeková and Brouder Šebeková, 2018). The missing information regarding melanoidins is how they interact with different molecules and microbiota inside the gastrointestinal tract (ALjahdali and Carbonero, 2017). It is known that MRPs reduce the activity of oxidoreductases (tyrosinases and polyphenol oxidases -PPOs). The mentioned enzymes oxidize endogenous phenolic compounds present in fruits. Activity of some MRPs preserves these phenolic compounds and prevents enzymatic browning in food. Moreover, phenolic compounds such as flavones and flavonoids are inhibitors of mutagensheterocyclic amines -which are produced in the Maillard Reaction too. In some food products the addition of sugar makes them more resistant to oxidative rancidity (  The chemical pathways of MRPs in food products are relatively well-known, but so far the biological aspects and the impact of these compounds on the human organism contains many gaps to be filled. As fruit and fruit derivatives are recognized as an element of well-balanced diet, it is important to evaluate the influence of Maillard reaction on their safety. Fruit-rich diet is linked to a lower chronic diseases risk and reduced cancer risk ( Among variety of Maillard Reaction Products, desirable and undesirable compounds can be found. The presence of some mutagenic compounds depends significantly on the pathway taken by the reaction. An understanding of the process is therefore a key to estabilish the optimal conditions in the processing of fruit (Martins et al.,  2000).

CONCLUSION
The Maillard Reaction is an elaborate network of chemical transformations which leads to the formation of many MRPs, including melanoidins, brown substances responsible for darkening food during e.g. roasting. Some MPRs are believed to pose a risk to health in high concentrations. The reaction itself leads to lowering nutritional value of foodstuffs. Therefore, research in field of Maillard Reaction occurrence in fruit products is reasonable, as many of them are an essential part of a balanced diet and children nourishment. While Maillard reaction was widely evaluated in many food products, few studies have focused on jams and fruit-based baby food.
Fruit jams and fruit-based baby food contain ingredients that undergo Maillard Reaction during their preparation and storage. Long heating periods and the abundance of sugar typical for these products favor Maillard reaction occurrence. At low pH the characteristic for fruit the MR pathway leads to hydroxymethylfurfural formation in jams and other fruit derivatives. Thus, HMF can be used as Maillard reaction marker in fruits products, along with other compounds: furosine and furfural.