The unique manuka effect: why New Zealand manuka honey fails the AOAC 998.12 C-4 sugar method

A new approach to detecting sugar syrup addition to honey: Stable isotope analysis of hexamethylenetetramine synthesised from honey monosaccharides (fructose and glucose)

One of the most common types of adulteration of honey involves the addition of invert sugar syrups. A new method was developed to measure the stable isotope ratios of carbon and carbon-bound non-exchangeable (CBNE) hydrogen from specific molecular positions in fructose and glucose in honey. This was achieved through periodate oxidation of the sugars to produce formaldehyde, followed by reaction with ammonia to form hexamethylenetetramine (HMT). The preparation was simplified, optimized, and validated by isotopic analysis of replicate syntheses of HMT from fructose, glucose, sugar syrup and a representative authentic honey sample. The optimized method had a repeatability standard deviation from 1.5‰ to 3.0‰ and from 0.1‰ to 0.4‰ for δ2H and δ13C, respectively. This methodology has advantages over alternative isotopic methods, for measuring CBNE hydrogen isotope ratios in sugars, in terms of time, sensitivity and operability and offers a complementary method to differentiate authentic honey from invert sugar syrups.

Application of multiple stable isotopes to aid identification of the origin of regional and organic animal products in Hesse, Germany

There is an increasing global demand for regional and organic produce. However, the growth of these markets depends on consumers' trust. Thus, novel methods must be developed to aid the verification of the origin of produce. We built on our previous study to identify the geographical origin and production method of animal-derived food products. Thirty-samples of eggs, 99 of milk, 34 of beef, and 62 of pork were collected from different regions in central Germany and analysed for their stable isotopic composition. The analysis followed a single-variate authentification approach using five isotope signatures, δ18O, δ2H, δ13C, δ15N, and δ34S. The best-performing indicators for verification of the geographical origin were δ15N and δ34S for beef; δ18O, δ2H, and δ13C for milk, and δ2H and δ13C for pork. These tracers indicated statistically significant differences among regions with the exception of pork; the results recorded for eggs were inconclusive. It was possible to distinguish between production methods by means of δ15N and δ34S (beef); all five tracers (eggs), and δ13C, δ15N, and δ34S (milk). This study demonstrated how the analysis of stable isotopes can be employed to determine the geographic region of origin and production method of animal-derived products in Germany.

Identification of novel unique mānuka honey markers using high-resolution mass spectrometry-based metabolomics

Mānuka honey is a valuable commodity produced by bees foraging the flowers of Leptospermum scoparium, a bush native to New Zealand and Australia. Due to its high value and proven health benefits, authenticity fraud in the sale of this food is a significant risk, as recounted in the literature. Four compulsory natural products must be present at minimum concentrations to authenticate mānuka honey (3-phenyllactic acid, 2'-methoxyacetophenone, 2-methoxybenzoic acid, and 4-hydroxyphenyllactic acid). However, spiking other kinds of honey with these compounds and/or the dilution of mānuka honey with other varieties may result in fraud going undetected. In this work, liquid chromatography coupled with high-resolution mass spectrometry and a metabolomics-based strategy has allowed us to tentatively identify 19 natural products -putative mānuka honey markers-, nine of which are reported for the first time. Chemometric models applied to these markers allowed the detection of both spiking and dilution fraud attempts of mānuka honey, even at 75% mānuka honey purity. Thus, the herein-reported methodology can be employed in the prevention and detection of mānuka honey adulteration even at low levels, and the tentatively identified markers presented in this work proved valuable for mānuka honey authentication procedures.

Methylglyoxal binds to amines in honey matrix and 2'-methoxyacetophenone is released in gaseous form into the headspace on the heating of manuka honey

Reports on the thermal stability of manuka honey in terms of food processing have been few. This study investigated changes in nine characteristic chemicals of manuka honey during heating. Among these, methylglyoxal (MGO) and 2'-methoxyacetophenone (MAP) were significantly decreased by heating at 90 °C. To elucidate the mechanism for this decrease, artificial honey was prepared from sugars and water with MAP or MGO and then heated. The decrease of MGO was enhanced with l-proline, lysine, or arginine derivatives, accompanied by formation of 2-acetyl-1-pyrroline, MGO-derived lysine dimer, or argpyrimidine, respectively, suggesting that an amino-carbonyl reaction is one pathway for the loss of MGO. The decrease of MAP in the artificial honey depended on the volume of headspace in a vessel. MAP from heated manuka honey was also detected in the gas phase, indicating that MAP was vaporized. Heating could thus reduce the beneficial and/or signature molecules in honey.

