Study of the interaction between water-kefir grains and metals in solution

XXVI Congresso Nazionale della Società Chimica Italiana XXX Y000

Study of the interaction between water-kefir grains and metals in solution

Vincenzo Zelano

Dipartimento di Chimica, Università di Torino Via Pietro Giura 5; marco.ginepro@unito.it

Kefir grains are a consortium of symbiotic microorganisms, mainly consistof different lactic acid bacteria (LAB) and yeasts. The grains consist of a sugar polymer (dextran), that host the microbial consortium, and the single grains have a cauliflower like shape of 5–20 mm in diameter, a transparent appearance and an elastic structure (1). The kefir grains ferment hexose sugars (such as sucrose) producing primarily lactic acid, ethanol and metabolites, that are released in the growth media. So, the final liquid drink, known as water-kefir too, contains many other interesting substances, among these acetic acid, glycerol, mannitol, vitamins, essential amino-acids and mineral elements. Because of nutraceutical benefits, studies concerning kefir are increasingly frequent in the last years. In addition, new researches have also showed how kefir grains are able to complex metallic cations present in the environment of fermentation but the mechanism is not fully understood.

Therefore, the main goal of this study is deeper understanding of kefir complexing ability in order to assessment possible applications in health and environmental fields. To this end, the interaction between water-kefir grains and metallic ions was studied.

Assuming that the proteins present on the superficial layer of the granules (S-layer proteins) are responsible of the complexing of the metallic cation, the research was initially focused into the nature of complexes, and studies of Raman spectroscopy and electronic paramagnetic resonance (EPR) were employed to investigate this aspect. Moreover, kinetic studies were carried out. The metals studied were mainly Pb, Ni, Cu, Mn, Cr(III) and Cr(VI). Pb, Ni and Cr are toxic elements, while Cu and Mn are paramagnetic elements present in natural water. In addition, other metallic species were investigated, such as Ca, K, Mg and Na. These elements are found in significant amount in the kefir grains and in water used for the water-kefir preparation. The study of the interaction between kefir grains and Cr(III) and Cr(VI) intends to understand if a different sequestering capacity towards Cr(III) and Cr(VI) exist, and if kefir grains present a reducing capacity towards Cr(VI).

The studies of kinetic related to each metal were carried out putting the water-kefir colonies in sucrose and water solutions with different concentrations of single metal. The pH and metal concentrations were measured at different intervals of time and this last analysis was conducted by ICP-OES. The obtained data showed the different exchange rate of metals and the different variation of acidification rate. It can be seen that the presence, the metal type and its concentration cause metabolic interference, modifying the physiological trend of pH. This effect is more evident in the first hours after addition of the metal, afterwards to its absorption, especially at high concentrations of cations. However, the adsorption process is reversible, since the metal is partially released after long contact times (24 h) and pH reaches the physiologic value (approx. pH= 3).

References: 1. Arslan, S. A review: chemical, microbiological and nutritional characteristics of kefir. CyTA - J. Food 13, 340–345 (2014).