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ISSN: 2157-7110
Journal of Food Processing & Technology

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Concentration of Apple Juice Using Spinning Disc Reactor Technology

School of Food Science and Nutrition, University of Leeds, Leeds LS2 9JT, UK
Corresponding Author : Dr. Mahmood Akhtar
School of Food Science and Nutrition
University of Leeds
Leeds LS2 9JT, UK
Tel:+44 (0) 113 3432952
Fax: +44 (0)1133432982
E-mail: [email protected]
Received January 20, 2011; Accepted March 25, 2011; Published March 27, 2011
Citation: Akhtar M, Chan P, Safriani N, Murray B, Clayton G (2011) Concentration of Apple Juice Using Spinning Disc Reactor Technology. J Food Process Technol 2:108. doi:10.4172/2157-7110.1000108
Copyright: © 2011 Akhtar M, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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Abstract

The use of spinning disc reactor (SDR) technology for the concentration of apple juice was investigated. The apple juice was passed over the SDR disc spinning at 2000rpm, heated at 90-120°C, and at a flow rate of 7mL s -1 . Experimental results showed that the SDR has no significant detrimental effects on the physicochemical properties and quality of the apple juice after the concentration process, despite using high processing temperatures (90- 120°C). Due to the short residence time of the SDR, the thermally-induced colour change of the concentrates has been minimized. All apple juice concentrate samples exhibited narrow particle size distributions with the average particle size in the range 0.1 to 12μm. The SDR-made reconstituted apple juices are comparable to both the original pure-pressed apple juice sample and the commercial reconstituted product. Volatile components (ester, aldehyde and alcohol) collected during the concentration process were analyzed with GC-MS, and the analysis suggests the formation of new aroma compounds (ethyl acetate, n-butyl alcohol and 2-hexenal) which can be added back to the reconstituted apple juice to enhance its sensory quality.

