Optical properties of honey: FTIR spectroscopy and refractometry

Abstract

115 samples of honey of various botanical types, geographical origin and harvest year (2019– 2021) were analysed using attenuated total reflection infrared spectroscopy of impaired total reflection, refractometry and biochemical analysis. Initial honey samples in liquid and crystallised states were investigated. Crystalline D-glucopyranose (glucose), D-fructopyranose (fructose), their 40% solutions and invert sugar were used as auxiliary substances. Biochemical analysis was used to determine the glucose content in honey samples. Based on the obtained data, a relationship between the results of biochemical analysis and refractometry (refractive index, the content of invert sugars, humidity) was established. We deduced equations that allow the content of glucose and fructose in honey to be evaluated by the refractive index. Studying honey by IR spectroscopy showed that all investigated samples, regardless of the botanical and geographical origins, can be classified into three groups dominated by: I – glucose, II – fructose and III – mixed, with a close content of two monosaccharides. This allowed the bands characteristic of α- and β-pyranose forms of glucose and fructose to be identified, as well as the nature of their changes depending on the ratio of both monosaccharides in honey as a result of their crystallisation to be assessed. It is noted that the ratio of monosaccharides determines not only the stability of the liquid crystal structure and crystallisation rate in honey but also their optical, biochemical and nutritional properties, which are important for the preferred use of honey in medical and pharmacopoeial practice, dietetics and cosmetology.