Application of membrane filtration methods for must processing and preservation

The aim of this study was to investigate membrane filtration for must processing and to carry out a complex method for preservation and concentration of grape must. In the experiments white and red grape-juice samples were used. After the transportation of the samples in to the laboratory the impact of the pre-treatment and process parameters were examined, and in the case of red grape must also the degree of anthocyanin retention was measured as well. Microfiltration (MF) resulted in the clarification and sterilization of the samples. After microfiltration permeates were concentrated by reverse osmosis (RO). During filtration the sugar content remained in the retentate. Very low anthocyanin content was measured in the red grape juice permeate, which proves good anthocyanin retention of the membrane. With the combination of the two processes a new complex method can be established that results in a sterile concentrate with a higher sugar content. This method gives a possibility to handle must excess and to develop a new product without preservatives.

In the first step of our study microfiltration was used for clarification and sterilization of the three must samples: Furmint and Szamorodni (white grape juice) and Kékfrankos (red grape juice). The membrane modules were hollow fibre and multitube. The SS content of the must samples was retained by the membranes applied. Sensory analysis was used to compare the appearance and the taste of the original must samples and the permeates. The assessors preferred the samples that were filtered, they found a colour intensity decrease. At the same time they found that through the filtration procedure must samples lost a bit of their typical odour. Based on the microbiological experiments we can state that microfiltration is an effective process for clarification (e.g. removing SS content from the samples) and sterilisation of the must samples, because the total cell number decreased with 6 order of magnitude. For the second step we applied reverse osmosis for the concentration of the microfiltered must. Two membranes were studied for the concentration (HR-30 and ACM-2). First test measurements were carried out to find the most proper operating parameters: the operating temperature, pressure and the recycle flow rate. The temperature was 35°C and pressure was 50 bar, depending on the membrane resistance (HR-30 and ACM-2). Examining the recycle flow rate there was no significant deviation in the permeate flux at 300 and 400 L/h. To prevent the damage of the pump, the measurements were carried out at the lower (300 L/h) flow rate. Furmint grape must was used to compare the two reverse osmosis membranes (HR-30 and ACM-2). We observed a significant deviation in the permeate flux. The different initial concentrations of the two feeds (Furmint and Kékfrankos) have not caused differences in the permeate flux by the same refractions. In the case of both must samples the final concentration was reached successfully. Membranes are supposed to retain anthocyanin content of the red grape must, therefore the absorbance of the original must sample and the permeate was measured at 420 nm and 520 nm. Colour intensity was determined from these values. The anthocyanin retention was 99.5%.