Red fruit juices

Red fruits are among the most important dietary source of polyphenols, such as anthocianins, flavonols, flavan-3-ols, benzoic and hydroxicinnamic acid derivates.

Numerous in vitro studies have reported their high antiradical activity and capacity to inhibit the human low-density lipoprotein and liposome oxidation. Biochemical and pharmacological activities have been attributed to free radical scavenging, effects on immune and inflammatory cell functions, anti-carcinogenic and antitumor proper-ties [ 78 ]. Within the group of red fruits, pomegranate ( Punica granatum ), redcurrant ( Ribes rubrum L .), blood orange ( Citrus sinensis L. ), blackcurrant ( Ribes nigrum L. ) and cherry ( Prunus avium L. ) juices are among the richest in anthocyanins, which are responsible of the bright red color and the strong antioxidant capacity, gaining huge interest as ingredients in the design of functional juices. In order to better preserve the properties of red fresh fruits, several new “ mild ” technological processes have been proposed in the last years.

A multi-step membrane process on a laboratory- and large-scale was proposed by Kozak et al. [ 79 , 80 ] for the treatment of blackcurrant juice. The integrated system consis-ted of a MF step to clarify the juice, a RO unit to pre-concentrate the juice up to 26 ° Brix and a final OD process to concentrate the juice up to 63 – 72 ° Brix. Experiments were per-formed, at first, at a laboratory scale to determine the optimal operating parameters.

The large-scale measurements were carried out on the basis of the laboratory results.

In large-scale experiments the depectinised juice was prefiltered through a 100 μ m bag filter and then preconcentrated through a RO flat-sheet membrane module (MFT-K ö ln) at an operating pressure of 51 bar and a temperature of 24 ° C. The concentration was carried out by using a PP hollow fiber membrane module (MD 150 CS 2N, Micro-dyn) with an average pore size of 0.2 μ m. In the concentrated juice the anthocyanin content was three times higher than the raw juice. The sensory analysis showed a little loss of aroma compounds in the reconstituted juice when compared to the raw juice, while the color intensity and the acidic flavor intensity remained unchanged.

An interesting process design based on the use of integrated membrane systems for the blackcurrant juice concentration was proposed by Sotoft et al. [ 81 ] to replace traditional multiple step evaporators and aroma recovery. The processes consisted of a VMD unit for aroma recovery and water removal by a combination of NF, RO and DCMD. In particular, a preliminary concentration of the raw juice up to 45 ° Brix is

3.4 Integrated membrane operations in fruit juices production   75

obtained using a combined NF/RO process: combining these two techniques it was possible to utilize the high rejection of RO membranes and the high concentration factor of NF membranes in order to overcome the high osmotic pressure limitations typically encountered in RO. The raw juice was first treated by RO with a dense mem-brane that had a high degree of sugar retention (99.7%). The microbial quality of the water in the permeate stream was very high, suggesting a potential reuse as a source of drinking water or process water in food production. The RO retentate was processed by NF. The NF permeate was recirculated back to the RO unit, while the retentate stream was submitted to the final concentration DCMD step, producing a concentrated juice with a TSS content of 65 – 70 ° Brix ( Figure 3.4 ). The production of the proposed system was fixed at 17,283 ton of 66 ° Brix concentrated juice/year with a production price of 0.40 € /kg assuming a membrane lifetime of 1 year. The estimated operation cost is lower than the price of a traditional process by about 43%: therefore the economical potential of the process is very promising in order to replace conven-tional evaporators.

Galaverna et al. [ 82 ] investigated two membrane-based configurations for the pro-duction of highly concentrated blood orange juices. The process included an initial clarification of the freshly squeezed juice by UF, in order to separate the liquid serum from the pulp. The clarified juice was successively concentrated using two consecu-tive processes: (i) first RO, as a preconcentration step up to 25 – 30 ° Brix, (ii) then OD to obtain a final concentration of 60 ° Brix. Alternatively, the clarified juice was directly concentrated by OD up to 60 ° Brix. The proposed system was very efficient in preser-ving the TAA of the juice, even at high concentrations (60 ° Brix). Among the different antioxidant components a slight decrease in the OD retentate was observed for ascor-bic acid (15%) and anthocyanins (23%), whereas flavanones and hydroxycinnamic acids were very stable. The final TAA value obtained in both configurations was not significantly different from that observed in the traditional thermal treatment. The concentrated juice retained its bright red color and its pleasant aroma, which was, on the contrary, completely lost during thermal concentration.

