NIR detection of honey adulteration reveals differences in water spectral pattern

Highlights

• NIR spectroscopy is applicable to detect a dominant form of honey adulteration.

• Best regression is achieved with spectral intervals of water and carbohydrates.

• Aquaphotomics describes molecular structures of water in adulterated honey.

• Highly organized water is more dominant in honey than in industrial sugar syrup.

• Adulteration by sugar syrup causes gradual reduction of organized water structures.

Abstract

High fructose corn syrup (HFCS) was mixed with four artisanal Robinia honeys at various ratios (0–40%) and near infrared (NIR) spectra were recorded with a fiber optic immersion probe. Levels of HFCS adulteration could be detected accurately using leave-one-honey-out cross-validation (RMSE_CV = 1.48; R²_CV = 0.987), partial least squares regression and the 1300–1800 nm spectral interval containing absorption bands related to both water and carbohydrates. Aquaphotomics-based evaluations showed that unifloral honeys contained more highly organized water than the industrial sugar syrup, supposedly because of the greater variety of molecules dissolved in the multi-component honeys. Adulteration with HFCS caused a gradual reduction of water molecular structures, especially water trimers, which facilitate interaction with other molecules. Quick, non-destructive NIR spectroscopy combined with aquaphotomics could be used to describe water molecular structures in honey and to detect a rather common form of adulteration.

Introduction

Honey is an important commodity in the food market and is acknowledged as a healthy natural sweetener, which is consumed alone or as an ingredient in a variety of sweet products. The world’s average annual honey production was 1.5 billion kg between 2005 and 2010 (FAOSTAT, 2013). Since consumption of honey in Hungary is very low (0.24 kg/capita/year), the country has become the second biggest honey exporter in the European Union, delivering 19 million kg per year to the market, covering more than 18% of the total EU export of honey. China and many European countries, particularly in the East, produce considerable amounts of False Acacia (Robinia pseudoacacia L.) unifloral honey, which is one of the most valuable honey products on the European market. However, the characteristics of Robinia honey (i.e. liquid due to the high fructose content – glucose:fructose = 1:1.4–1.7, very light colored and flavored) can be easily counterfeited (Persano Oddo & Piro, 2004).

The European Commission has defined honey (European Commission, 2002) and stipulated that nothing should be added to it. However, since there is a great demand for high quality honey and consumers intend to pay high prices for these products, valuable honeys have become a target of adulteration with cheap sweet foreign components worldwide. Adulterated honeys are often labeled as natural and priced the same as pure honey, which is both fraudulent and unfair to consumers (Chen et al., 2011). In general, the adulteration of honey does not pose a health risk. However, it does influence market growth negatively by damaging consumer confidence (Mehryar, 2011). Consequently, identification and authentication of unadulterated honey is important for processors, retailers and consumers, as well as regulatory authorities (Chen et al., 2011).

Large-scale honey adulteration appeared on the world market in the 1970s with the introduction of high-fructose corn syrup (HFCS) by the industry (Mehryar, 2011). Based on the pioneering investigations of Doner and White (1977), HFCS content in natural honey samples was described using mass spectroscopy (Doner and White, 1978), gas–liquid chromatography (Doner, White, & Phillips, 1979) and high pressure liquid chromatography (White, 1980). Although these methods and their further developed applications can accurately detect honey adulteration, they have a number of disadvantages that discourage practical application in screening systems, i.e., the need for highly skilled labor, time-consuming, destructive and ultrapure preparation and large instrumental analysis. Thus, there is an obvious demand for developing easy to use, rapid, non-destructive, and low-cost analytical methods to detect and quantify HFCS adulteration in commercial honey lots.

Near infrared (NIR) spectroscopy is a rapid, highly accurate, versatile, multi-analytical technique based on the electromagnetic absorption of organic compounds in the short wavelength infrared range (780–2500 nm). It can be applied to qualitative or quantitative analyses of multiple components of a sample by a single measurement. Moreover, no reagents are required and no hazardous waste is produced. The NIR technique has been applied in a wide range of fields in the last decades and several of these applications are currently in use as routine analyses, even in on-line monitoring systems, both in agriculture and the food industry (Downey, 1996, Huang et al., 2008, Prieto et al., 2009).

The use of the NIR technique for food authentication was reviewed by Reid, O’Donnell, and Downey (2006). In regards to honey, Cho and Hong, 1998, Ha et al., 1998 developed NIR prediction method for conventional quality parameters of Korean honeys using transflectance cuvettes with gold and aluminum reflector back. García-Alvarez, Huidobro, Hermida, and Rodríguez-Otero (2000) analyzed fructose, glucose and moisture content of honeys, and reported proper results in calibration and independent validation for all three components when the calibration was generated using transflectance spectra. As an authentication tool, NIR spectroscopy has been applied successfully for rapid determination of the floral origin of honeys (Chen et al., 2012, Escuredo et al., 2015). Transflectance NIR spectra of Irish artisanal honeys adulterated with fructose and glucose were acquired by Downey, Fouratier, and Kelly (2003) using a gold back reflector with 0.1 mm sample thickness, and accurate classification models were generated to reveal adulteration. Kelly, Petisco, and Downey (2006) investigated the detection of honey adulteration with sugar-beet invert syrup and HFCS using both qualitative and quantitative NIR models. Applying a fiber optic probe, successful classification was performed by Chen et al. (2011) using FT-NIR spectra of unadulterated and adulterated Chinese honeys collected from apiaries and purchased in local groceries.

Water is one of the most studied compounds in physics and chemistry, the properties of which are explained by several different structural models based on the number, nature and strength of hydrogen bonds (Segtnan, Sasic, Isaksson, & Ozaki, 2001). Aquaphotomics, as a new approach in NIR spectroscopy, considers water as a multi-element system that can be described by its multi-dimensional NIR spectra (Tsenkova, 2009). Since water-related H-bonds are present in most natural samples, this analytical approach, using perturbed water in different environments as a mirror for the rest of the molecules in the sample, can be applied effectively in various fields (Kinoshita et al., 2012, Tsenkova et al., 2009).

The objective of the present study was to develop an applicable NIR model for screening unifloral Robinia honey using a fiber-optic probe. Recent findings of aquaphotomics were implemented to interpret the chemometric calibration models for measuring the level of HFCS adulteration, and to extract important information on functionality of the honey related to its water structures.