One of the most popular products provided by nature, or more precisely, bees, is honey. The chemical composition of honey is very complex [
1]. There have been detected more than 500 chemicals in various honeys. Due to its exceptionally extensive chemical composition, honey has a considerable impact on the human body. However, like every product, it has its advantages and disadvantages. The positives are: it is beneficial for people struggling with anemia, increases immunity; protects plasma (the water part of the blood); has antibacterial properties, which may reduce the occurrence of caries; protects against gastric injury (stomach); prevents, heals some digestive disorders, e.g. ulcers, gastroenteritis; has been used since ancient times to treat eye diseases, e.g. blepharitis, conjunctivitis; reduces the risk factors of cardiovascular and metabolic diseases; natural honey is a powerful antibiotic [
2]. Honey, as a whole natural food, is exposed to several pollutants, such as antibiotics, pesticides, heavy metals and other toxic compounds. Their occurrence may result from residual bee treatment, accidental exposure, environmental hazards, or hostile competition practices. These residues of the mentioned compounds harm the human body. Apart from the above-mentioned impurities/ chemical hazards found in honey, is also very common, and among them are 5-hydroxymethylfurfural (HMF). All reduce the health benefits and quality of honey.. 5-hydroxymethylfurfural deserves interest due to its specific properties. On the one hand, it belongs to carcinogenic compounds, has a cytotoxic effect, and hinders or prevents DNA replication/ repair [
3,
4,
5]. On the other hand, it can be used in high mountain diseases, and brain hypoxia and can remove free radicals [
5,
6,
7]. HMF is a six-carbon heterocyclic organic compound containing both an aldehyde and alcohol (hydroxymethyl) functional group.
Figure 1 shows the structure of HMF.
5-Hydroxymethylfurfural is a solid, yellow substance that exhibits a low melting point and high water solubility [
8]. It is a natural toxicant, which is formed from hexoses by heating and storing foods containing high amounts of sugar. There are two main ways in which 5-hydroxymethylfurfural is formed. The first is the caramelization of sugars at temperatures exceeding 150°C. The process occurs most easily in sugar reduction reactions, incl. glucose, fructose, or ribose. Also, it is catalysed by an acidic environment. The second method of HMF formation is the Maillard reaction, i.e. a non-enzymatic browning reaction. The reaction takes place as a result of the product’s long storage time or its heating. The name “Maillard reaction” comes from the French chemist Louis Maillard, who was the first to describe the reactions between sugars and amino acids in 1912. The aldehyde group of a carbohydrate reacts with the amino group derived from an amino acid or protein to form colorless intermediates, Amadori compounds, which in turn, when heated, form HMF. As a result of further changes taking place, many colored products are created. The products of the Maillard reaction reduce the nutritional value of food by blocking or destroying certain amino acid residues (lysine, cysteine, methionine, tryptophan) and reducing their bioavailability [
9]. Natural bee honey, freshly prepared, contains 2-5 mg/kg HMF. After one year, the quantity of HMF changes, and its content increases to 7-10 mg/kg. After two years it is 20-25 mg/kg, while longer storage of honey causes even a further increase in HMF content, up to 50-100 mg/kg [
10]. Scientists recommend that honey, regardless of its origin, should be consumed within a year [
11]. Permissible HMF content in honey established by the European Council Directive 2001/110/EC of 20 December 2001 [
12] relating to honey is up to 40 mg/kg for products from the European Union. In turn, for honey produced in tropical regions, the limit is higher and amounts to 80 mg/kg. The fact that the quantity of HMF has been exceeded indicates that honey is adulterated or overheated [
12]. HMF is easily formed at low temperatures, however it must be supported by the second condition, low pH. Additionally, long-term storage and high temperature increase its concentration. In addition to temperature and pH, the rate of HMF formation in the product also depends on the moisture content of honey, so there are frequently taken steps to keep moisture content low [
13].Based on the conducted research, numerous scientists have concluded that both temperature and the duration of heating affect the formation of HMF in honey. The work [
14] shows the results of honey research collected in Anatolia, Turkey. Honey samples were heated at 135°C for 100s, which produced a similar quantity of HMF as when the samples were heated at 150°C for 40 s. Whereas showed that honey, regardless of its origin, should be consumed within a year [
15]. They researched Malaysian honey. In their research, they showed that HMF content in honey changes accordingly with storage time. In honey samples stored for 3-6 months, the level of HMF ranged from 2.80 to 24.87 mg/kg, which was within the limits set by the European Commission (up to 80 mg/kg for honey from tropical regions) [
12]. On the other hand, the extension of the storage time from 12 to 24 months resulted in exceeding the permissible standard for HMF levels and ranged from 128.19 to 1131.76 mg/kg. HMF has both harmful and positive effects on the human body. Most of the studies and observations in this regard have been carried out primarily on mice and rats under laboratory conditions. HMF and its derivatives are genotoxic, mutagenic, carcinogenic, and destructive to DNA. The compounds also inhibit the work of enzymes in organisms. HMF is an indirect mutagen. It is converted into sulfuric acid ester (VI): 5-sulfoxymethylfurfural (SMF) as a result of activation by sulfotransferases in the liver. SMF is a highly mutagenic compound [
3]. Hence, HMF and its derivative SMF are strong carcinogens, a fact proven in a great number of studies. HMF is cytotoxic in high concentrations, it irritates mucous membranes, skin, eyes, and upper respiratory tracts [
5]. It is a selective inhibitor of a DNA polymerase enzyme, which disturbs or completely impedes a DNA synthesis during DNA replication or repair [
4]. HMF has been shown to have a positive effect on human health. Zhao et al. showed in their studies that HMF can remove free radicals depending on the dose (0.8-6.4 mM) [
16]. HMF also increases survival when oxygen levels are low resulting from, for example, hypoxia caused by staying at high altitude (mountain climbing), atherosclerosis, or cancer. Therefore, it can be a therapeutic agent used for treating mountain sickness, cerebral edema, or pulmonary edema at high altitudes [
6,
7]. HMF measurement is used to assess the quality of honey. It is generally present in trace amounts or does not occur in fresh honey. There are many methods used to determine HMF levels in honey. 5-hydroxymethylfurfural content is determined in a clear, filtered, aqueous solution of honey using a method based on the HPLC technique with a reversed phase and UV detection [
6,
7,
17]. Whereas the International Honey Commission recommends the use of three methods: two spectrophotometric methods White & Winkler, and the HPLC method [
18]. In addition, other methods are used to determine HMF in various types of samples, not necessarily honey [
19]: HPLC with refractive index detector (HPLC-RID), HPLC coupled with mass spectrometry (LC-MS), gas chromatography coupled with mass spectrometry (GC-MS), [
20] electrochemical biosensors [
21], micellar electro-kinetic capillary chromatography (MECK) [
22].
The objective of this study was to determine 5-hydroxymethylfurfural (HMF) levels in honeys available on the Polish market with three methods: two spectrophotometric by White and Winkler, and one developed for this work based on the LC-MS/MS technique. The indirect aim of the study was to assess the usefulness of all three methods for determining HMF content in honey.