Correlation between chemical characteristics and optical spectra of Spirulina commercially available on the Bulgarian market

The aggregate of various chemical substances useful for the functioning of the human body are known as nutrients. Spirulina has been present in human nutrition since ancient times, but in recent years the interest in it has been particularly increased due to the emergence of numerous alternative methods of nutrition. This study aimed to compare the functional and elemental composition as well as the optical properties of commercial Spirulina products available on the Bulgarian market. For this purpose, fluorescence spectroscopy in the ultraviolet and visible range, fourier transform infrared spectroscopy and inductively coupled plasma optical emission spectroscopy were used. The basic components of the analyzed Spirulina samples are proteins (1657 and 1537 cm-1) and carbohydrates (1069 and 1054 cm-1) and no meaningful differences between the IR spectra of the samples. Concentrations of important microelements Mg, Fe, Cu, Zn, and Mn varies with the manufacturer. The highest levels for Mg (6.69 g kg-1) were measured in samples from USA, while the Spirulina fabricated in Bulgaria exhibits the highest contents of Zn (242 mg kg-1) and Cu (25.4 mg kg-1). All samples followed the tendency Mg>Fe>Mn>Zn>Cu. Making use of a fiber optic spectrometer the fluorescence spectra of the studied samples of Spirulina platensis for an excitation wavelength of 380 nm were measured. In these spectra we observe three fluorescence maxima: at 465 nm – nicotinamide dinucleotide phosphate, 640 nm chlorophyll a, and 736 nm due to similar to chlorophyll pigments. A strong positive correlation between the contents of Zn and Cu on the one side and the second fluorescence peak (λ=640 nm) for excitation wavelength at 380 nm. In contrast, a high negative correlation for Fe and the third fluorescence maximum (λ= 736nm) is observed for all excitation wavelengths. The correlation dependencies were obtained with the least squares method with a significance level of p ≤ 0.05.