Clarification and concentration of pomegranate juice (Punica granatum L.) using membrane processes
Highlights
► The production of concentrated pomegranate juice was investigated by using an integrated membrane process. ► The fresh undepectinized juice was clarified by hollow-fiber ultrafiltration (UF) membranes. ► The concentration step of the clarified juice was performed by using an osmotic distillation (OD) apparatus. ► The performance of UF and OD operations was evaluated in terms of productivity and quality of the processed juice.
Introduction
Significant progresses have been made over the past 10 years towards a much more comprehensive understanding of some pharmacologic components of pomegranate fruit (Punica granatum). Clinical research studies suggest that some compounds of the pomegranate juice are characterized by anticarcinogenic, antimicrobial, antioxidant and antiviral activities (Malik et al., 2005, Noda et al., 2002, Braga et al., 2005, Vidal et al., 2003). Further, as reported in biological studies, pomegranate juice is enriched in anti-atherosclerotic and anti-atherogenic compounds which have been shown to reduce blood pressure and low density lipoprotein (LDL) oxidation (Aviram and Dornfeld, 2001). These activities have been attributed to its phenolic fraction containing a significantly high level of hydrolyzable tannins, as well as anthocyanins (delphinidin, cyanidin and pelargonidin 3-glucosides and 3,5-diglucosides), which exhibit high antioxidant activity (Lansky and Newmann, 2007, Visioli and Hagen, 2007). Besides, different studies have shown that pomegranate juice contains much more antioxidant compounds than other fruit juices and beverages (Gil et al., 2000).
Due to these characteristics and increasing public awareness about nutritional food, the demand for the pomegranate fruit and its by-products has increased significantly in the last years. Consequently, many industries producing pomegranate juice, as well as pharmaceutical companies extracting health beneficial compounds from the fruit, have been developed (Seeram et al., 2006).
Traditional methods of processing fruits limit the possibility to obtain products able to retain as much as possible the peculiarity of the fresh fruit and its high content in health-beneficial compounds. For instance, conventional juice clarification processes are based on the use of fining agents (gelatine, bentonite, silica sol, diatomaceous earth, etc.) which create serious problems of environmental impact due to their disposal (Vaillant et al., 1999). Moreover, the concentration of fruit juices, aiming at ensuring a longer storage life and an easier transportation, is performed by thermal evaporation methods resulting in color degradation and reduction of most thermally sensitive compounds, with a consequent remarkable qualitative decline (Jiao et al., 2004).
Product quality improvement and energy savings have guided the development of minimal processing techniques. Compared with traditional juice processing methods, membrane processes are low-cost and athermal separation techniques which involve no phase change or chemical agents. These features are becoming very important factors in the production of new fruit juices with natural fresh tastes and additive-free.
Ultrafiltration (UF) membranes are commonly used to clarify and stabilize fruit juices and commercial processes have already implemented on industrial scale for juices such as apple and orange (Girard and Fukumoto, 2000). They are able to retain large species such as microorganisms, lipids, proteins and colloids while small solutes such as vitamins, salts, and sugars flow together with water. Advantages of the UF over conventional fruit juice processing are in terms of: increased juice yield; possibility of operating in a single step; possibility of avoiding the use of gelatins, adsorbents and other filtration aids; reduction in enzyme utilization; easy cleaning and maintenance of the equipment; elimination of needs for pasteurization; better juice clarity; reduction of filtration times and waste products (Fukumoto et al., 1998).
Concentration processes of the clarified pomegranate juice by using thermal evaporation have already been proposed. In particular, Chen and Song (1994) developed a fresh-preserving concentrated pomegranate juice producing method adopting a low temperature vacuum process for the concentration of the filtered juice. Pang et al. (2008) patented a method in which the clarified juice was concentrated by thermal evaporation and then submitted to pasteurization before the aseptic packaging. The concentrated juice is characterized by a pH value between 2.8 and 3.7, turbidity from 0 to 25 NTU, a transmittance (T625) equal to or greater than 90% and a color ratio (A520nm/A430nm) equal to or greater than 1.0.
Membrane concentration processes, such as reverse osmosis (RO), membrane distillation (MD) and osmotic distillation (OD), present some attractive options to overcome limitations associated with vacuum evaporation. In particular, OD is a novel athermal membrane-separation process based on the use of a macroporous hydrophobic membrane separating two aqueous solutions having different solute concentration: a dilute solution on one side and a hypertonic salt solution (concentrated brine stripper) on the opposite side. The driving force of the process is given by a water vapor pressure gradient across the membrane generated by the difference in water activity between the two sides of the membrane. The hydrophobic nature of the membrane prevents penetration of the pores by aqueous solutions, creating air gaps within the membrane. The water vapor pressure gradient across the membrane determines a transfer of vapor across the pores from the high-vapor pressure phase to the low one (Hogan et al., 1998, Nagaraj et al., 2006). This migration of water vapor results in the concentration of the feed and dilution of the osmotic agent solution. Since the OD process is operated at room temperature and atmospheric pressure, products can be concentrated without mechanical and thermal damage. Therefore OD is an attractive process for the concentration of solutions containing thermo-sensitive compounds such as fruit juices and pharmaceuticals.
The concentration of clarified fruit juices by OD has been investigated by different authors (Cassano and Drioli, 2007, Cassano et al., 2007, Hongvaleerat et al., 2008, Rodrigues et al., 2004, Shaw et al., 2001, Kozák et al., 2006) and its potential for the better preservation of the quality of the raw material has been clearly confirmed.
The present study was undertaken to investigate the potentiality of an integrated membrane process in the clarification and concentration of pomegranate juice, as an alternative to conventional processing methods. In particular, the fresh juice, with an initial total soluble solids (TSS) content of 162 g kg−1, was preliminarily clarified by ultrafiltration (UF). Then the clarified juice was submitted to a concentration step, at room temperature, by using osmotic distillation (OD) up to a final TSS content of about 520 g kg−1. The performance of UF and OD processes was evaluated in terms of both productivity (permeate and evaporation fluxes) and quality of the processed juice.
Section snippets
Juice extraction
Pomegranates of Sicilian origin (Punica granatum L. cv Selinunte) were supplied by Citrech Snc (Messina, Italy). Fruits were washed in cold tap water and drained. They were manually cut-up and the outer leathery skin which encloses hundreds of fleshy arils was removed. Arils were pressed by using an electric tomato crusher: the extracted juice, having a deep-red color, was prefiltered with a stainless steel filter. Then it was stored in a refrigerator cell at −17 °C and defrosted before use.
UF equipment and procedures
The
Pomegranate juice clarification
The evolution of the permeate flux during the UF of pomegranate juice in the selected operating conditions is illustrated in Fig. 1. The initial permeate flux of about 29 L m−2 h−1 decreased gradually with operating times due to concentration polarization and fouling phenomena to reach a steady-state value of about 20 L m−2 h−1.
After the cleaning with water the hydraulic permeability of the membrane was 97.7 L m−2 h−1 bar−1, about 85% lower than the initial value (655.6 L m−2 h−1 bar−1). The cleaning with a
Conclusions
The integrated membrane process involving the clarification of the juice by UF and the concentration of the clarified juice by OD was observed to be an attractive process for the production of a concentrated juice with high antioxidant activity which can be employed for the formulation of pharmaceuticals, cosmetics and food products (flavoring for beverages, juice blendings, alcoholic beverages, food colorants, sweets, baked products, etc.).
Clarification by UF essentially enables the removal of
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