Investigation of kefirs produced in different hardnesses with spectroscopy and rheology techniques

In this thesis study, qualitative analyses of sweet, medium-hard and extra-hard kefirs produced by traditional methods were carried out using spectroscopy (UV-Vis and FTIR) and rheology methods. It was determined that the fermentation time, i.e. the degree of hardness, was effective on the optical properties of kefirs. It was determined that the UV spectra of kefirs with different hardness had peak values in two different wavelength regions depending on the bioactive compound concentration and the presence of protein or polypeptide. Optical energy gap (Eg) values for sweet, medium-hard and extra-hard kefirs were recorded as 5.60, 5.05 and 4.91 eV, respectively. It was concluded that the concentration of the bacterial strains in the structure of kefirs and the organic molecules they produced during their survival cycles were effective on these different values. It was determined that the rheological properties of kefirs with different hardness were compatible with the power law model and exhibited non-Newtonian behavior. Moreover, the flow properties of all kefirs were of viscoelastic nature. The FTIR spectra of kefirs with different hardness were recorded in the mid-wavelength infrared region in the wavenumber range of 4000 cm-1 to 400 cm-1 and were found to be very close to each other. The different peak values in the wavenumbers range of 3500 cm-1 to 3000 cm-1 and 2000 cm-1 to 1500 cm-1 of kefirs with different hardness were due to the presence of functional groups and structural differences of the molecules of bioactive compounds, respectively.