A similar UF/OD integrated membrane process was proposed and investigated for the clarification and concentration of pomegranate juice [ 83 ]. The raw juice, with an initial TSS content of 16.2 ° Brix, was clarified by hollow fiber UF membranes and then concentrated by using a Liqui-Cell Extra-Flow 2.5 × 8 ″ membrane contactor (Memb-rana, Charlotte, NC, USA) up to a TSS value of 52 ° Brix. The analytical measurement performed on clarified and concentrated samples showed that sugars, organic acids (malic, ascorbic and citric acids), polyphenols and anthocyanins were well-preserved during the process independently on the TSS content ( Table 3.1 ). The evaluation of the TAA in the OD samples confirmed the validity of the proposed process in preserving juice bioactive compounds. In particular, the concentrated juice at 52 ° Brix showed only a 10% reduction of the TAA when compared with the clarified juice. The integ-rated process for the clarification and concentration of pomegranate juice is depicted in Figure 3.5 .

1 2 3 4

RO NF

VMD DCMD

Distillation

RO permeate

VMD retentate

Filtered macerated juice CondenserPermeate VacuumAroma concentrate Permeate inPermeate out

Concentrated juice Figure 3.4: Integrated membrane process for aroma recovery and blackcurrant juice concentration. (1) aroma recovery by vacuum membrane distillation (VMD) followed by distillation; (2) preconcentration by reverse osmosis (RO); (3) preconcentration by nanofiltration (NF); (4) final concentration by direct contact membrane distillation (DCMD) (adapted from Sotoft et al. [ 81 ])

3.4 Integrated membrane operations in fruit juices production   77

Table 3.1: General composition of pomegranate juice clarified and concentrated by integrated membrane process

Fresh juice Permeate UF Retentate UF Retentate OD

Suspended solids, %(w/w) 4.8 0 5.3 –

pH 3.75 3.78 3.74 –

Total acidity (g/l) 0.41 0.35 0.44 –

Total soluble solids ( ° Brix) 16.2 16.2 – 52.0 Total antioxidant activity (mM

Trolox)

12.9 10.6 14.1 10.1*

Ascorbic acid (mg/l) 68.0 47.0 71.0 44.0*

Malic acid (g/l) 1.90 1.82 2.01 1.80*

Citric acid, (g/l) 1.47 1.45 1.24 1.26*

Total polyphenols (g catechin/l)

1.57 1.31 1.70 1.22*

Total anthocyanins (mg/l) 102.8 90.7 100.6 75.85*

UF, ultrafi ltration; OD, osmotic distillation *value referred to a TSS content of 16.2 ° Brix.

Pomegranate fruits

Washing

Peeling

Arils

Squeezing

Seeds

Pressing Washing

Incubation with pectinase (1%, room temperature, 4 h)

Osmotic distillation Sieving (200 mm) Arils juice

Ultrafiltration

Concentrated juice

Retentate Permeate

Figure 3.5: Integrated membrane process for the production of pomegranate juice concentrate

The possibility to concentrate different red fruits juices, by using a coupled operation of MD and OD, referred to as “ membrane osmotic distillation ” (MOD), was evaluated by Koroknai et  al. [ 84 ]. In the proposed system, three different red fruits juices (chokeberry, redcurrant and cherry), were firstly clarified by UF and then con-centrated by MOD. The clarification step improved the efficiency of the MOD process, providing a less viscous feed stream with significantly lower fouling behavior during the concentration, at the same time excluding the possibility of microbiological con-tamination in the further concentration process. During the concentration process, the integration of OD and MD processes permitted an increase of the driving force, which resulted in enhanced water flux during the operation. The resulting process was more effective than MD or OD alone [ 85 ]. The obtained evaporation fluxes were in the range of 4.51 – 5 kg/m ² h for all the investigated juices. An excellent preservation of the valuable antioxidant capacity ( > 97%) was observed, indicating the validity of the process in preserving the original nutritional value of the fresh fruits.