Ultrafiltration to Increase the Consistency of Fruit Pulps: The Role of Permeate Flux
Abstract
:1. Introduction
2. Materials and Methods
2.1. Experimental Set-Up
2.2. Experimental Runs
- Rinsing with tap water for 30 min, to remove the pulp residues still present inside the set-up.
- Cleaning with NaOH solution (pH = 11) and NaClO (200 mg/L active chlorine) at 50 °C for one hour.
- Rinsing with water at 50 °C for one hour.
2.3. Analysis of Pulp and Permeate
2.3.1. Consistency as Bostwick Degree (Bo)
2.3.2. Total Acidity (TA)
2.3.3. Sugar Content as Brix Degree
3. Results and Discussion
3.1. Permeate Flux
- The kind of fruit strongly influences the permeate flux: pear pulp shows values of the permeate flux 60–100% higher than the values achieved with apricot pulp, even when the flow rate is just 3.9 m3/h, to say, lower than the value used for apricot pulp (5.1 m3/h). In terms of tangential velocity, these flow rates correspond to 2.1 m/s and 2.8 m/s, respectively. The data demonstrate that the kind of fruit influences the permeate flux more than the pulp flow rate, which is responsible for the tangential velocity. Satyannarayan and Kumar [17] found a similar influence by flow rate and kind of fruit. They processed lime and pineapple juice and evidenced that the permeate flux was different for the juices at the same flow rate. Moreover, they showed the need to reduce the pectin content to increase the permeate flux, demonstrating that different juice compositions influence the filtration performance, especially when colloids or macromolecules are present.
- For apple and apricot pulps, the results show a gap, too, with the permeate flux for pear pulp 10–30% higher than the flux achieved with apple pulp. The permeate flux for apple pulp shows a constant trend during the ultrafiltration time. These data are in line with the findings achieved by He et al. [21] with apple juice ultrafiltered at a tangential velocity equal to 2.5 m/s and MWCO equal to 50 kDa. These authors showed that raw apple juice did not evidence any permeate flux decline in the first minutes of ultrafiltration compared to the results obtained with pasteurized and pasteurized+hydrolyzed apple juices. The hypothesis was a very fast adsorption of colloids on the membrane that occurred when the apple juice was raw, and it was not appreciable by monitoring the permeate flux. A similar hypothesis was made by Bruijn and Borquez [22].
- The high values for pear pulp could be attributed to the cleaning action done on the membrane by the sclereids, the so-called “stone cells”. In membrane processes, the performance is heavily influenced by the material deposited on the membrane surface. For fruit pulps, this drawback is caused by fibers and pectins, more and more accumulated on the membrane and partially linked to it in a tight way, constituting one additional barrier to the membrane crossing to permeation (fouling). In this situation, the cleaning action is fundamental to keeping the membrane performance as constant as possible, notwithstanding the natural decrease in separation efficiency.
- The trend of the permeate flux of apricot pulp slowly decreases with time, whereas this does not occur for the other two pulps. One hypothesis is the fouling due to the deposition of pulp compounds on the membrane itself. At a local level on the membrane, jellying and/or strong bonds could occur, especially when the total acidity is high, like for apricot pulp.
3.2. Consistency
- Pear pulp gives the best results, whereas apricot pulp shows the lowest efficacy, whichever parameter is considered for the comparison.
- The flow rate influences the filtration efficacy, but not to a large extent, except for pear pulp.
- The temperature always plays a relevant role for all the pulps, being responsible for a decrease in fluid viscosity. Its influence is higher than the flow rate.
- For apricot pulp, the consistency increase is limited compared to the values shown by the other pulps. The limitation is more evident with the highest total acidity. As aforesaid, this could be caused by the jellying of pectins on the membrane. Pectins can form when the concentration of sugars and acids has particular values. If this happens locally on the membrane, the permeation occurs with enormous difficulties due to the additional resistance generated by them.
3.3. Taste
3.4. Influence of Working Time on Membrane Performance
3.4.1. Apricot
3.4.2. Pear
4. Conclusions
- For the tested pulps, the selection of MWCO equal to 200 kDalton was suitable to avoid changes to the composition of the final products compared to the raw ones.
- Permeate flux is always enhanced by high flow rates, due to the high cross-flow velocity. The operative limit is due to the growth of head losses and, therefore, the maximum pressure of the pumping system, which cannot be higher than the maximum value tolerated by the membranes (to avoid failures, a safety valve is placed on the discharge pipe). In this study, the best results were achieved with a cross-flow velocity in the order of 2.6 m/s.
- In addition, a temperature higher than the room temperature is beneficial due to the fluid viscosity decrease. However, this parameter must be chosen considering the organoleptic damage that it can cause to the pulp if its value grows too much and the maximum temperature acceptable for the membrane. A value around 50 °C is the maximum applicable value.
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Conflicts of Interest
References
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Apple Pulp | Apricot Pulp A | Apricot Pulp B | Pear Pulp | |
---|---|---|---|---|
Brix degree | 11 | 10.1 | 11.6 | 8.2 |
Total acidity (% citric acid monohydrated) | 0.31 | 1.27 | 1.93 | 0.38 |
pH | 3.7 | 3.3 | 3.1 | 3.8 |
Density (kg/m3) | 1060 | 1020 | 1020 | 1040 |
Apple Pulp | Apricot Pulp (*) | Pear Pulp | |
---|---|---|---|
Transmembrane pressure (Pa) | 2 × 105–3.3 × 105 | 1.4 × 105–1.8 × 105 | 1.4 × 105–2.8 × 105 |
Range of permeate flux (l/(h m2)) | 31.5–32.5 | 15–22 | 34–41 |
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Chiampo, F. Ultrafiltration to Increase the Consistency of Fruit Pulps: The Role of Permeate Flux. ChemEngineering 2024, 8, 3. https://doi.org/10.3390/chemengineering8010003
Chiampo F. Ultrafiltration to Increase the Consistency of Fruit Pulps: The Role of Permeate Flux. ChemEngineering. 2024; 8(1):3. https://doi.org/10.3390/chemengineering8010003
Chicago/Turabian StyleChiampo, Fulvia. 2024. "Ultrafiltration to Increase the Consistency of Fruit Pulps: The Role of Permeate Flux" ChemEngineering 8, no. 1: 3. https://doi.org/10.3390/chemengineering8010003