This article provides an overview of recent developments and the published literature in membrane technology with regard to fruit juice processing and considers the impact of such technology on product quality. In the fruit juice industry, membrane technology is used mainly to clarify the juice by means of ultrafiltration and microfiltration and to concentrate it by means of nanofiltration and reverse osmosis. We look at enzyme immobilization techniques to improve filtration performance and operation methods to quantify fouling. Membrane fouling is a critical issue and inhibits the broader application of membranes in the fruit production industry. Pectin and its derivatives form a gel-like structure over the membrane surface, thereby reducing the permeate flux. In order to degrade pectin, the raw juice is usually subjected to an enzymatic treatment with pectinase, which hydrolyses pectin and causes its protein complexes to flocculate. The resulting juice has reduced viscosity and a much lower pectin content, which is advantageous in the subsequent filtration processes.

The conversion of glycerol in supercritical water (SCW) was studied at 510 -550°C and a pressure of 350 bars using both a bed of inert and non-porous ZrO 2 particles (hydrothermal experiments), and a bed of 1 % Ru/ZrO 2 catalyst particles. Experiments were conducted with a glycerol concentration of 5 wt% in a continuous isothermal fixed-bed reactor at a residence time between 2 and 10 s. Hydrothermolysis of glycerol formed water-soluble products such as acetaldehyde, acetic acid, hydroxyacetone and acrolein, and also gases like H 2 , CO and CO 2 .The catalyst enhanced the formation of acetic acid, inhibited the formation of acrolein, and promoted the gasification of the glycerol decomposition products. Hydrogen and carbon oxides were the main gases produced in the catalytic experiments, with only minor amounts of methane and ethylene. Complete glycerol conversion was achieved at a residence time of 8.5 s at 510 °C, and at around 5 s at 550 °C with a 1 wt% Ru/ZrO 2 catalyst. The catalyst was not active enough to achieve complete gasification, since high yields of primary products like acetic acid and acetaldehyde were still present. Carbon balances were between 80 and 60 % in the catalytic experiments, decreasing continuously as the residence time was increased. This was attributed partially to the formation of methanol and acetaldehyde, which were not recovered and analyzed efficiently in our set-up, but also to the formation of carbon deposits. Carbon deposition was not observed on the catalyst particles but on the surface of the inert zirconia particles, especially at high residence time. This was related to the higher concentration of acetic acid and other acidic species in the catalytic experiments, which may polymerize to form tar-like carbon precursors. Because of carbon deposition, hydrogen yields were significantly lower than expected; for instance at 550 ºC the hydrogen yield potential was only 50 % of the stoichiometric value.

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