Investigation of Temporal Apparent C4 Sugar Change in Manuka Honey

New Zealand manuka honeys are known for their propensity to increase apparent C4 sugar content during storage. Depending on the particular storage regime and the initial content of dihydroxyacetone (DHA) in honey, the ready-to-market product often fails the C4 sugar test because of the above phenomenon. We have used DHA labeled with a radioactive ¹⁴C isotope in a set of honeys subject to an incubation experiment. These honeys were analyzed for DHA, methylglyoxal (MG), hydroxymethylfurfural (HMF), apparent C4 sugars, and ¹⁴C scintillation counts over a period of 18 months. The major conclusion of this experiment is that neither DHA nor MG is responsible for the δ¹³C shift in the honey protein extract. There must be some other yet unknown substance of manuka honey, which binds to the protein and causes negative δ¹³C shift. One identified candidate for such a binding is carbon dioxide.

Authenticity and geographic origin of global honeys determined using carbon isotope ratios and trace elements

Honey is the world's third most adulterated food. The addition of cane sugar or corn syrup and the mislabelling of geographic origin are common fraudulent practices in honey markets. This study examined 100 honey samples from Australia (mainland and Tasmania) along with 18 other countries covering Africa, Asia, Europe, North America and Oceania. Carbon isotopic analyses of honey and protein showed that 27% of commercial honey samples tested were of questionable authenticity. The remaining 69 authentic samples were subject to trace element analysis for geographic determination. One-way ANOVA analysis showed a statistical difference (p < 0.05) in trace element concentrations of honey from Australian regions and different continents. Principal component analysis (PCA) and canonical discriminant analysis (CDA) coupled with C5.0 classification modelling of honey carbon isotopes and trace element concentrations showed distinct clusters according to their geographic origin. The C5.0 model revealed trace elements Sr, P, Mn and K can be used to differentiate honey according to its geographic origin. The findings show the common and prevalent issues of honey authenticity and the mislabelling of its geographic origin can be identified using a combination of stable carbon isotopes and trace element concentrations.

1H-NMR Profiling and Chemometric Analysis of Selected Honeys from South Africa, Zambia, and Slovakia

Honey is the natural sweet substance produced by honeybee from nectar or honeydew, exhibiting several nutritional and health benefits. It contains a complex mixture of compounds in different proportions, with sugars being the main component. The physicochemical characteristics of ten honeys were evaluated; represented by five, three, and two from South Africa, Slovakia, and Zambia, respectively. The range of values for the pH (3.75–4.38), electrical conductivity (99–659 µS/cm), and moisture content (14.2–17.7%) are within the recommended limits for quality honeys. ¹H-NMR (Nuclear Magnetic Resonance) profiling of the honeys in D₂O was determined, and the data were analysed by chemometrics. This method is fast, reproducible, and sample pre-treatment is not necessary. The ¹H-NMR fingerprints of various chemical shift regions showed similarity or dissimilarity across geographical origins that are useful for identification, detection of adulteration, and quality control. The principal component analysis PCA and partial linear square discriminant analysis PLS-DA of the ¹H-NMR profiles successively categorises the honeys into two chemically related groups. The R² values are higher than the corresponding Q² values for all samples, confirming the reliability of the model. Honeys in the same cluster contain similar metabolites and belong to the same botanic or floral origin.

Authenticity determination of honeys with non-extractable proteins by means of elemental analyzer (EA) and liquid chromatography (LC) coupled to isotope ratio mass spectroscopy (IRMS)

The present work aims to systematically demonstrate the authenticity of honeys with non-extractable proteins for the first time, by means of EA-IRMS and LC-IRMS. Fifty-three pure honeys of various botanical and geographical origins were studied and a criterion on the basis of the stable carbon isotope ratio characterization of total honey and the main sugars was established for pure honeys. Parameters such as δ¹³C values of total honey and the main sugars were well utilized to identify honeys with non-extractable proteins. Thirty-five honeys from which protein could not be extracted were all identified as adulterated with C-4 sugars or C-3 sugars. The use of isotopic compositions and some systematic differences permit the honeys with non-extractable proteins to be reliably identified. The findings obtained in this work could supplement the AOAC 998.12 C-4 sugar method, with regard to honeys from which protein cannot be extracted.

