Anatomía vertebral en cocodrilos

– NFC orange juice is processed and pasteurised by flash heating immediately after squeezing the fruit, without removing the water content from the juice. It can be stored freezed or chilled for at least a year.

– RECON is a juice that has been processed to obtain the frozen concentrate and then reconstituted by adding back the water that had been originally removed.

Reconstituted single-strength juice is normally reconditioned by the packager and sold as a ready-to-serve product, either chilled or in an aseptic form without the need of refrigeration.

Traditional technologies involved in the production of all these juices are characte-rized by some drawbacks concerning the quality of the product, the energetic con-sumption and the environmental impact. It is known that the thermal treatment by pasteurization and/or thermal concentration produces a severe loss of the volatile organic flavor/fragrance components as well as a partial degradation of ascorbic acid and natural antioxidants, accompanied by a certain discoloration and a consequent qualitative decline [ 7 , 8 ].

Membrane technologies, as “ mild technologies ” , represent very efficient systems to preserve the nutritional and organoleptic properties of citrus fresh products because of the possibility of operating at room temperature with low energy consumption. In addition, they offer interesting alternatives to the traditional techniques for the reco-very of high added value compounds from citrus by-products. A full exploitation of the potential of these techniques may be achieved through the integration of different processes.

This chapter will provide an overview of membrane operations that can subs-titute traditional operations in the clarification, concentration and aroma recovery of citrus juices as well as in the recovery of bioactive compounds from by-products of citrus juice production. Integrated membrane processes, which can contribute to redesign the traditional industrial transformation of citrus fruits within the PI strat-egy, are also presented and discussed.

4.2   Clarification of citrus juices

The traditional methods of citrus juice clarification are based on the use of different technologies, including centrifugation, depectinisation, fining agents (such as bento-nite and gelatin) and filtration by diatomaceous earth. The use of microfiltration (MF) and ultrafiltration (UF) membranes presents many advantages over conventional clarification, including the possibility to operate at room temperature (thus avoiding pasteurization and sterilization), increased juice yields, reduced labor and capital costs, elimination of filter aids, reduction of waste products, easy maintenance of the equipment [ 9 ].

Permeate fluxes and permeate quality are the most important aspects for the selection of a proper membrane. The build-up of macromolecular or colloidal species at the upstream interface of the membrane such as pectins, proteins, tannins and fibers determines a rapid permeate flux decay followed by a long and gradual decline towards a steady-state limit value. This phenomenon is known as concentration pola-rization. Fouling mechanisms, such as adsorption of particles on the membrane pore walls and pore plugging, are additional phenomena.

Pretreatment methods can reduce the particulate matter in the juice, leading to a remarkable improvement of permeate fluxes and the attainment of higher concen-tration factors.

Rai et al. [ 10 , 11 ] studied the effect of seven different pretreatment methods on the performance of a 50 kDa thin-film composite polyamide membrane in the clarifica-tion of mosambi juice including centrifugaclarifica-tion, fining by gelatin, fining by bentonite, fining by bentonite followed by gelatin, enzymatic treatment with pectinase, enzyma-tic treatment followed by centrifugation and enzymaenzyma-tic treatment followed by fining with bentonite. Among the various pretreatment methods the enzymatic treatment followed by bentonite was found the best to maximize the permeate flux.

Permeate fluxes in MF and UF processes depend strongly on operating and fluid dynamic conditions and on the nature of the membrane and feed solutions. Cassano et  al. [ 12 ] evaluated the effect of operating parameters on membrane fouling and juice quality in the clarification of depectinised blood orange juice by using a poly-vinylidene fluoride (PVDF) UF tubular membrane module with a NMWCO of 15 kDa (Koch Membrane Systems Inc., Wilmington, MA, USA). Permeate fluxes increased with transmembrane pressure (TMP) up to a limiting value (TMP _lim ) depending on the physical properties of the juice and axial velocity. An increase in the feed flow rate produced a linear increase of the permeate flux caused by the effect of the shear stress at the membrane surface, which enhanced the rate of removal of deposited particles reducing the polarized layer thickness. The increase in the operating tem-perature produced a reduction of juice viscosity, together with an increase of the diffusion coefficient of macromolecules with a consequent enhancing of the perme-ate flux. In optimized operating conditions (TMP 0.85 bar, feed flow rperme-ate 800 l/h and temperature 25 ° C) the initial permeate flux of 19 l/m ² h decreased at a steady-state value of about 11 l/m ² h when the VRF reached a value of 3 and remained constant up to the final VRF value of 6. In the fixed operating conditions of TMP and tempe-rature, the fouling mechanism evolved from a partial to a complete pore blocking condition in dependence of the axial feed velocity. Ascorbic acid, anthocyanins, narirutin and hesperidin, which contribute to the TAA of the juice were recovered in the clarified juice, while suspended solids were completely removed by the UF membrane. Thus, the flux decline during the UF process was attributed to the for-mation of fouling layers through a combination of suspended particles and adsor-bed macromolecular impurities. A mass balance of the UF process is depicted in Figure 4.2 .

