Does your food meet the standard?

UV-Visible spectrophotometry is a fast, simple, non-destructive analytical technique for identifying and measuring certain properties of food and beverage samples - ideal for quality control

UV-Visible (UV-Vis) has long been selected as a first-line, go-to analytical technique because of its ability to strip away layers of complexity that many techniques require. Advancements in computing speed, method development and instrument spectral resolution have made UV-Vis spectrophotometry a critical technology for solving the at-line, in-line and related analytical problems of the food and beverage industries. Accuracy, precision, short time of analysis and limited sample preparation make this an ideal method for routine analysis. It also replaces time consuming, expensive and hazardous primary analysis that is typically run on food and beverage products.

A wide range of sample types can be analysed for quality control using UV-Vis spectrophotometry, including coarse solids, powders, pellets, gels, pastes, slurries, opaque and clear liquids. When it comes to the analysis of liquids, in particular, the colour of a sample can be determined by interpretation of the absorbance of the sample at various wavelengths. By utilising advanced photodiode array technology, sophisticated UV-Vis spectrophotometers offer significant advantages with regards to full-spectrum data acquisition and sample throughput compared to traditional dispersive, monochromator-based spectrophotometers. Full spectrum data is acquired at less than one second for routine data applications and can be acquired at nearly 50 points per second for advanced kinetics applications.

Recent advancements in spectrometer design provide simultaneous detection of all wavelengths throughout the UV and visible regions of the spectrum, allowing a nearly instantaneous display of a full absorbance spectrum from 190-1100nm. Full spectrum analysis of every standard and sample allows users to create standard curves, plot 3D graphical displays and examine samples at any wavelength at any time, greatly enabling analytical method development.

Two experiments were performed using UV-Vis to demonstrate the efficiency and reliability of the method for quality control of food and beverage samples.

Figure 1

Determination of HMF in honey

HFM (hydroxymethylfurfural) is an aldehyde that is generated by the decomposition of fructose in acidic conditions. It occurs naturally over time in most honeys and very quickly when honey is heated. As a result, the amount of HMF present in honey is used as an indicator for storage length and the amount of heating which has taken place, determining the quality of honey. High levels of HMF may be the result of inadequate storage, adulteration with sugar additives or severe heat treatment during the extraction process1.

Although HMF is not considered as a harmful substance, food standards in many countries regulate HMF levels in honey. Council directive 2001/110/EC2 specifies that honey in general, except baker's honey, must contain no more than 40mg/kg HMF, while honeys of declared origin from regions with tropical climate and blends of these honeys must contain no more than 80mg/kg HMF. Similarly, the Korean food regulatory standards specify maximum concentration of HMF in honey no greater than 80mg/kg.

A UV-Vis spectrophotometer (Thermo Scientific Evolution Array) was used to determine the HMF content of honey according to the White method3. The absorbance of a clarified aqueous honey solution was measured against a reference solution of the same honey in which the 284nm chromophore of HMF was destroyed by bisulphite. The HMF content of honey was then calculated using the following equation:

HMF (mg/100g of honey) = (A₂₈₄ –A₃₃₆) x Factor w Where: w = weight of sample in grams Factor = 126 x 100 x 1000 x 100 X 74.87 19830 x1000 and 126 = the molecular weight of honey and 16830 = molar absorptivity of HMF at 284nm.

Five grams of honey sample were dissolved in 25ml of deionised water. Subsequently, 0.5ml of Carrez Solution I (150mg/ml potassium ferrocyanide) and 0.5ml of Carrez Solution II (300mg/ml zinc acetate) were added to the sample and mixed well. The sample was brought to a final volume of 50ml with deionised water using a drop of alcohol to suppress surface foam. It was then filtered and the first 10ml of filtrate were discarded, with 5ml of the remaining filtrate transferred into each of two test tubes. A reference sample was prepared by adding 5ml of 0.20% sodium bisulphite to one of the test tubes of filtrate. A test sample was prepared by adding 5ml of deionised water to the other test tube of filtrate. Both samples were mixed well using a vortex mixer. The absorbance of the test sample was measured against the reference sample at 284nm and 336nm.