Rapid and Reliable HPLC Method for the Simultaneous Determination of Dihydroxyacetone, Methylglyoxal and 5-Hydroxymethylfurfural in Leptospermum Honeys

A reliable determination of dihydroxyacetone, methylglyoxal and 5-hydroxymethylfurfural is essential to establishing the commercial value and antimicrobial potential of honeys derived from the Leptospermum species endemic to Australia and New Zealand. We report a robust method for quantitation of all three compounds in a single HPLC run. Honey samples (n = 6) that are derivatized with o-(2,3,4,5,6-Pentafluorobenzyl) hydroxylamine were quantitated against a stable anisole internal standard. Linear regression analysis was performed using calibration standards for each compound (n = 6) and results indicated a high degree of accuracy (R² = 0.999) for this method. The reliability of some commercial methylglyoxal solutions were found to be questionable. Effective quantitation of methylglyoxal content in honey is critical for researchers and industry, and the use of some commercial standards may bias data. Two accurate methylglyoxal standards are proposed, including a commercial standard and a derivative that can be prepared within the laboratory.

Adulteration Identification of Commercial Honey with the C-4 Sugar Content of Negative Values by an Elemental Analyzer and Liquid Chromatography Coupled to Isotope Ratio Mass Spectroscopy

According to the AOAC 998.12 method, honey is considered to contain significant C-4 sugars with a C-4 sugar content of >7%, which are naturally identified as the adulteration. However, the authenticity of honey with a C-4 sugar content of <0% calculated by the above method has been rarely investigated. A new procedure to determine δ(13)C values of honey, corresponding extracted protein and individual sugars (sucrose, glucose, and fructose), δ(2)H and δ(18)O values, sucrose content, and reducing sugar content of honey using an elemental analyzer and liquid chromatography coupled to isotope ratio mass spectroscopy, was first developed to demonstrate the authenticity of honey with a C-4 sugar content of <0%. For this purpose, 800 commercial honey samples were analyzed. A quite similar pattern on the pentagonal radar plot (isotopic compositions) between honey with -7 < C-4 sugar content (%) < 0 and 0 < C-4 sugar content (%) < 7 indicated that honey with -7 < C-4 sugar content (%) < 0 could be identified to be free of C-4 sugars as well. A very strong correlation is also observed between δ(13)C honey values and δ(13)C protein values of both honey groups. For the δ(18)O value, the C-4 sugar content (%) < -7 group has lower (p < 0.05) values (16.30‰) compared to other honey, which could be a useful parameter for adulterated honey with a C-4 sugar content (%) < -7. The use of isotopic compositions and some systematic differences permits the honey with a C-4 sugar content of <0% to be reliably detected. The developed procedure in this study first and successfully provided favorable evidence in authenticity identification of honey with a C-4 sugar content of <0%.

Plausible authentication of manuka honey and related products by measuring leptosperin with methyl syringate

Manuka honey, obtained from Leptospermum scoparium flowers in New Zealand, has strong antibacterial properties. In this study, plausible authentication of the manuka honey was inspected by measuring leptosperin, methyl syringate 4-O-β-D-gentiobiose, along with methyl syringate. Despite a gradual decrease in methyl syringate content over 30 days at 50 °C, even at moderate 37 °C, leptosperin remained stable. A considerable correlation between nonperoxide antibacterial activity and leptosperin content was observed in 20 certified manuka honey samples. Leptosperin and methyl syringate in manuka honey and related products were analyzed using HPLC connected with mass spectrometry. One noncertified brand displayed significant variations in the leptosperin and methyl syringate contents between two samples obtained from different regions. Therefore, certification is clearly required to protect consumers from disguised and/or low-quality honey. Because leptosperin is stable during storage and specific to manuka honey, its measurement may be applicable for manuka honey authentication.

Investigating C-4 sugar contamination of manuka honey and other New Zealand honey varieties using carbon isotopes

Carbon isotopes (δ(13)C honey and δ(13)C protein) and apparent C-4 sugar contents of 1023 New Zealand honeys from 15 different floral types were analyzed to investigate which New Zealand honey is prone to failing the AOAC 998.12 C-4 sugar test and evaluate the occurrence of false-positive results. Of the 333 honey samples that exceeded the 7% C-4 sugar threshold, 324 samples of these were New Zealand manuka honey (Leptospermum scoparium, 97.2% of all fails found in the study). Three monofloral honeys (ling, kamahi, and tawari) had nine samples (2.8% of all fails found in the study) with apparent C-4 sugars exceeding 7%. All other floral types analyzed did not display C-4 sugar fails. False-positive results were found to occur for higher activity New Zealand manuka honey with a methylglyoxal content >250 mg/kg or a nonperoxide activity >10+, and for some ling, kamahi and tawari honeys. Recommendations for future interpretation of the AOAC 998.12 C-4 sugar method are proposed.