4.2 Clarification of citrus juices   91

An improvement of color and clarity of mandarin juice through the removal of suspended solids was also achieved by using modified poly(ether ether ketone) (PEEKWC) and polysulfone (PS) hollow fiber (HF) membranes prepared through the phase inversion process [ 13 ]. PEEKWC membranes showed a lower rejection towards TSSs, total phenolics and TAA in comparison with the PS membranes in agreement with the lower rejection observed for PEEKWC membranes towards dextrans with spe-cific molecular weight.

The analysis of membrane fouling in the clarification of orange juice with PVDF MF membranes (Tri-Cor 2-MFK, Koch Membrane Systems Inc.) with a pore size of 0.3  μm revealed that the separation process is controlled by cake filtration mecha-nisms at relatively low velocity (i.e., Reynolds number  = 5000) and low TMPs. At higher Reynolds numbers an increase of applied TMP allows an increase in the limit permeate flux by a factor of about 4. In these conditions the filtration process is cont-rolled by a complete pore blocking mechanism and flux decay is negligible [ 14 ].

The use of electric fields in UF to reduce fouling and concentration polarization has been also investigated in the treatment of citrus juices. Pectin is negatively charged and its charge density depends on pH and degree of methoxylation. Therefore, the

Suspended solids 933 g Soluble solids 211.95 g Ascorbic acid 1.00 g Anthocyanins 95.0 mg

Narirutin 73.41 mg

Hesperidin 48.88 mg

Suspended solids 0 g Soluble solids 869.12 g Ascorbic acid 4.98 g Anthocyanins 424.0 mg

Narirutin 363.09 mg

Hesperidin 261.27 mg Suspended solids 933 g

Soluble solids 1119.6 g Ascorbic acid 6.54 g Anthocyanins 564.0 mg

Narirutin 436.08 mg

Hesperidin 310.96 mg Depectinized

orange juice (9.33 I)

Clarified juice (7.76 I) Retentate (1.57 I)

UF

Figure 4.2: Mass balance of the UF process in the clarification of blood orange juice with tubular PVDF membrane

application of an external field with appropriate polarity can reduce the pectinous gel layer thickness due to the electrophoretic migration of pectin molecules away from the membrane surface. Sarkar et  al. [ 15 ] evaluated the effect of the electric field, applied in both constant and pulse modes, on the permeate flux in the UF of mosambi [ Citrus sinensis (L.) Osbeck] juice with a 50 kDa polyethersulfone (PES) membrane. The application of the electric field resulted in a significant improve-ment of the permeate flux, achieving a 22% reduction of total energy consumption per unit volume of permeate. In addition, PEF was more advantageous in terms of permeate flux improvement and energy consumption if compared with constant electric fields.

MF and UF membranes allow a complete removal of pulp and water soluble pectins from orange juice. In particular, the use of HF PS membranes with a MWCO of 50 kDa permits a complete separation of suspended solids from freshly squeezed orange juice: most of pectin materials are retained by membranes and the viscosity of the juice is appreciably reduced [ 16 ]. Oxygenated aroma compounds, such as alco-hols, esters and aldehydes, flow freely through the membrane while less polar aroma compounds like limonene and valencene tend to be associated with the retained pulp.

Todisco et al. [ 17 ] found that more water soluble compounds, such as aldehydes, esters and alcohols, pass through PVDF UF membranes while hydrocarbons and less polar aroma compounds like limonene are retained with the pulp fraction. Conversely, some esters such as methyl acetate, ethyl acetate, ethyl butyrate and methyl butyrate, which contribute to the “ top-note ” of citrus flavors are recovered in the clarified juice.

Consequently, even if UF membranes remove some volatile aroma compounds, the greatest contributors of orange flavors can be preserved, allowing the production of orange juice with an improved appearance.

The clarification of lemon juice was investigated using a 0.2 μm MF membrane in a flat-sheet configuration [ 18 ]. The clarified juice presented titrable acidity, pH and TSS values comparable with those of untreated fresh lemon juice. Optimal performan-ces were obtained at a TMP of 0.6 bar and a feed flow rate of 1 m/s.