Figure 2

Two different samples of honey were measured using this procedure. The spectra of both honey samples are detailed in Figure 1. The HMF content of honey was found to be 3.3635mg/100g for honey 1 and 1.6981mg/100g for honey 2 as shown in Table 1. The equation calculation mode in the UV-Vis spectrophotometer software (Thermo Scientific VISIONcollect) enabled fast and easy calculation of the HMF content of honey. The HMF content of honey 2 was found to be lower than that of honey 1. Both honey 1 and 2 meet the Korean directive for the maximum level of HMF, while only honey 2 meets the EC directive.

Wine colour analysis

Several important measures of wine quality can be evaluated by mathematical combination of absorbance values at multiple wavelengths. For the purposes of this experiment, three values were used, namely wine colour intensity, wine hue and CIE L*a*b*. Wine colour intensity is a simple measure of how dark the wine is using a summation of absorbance measurements in the violet, green and red areas of the visible spectrum (wine colour intensity = A420 + A520 + A620, where A? represents the absorbance at wavelength ?). Wine hue is a simplistic measure of the appearance of the colour. It is a ratio of the absorbance in the violet area to the absorbance in the green area (wine colour hue = A420/ A520).

Figure 3

CIE L*a*b* is one of the most widely used international scales for colour. It is calculated from a mathematical combination of absorbance data taken across the entire visible spectrum with standard functions for the illumination source and the observation angle. In this particular example, the illumination source specified for the colour calculation is D65 which is considered to be standard daylight. The observer function is specified as the CIE standard 10° observer. Of the three values L*, a* and b*: L* is a measure of luminosity or lightness (an L* value of 0 represents pure black whereas an L* value of 100 represents pure white; a* is a measure of the redness or greenness of the colour (positive values represent a redder value and negative values represent more green); and the b* value is a yellow-blue measurement (positive values represent a more yellow sample whereas negative values a more blue sample).

Experiments were carried out using a UV-Vis spectrophotometer (Thermo Scientific Evolution Array) running on the equation calculation mode of the software for wine colour intensity and wine hue, and or the colour analysis mode of the software for the CIE L*a*b* colour. Samples were measured in 1mm pathlength cuvettes against a deionised water blank. L*a*b* colour measurements were carried out in transmittance mode with the scan number method parameter set to 10 and the Integration method parameter set to 1.

Spectra of seven wine samples are shown in Figure 2. Table 2 shows the colour intensity and hue calculation results. The absorption spectra are similar in profile but the intensity of the absorption at the measurement wavelengths varies considerably. The expected difference between red and white wine is seen clearly in the range between 400 and 650nm, where red wine samples exhibit an absorbance peak due to absorption by anthocyanin that is absent in the white wines. This peak in the red wine samples may be used in separate methods to determine the anthocyanin content of the wine sample.

Figure 3 shows the transmittance spectrum of wine sample 3 and a control sample specified as the target – deionised water. Table 3 displays the measured colour parameters for all seven wine samples and the calculated colour difference between each and the target sample of deionised water. ?E is a common measure of colour difference, calculated as:

This value is reported in the centre column of the table. ?E is frequently used as a pass/fail measure for whether a sample complies with a standard established for a product. As expected, the red wine samples show the most divergence (higher ?E) from the target water sample.

Overall, the experimental results demonstrated that UV-Vis spectrophotometry offer fast customised calculation of analytical parameters for quality control in the wine industry.

UV-Vis spectrophotometry has long been established as a rapid, easy-to-use and non-destructive analytical technique for product quality control in the food and beverage industries. Recent technological advancements have seen the introduction of state-of-the-art UV-Vis spectrophotometers capable of facilitating full-spectrum data acquisition, increased sample throughput and simultaneous detection of all wavelengths throughout the UV and visible regions of the spectrum. Experimental results demonstrate the superior capabilities of the technique for reliable and fast quality control of food and beverage samples.