Clarification and concentration of citrus and carrot juices by integrated membrane processes

Abstract

The use of integrated membrane processes for the clarification and the concentration of citrus (orange and lemon) and carrot juices is proposed as an alternative to the traditional techniques of the agro-food industry.

The ultrafiltration (UF) process was studied on a pilot plant unit to clarify the raw juice. A limpid phase was produced in this step and it was used for concentration by successive membrane treatments.

The reverse osmosis (RO) process, performed on a laboratory plant unit, was used to preconcentrate the permeate coming from the UF step up to 15–20 g TSS/100 g.

A final osmotic distillation (OD) step yielded a concentration of the retentate coming from the RO up to 60–63 g TSS/100 g at an average throughput of about 1 kg h m⁻². This laboratory unit was mainly used to develop operational parameters for the design of a full-scale plant and, secondly for the production of sample concentrates for their testing and evaluation.

The results obtained in the experimental tests concerning the identification of membrane modules, the operating and fluid-dynamic conditions and cleaning procedures of membrane modules, are reported and discussed. The performance of the membrane modules was evaluated on the basis of productivity, quality of the product (evaluation of the antioxidant activity) and fouling characteristics.

On the blood orange juice samples it was demonstrated that the total antioxidant activity (TAA) of juices concentrated by evaporation is lower than that of the fresh juice. During the UF process TAA was maintained, with respect to the fresh juice, both for the permeate and for the retentate. A little decrease of the TAA was observed in the RO treatment, probably on account of the high pressure employed during the process. After this step the subsequent concentration treatment by OD did not induce any significant changes to TAA independently from the final concentration obtained.

Moreover, the juices concentrated by membrane technology retained their colour and large part of their aroma which is on the contrary lost during thermal concentration.

According to the results obtained, it is possible to suggest an integrated membrane process that, starting from the raw juice, yields concentrated juices of high quality and high nutritional value. The residual fibrous phase coming from the UF process could be submitted to a stabilising treatment and reused for the preparation of beverages enriched in fibres.

Advantages are in terms of: reduction of clarification times, simplification of the clarification process, increasing of clarified juice volumes, the possibility of operating at room temperature and preserving the juice freshness, aroma and nutritional value, improvement of the quality of the final product through the removal of extraneous substances and improvement of the production processes.

Introduction

Fruit and vegetable juices are beverages of high nutritional value since they are enriched with minerals, vitamins and other beneficial components for human health that are generally indicated as antioxidants. Unfortunately during the industrial transformation, a large part of the characteristics determining the quality of the fresh product undergoes a remarkable modification: the thermal damage and the chemical oxidation degrade more sensitive components reducing the quality of the final product.

Fruit juices present on the market are generally constituted by two types of products: fresh juices, obtained by simple squeezing and then submitted to a mild pasteurisation; juices reconstituted from concentrate. It is known that the thermal treatment by pasteurisation and/or thermal concentration produces modifications of some components with consequent degeneration of taste and chemical characteristics. In particular, concentration reduces the storage volumes (so reducing transport and storage costs) and facilitates the preservation; on the other hand, when the concentration is carried out by evaporation, most of the aroma compounds contained in the raw juice are lost and the aroma profile undergoes an irreversible change with a consequent remarkable qualitative decline (Maccarone, Campisi, Cataldi Lupo, Fallico, & Nicolosi Asmundo, 1996). Alternative techniques to the evaporation as the cryoconcentration, in which water is removed as ice and not as vapor, are not able to substitute far the evaporative concentration of products with large diffusion, as for example the citrus juices, since they require a remarkable energy consumption. Besides the achievable concentration (about 40 g TSS/100 g) is lower than the values obtained by evaporation (60–65 g TSS/100 g).

Concentration processes avoiding high temperature are interesting approaches to preserve the nutritional and organoleptic characteristics of fruit juices. Membrane processes are today consolidated systems in various productive sectors for their capacity to operate at room temperature and with low energetic consumption. Particularly, ultrafiltration (UF) and microfiltration (MF) processes are a valid approach for the clarification of fruit juices (Todisco, Tallarico, & Drioli, 1998; Wu, Zall, & Tzeng, 1990).

UF membranes retain large species such as micro-organisms, lipids, proteins and colloids while small solutes as for example vitamins, salts, sugars, flow through the membrane together with water. Therefore the possibility of microbial contamination in the permeate stream is minimised avoiding any thermal treatment and, consequently, loss of volatile aroma substances (Tallarico, Todisco, & Drioli, 1998).

Clarified juice coming from the UF process can be commercialised or submitted to a concentration process in order to obtain a product suitable for the preparation of juices and beverages.

Reverse osmosis (RO), membrane distillation (MD) and osmotic distillation (OD) are frequently used as concentration techniques (Álvarez, Riera, Álvarez, & Coca, 1998; Barbe, Bartley, Jacobs, & Johnson, 1998; Calabrò, Jiao, & Drioli, 1994; Drioli et al., 1996; Gostoli, Bandini, Di Francesca, & Zardi, 1994; Vaillant et al., 2001). The RO process permits to separate principally water from the juice but it is limited by high osmotic pressures; for this reason it is used as a preconcentration technique which permits concentration values of about 30 g TSS/100 g corresponding to osmotic pressures of about 50 bar. Aroma compounds and other important chemical constituents such as anthocyanins, vitamins, sugars, acids, calcium, potassium, magnesium and phosphorus are rejected in the process.

The limitation of high osmotic pressures can be reversed by continuing juice concentration by MD or OD.

OD is a new membrane process also called “isothermal MD” that can be used to remove selectively water from aqueous solutions under atmospheric pressure and at room temperature, avoiding thermal degradation (Courel, Dornier, Herry, Rios, & Reynes, 2000; Hogan, Canning, Peterson, Johnson, & Michaels, 1998; Kunz, Benabiles, & Ben-Aı̈m, 1996). It involves the use of a microporous hydrophobic membrane to separate two circulating aqueous solutions at different solute concentrations: a dilute solution and an hypertonic salt solution. If the operating pressure is kept below the capillary penetration pressure of liquid into the pores, the membrane cannot be wetted by the solutions. The difference in solute concentrations, and consequently in water activity of both solutions, generates, at the vapour–liquid interface, a vapour pressure difference causing a vapour transfer from the dilute solution towards the stripping solution. The water transport through the membrane can be summarised in three steps: (1) evaporation of water at the dilute vapour-liquid interface; (2) diffusional or convective vapour transport through the membrane pore; (3) condensation of water vapor at the membrane/brine interface.

During the OD process the stripping solution is diluted due to the water transfer from the feed stream. It can be reconcentrated by evaporation and in this sense it can be recycled and reused in the process.

The objective of the present work was to identify an integrated membrane process for the production of concentrated citrus and carrot juices with high nutritional value. In particular lemon, orange and carrot juices were clarified by UF and then submitted to a concentration step by a RO–OD sequence until concentrations of soluble solids between 60 and 63 g TSS/100 g were reached.