Influence of the Soluble Solids on the Zeta Potential of a Cloudy Apple Juice

The effect of soluble solids on the Zetapotential (ξ) of a cloudy apple juice was investigated. Apple juice was enzymatically treated with pectinolytic enzymes and diafiltred with a polysulphone UF membrane (100 kDa) to eliminate soluble solids. Most soluble solids were removed after 4 equivalent sample volumes (ESV) of washing water. During a second period, from 5 to 23 ESV, diafiltration rate was significantly reduced. ξ (mV) moderately increased its absolute value with diafiltration during the first diafiltration period, changing at a rate of -1.1 mV/oBrix up to 0.3 oBrix). Results also indicated that after the only restitution of sugars ξ remained practically constant. When malic acid was restituted to a diafiltred juice, remarkable changes in Zeta potential occurred. As particles in this type of colloids are strongly negative, pH modification had a significant influence. Keywords— Cloudy apple juice, diafiltration, soluble solids, Zeta-potential.