Keywords
Concentration; Spinning disc reactor; Apple juice concentrate; Physicochemical properties; GC-MS
Introduction
Fruit juices are often concentrated to reduce their weight and volume, and thus to reduce their packaging, transportation and storage costs. Concentrated juices are biochemically stable and have a long shelf life due to the reduction in the water activity [1]. However, conventional thermal concentration techniques are known to cause losses of flavour and aroma compounds, and have consequent undesirable effects on the flavour characteristics of the juice products [2].
The aroma components of apple juice usually comprise a mixture of volatile organic compounds, principally esters, aldehydes and alcohols, but also ethers, fatty acids, lactones, terpenes and ketones [3]. In general, the concentration of individual aroma components in the apple juice ranges from less than 1 to 20ppm [4,5]. Each of these compounds gives a typical character to the apple juice flavour. During conventional thermal concentration processing, many of these volatile compounds present in the apple juice are transformed (due to high temperature) or lost (via water vaporization). The detrimental effects were also reported in the manufacturing process of other fruit juice such as orange juice [6,7] and pineapple juice [8]. These undesirable effects can be significant, Peredi [9] reported more than half of the tested volatile compounds were lost compared to the unprocessed apple juice and Su and Wiley [10] demonstrated that almost all volatile compounds in apple juice are lost during the thermal processing. To generate a flavourful product, these aroma compounds must be recovered and added back to the concentrate [11]. Therefore, it is important to select the suitable technology for food processing, especially for heat sensitive compounds, to produce high nutritive value and appreciable organoleptic quality of the products.
In light of maximizing the efficiency of the concentration process while maintaining the nutritional and sensory qualities of the final product, the spinning disc reactor (SDR) could be an alternative technology in the production of fruit juice concentrates. The SDR has been widely used in emulsification, chemical and pharmaceutical industries [12-15]. Recently, we have used the SDR technology in pasteurizing carrot juice [12]. The results showed that the pasteurization process is more controllable than the conventional technologies. Not only was the processing time much reduced, but the quality of the juice was also retained. No significant colour change was observed in the SDR-processed juice compared to the fresh juice sample.
The SDR (type P201) used in this study (Figure 1) is comprised of a 20 cm diameter disc with a heating and cooling facility in the range of + 250°C to - 20°C by using heat transfer fluid (Paratherm OR) in a recirculation bath. The spinning disc has a speed range of 100 to 3000rpm with sample feed rates in the range 0.5 to 7mL s-1. The main vessel has been mechanically designed to withstand pressures up to 5bar. A selection of two standard pumps can be incorporated to the main controller, depending on the viscosity of the feed material or the liquid sample to be used. The SDR technology works on the principle that the feed liquid passes across the surface of a metal disc which can be programmed to spin at controlled speeds, subjected to heating and cooling at according to the product requirements. The centrifugal force causes the liquid to form a very thin (typically several µm in thickness) film on the disc, which gives very high heat transfer coefficients between the disc and the liquid, as well as very high mass transfer between the liquid and the gas phase above the liquid film. The residence time on the disc is short, typically less than a second, so that rapid pasteurization, followed by rapid cooling on the walls beyond the disc edge, can be achieved. An inert carrier gas can also be introduced to the system during the concentration process to facilitate evaporation (Figure 1). In addition, the SDR method can be applied as a continuous process on industrial scale.
In order to explore the potential of this new technology for the food and drink industry, the aim of this work was to employ SDR for the production of apple juice concentrate and to evaluate the efficiency of the concentration process. We also investigated the effect of processing temperature on the physicochemical properties of the apple juice concentrates, and characterized the SDR-made reconstituted apple juice compared to a pure-pressed sample and a commercial product. The objective of this study was to achieve a cost and energy efficient concentration process without compromising the quality of the product.
Materials and Methods
Apple juice
A 100% pure-pressed (not reconstituted) apple juice, purchased at a local supermarket (Sainsbury's Supermarkets Ltd., UK.), containing a measured value of 10.5°Brix, 2.07% acidity, pH 4.0 and a trace amount of added ascorbic acid, was used for this study. The sample had already been pasteurized by the manufacturer and was stored in a refrigerator at 5°C prior to testing.
Concentration of apple juice
A 500mL sample of apple juice was fed into the SDR at a flow rate of 7mL s-1, and passed continuously over the SDR disc rotating at 2000 rpm, set at 90°C - 120°C. A counter current nitrogen carrier gas (99.99% purity; 0.1psi) was applied to the system in order to facilitate water evaporation from the liquid sample, reducing the processing time. Throughout the concentration process, samples of water vapour containing the volatile aroma compounds of the apple juice were collected from the SDR unit with deionised water for gas chromatography-mass spectrometry (GC-MS) analysis. Apple juice concentrates, labelled as concentrate I and concentrate II, were collected at times when the volume of the apple juice sample had reduced to 350mL and 150mL (i.e., to 30% and 70% of the original volume), respectively. The times required to achieve such volume reduction were around 5 to 15 and 9 to 25min, respectively, depending on the processing temperature used. All samples were stored in airtight antiseptic containers at 5°C before analyses.
Reconstituted apple juice
Reconstituted apple juice was prepared by adding 350mL distilled water to the concentrate II to make up to 500mL, i.e., same volume as the unprocessed apple juice sample. The physicochemical properties of the reconstituted juice were then compared to another commercially manufactured apple juice (made from concentrate), containing a measured value of 10.6ºBrix, 3.02% acidity and a pH of 3.80.
Physicochemical properties
A digital pH meter (Hanna Instruments Ltd., Bedfordshire, England and UK.) was used to measure the pH of the apple juice samples at 5°C. The acid content of the juices was determined using an automatic titrator, Titration Excellence T50 (Mettler-Toledo AG, Schwerzenbach, Switzerland), equipped with a DGi111-SC titration sensor. An aliquot (40mL) of the sample was titrated against a 0.1M NaOH to an endpoint of 8.1. The soluble solids, expressed in ºBrix, were measured in a digital refractometer (Bellingham+Stanley Ltd., Kent, England, UK.) with 0.1 resolution. Each physicochemical parameter was measured in triplicate.
Rheological measurement
Steady-state viscosity of apple juices was determined using a Bohlin C-VOR rheometer (Malvern Instruments Ltd., Worcestershire, England, UK.), with a C25 cup and bob geometry. The sample was poured into the rheometer cell, which is surrounded by a temperature controlled vessel, and allowed to equilibrate at 5°C for 10 min prior to the measurement. Apparent viscosity was measured at shear-rates in the range 0.1-200 s-1 using continuous shear, with a 30s delay time and a 30s integration time at each shear rate. Duplicate measurements were made to check the reproducibility.
Particle size analysis
Particle size distributions of apple juice concentrates were measured using a Malvern Mastersizer MS2000 laser light-scattering analyser (Malvern Instruments Ltd., Worcestershire, England, UK.) with absorption parameter value of 0.01 and refractive index ratio of 1.53. The particle size measurements are reported as the volumeweighted mean diameter (d43 = Σi ni di4 / Σi ni di3), where ni is the number of particles of diameter di.. The d43 value was used to monitor changes in the particle-size distribution of the juice concentrates at 5°C.
Gas chromatography-mass spectrometry analysis
The aroma profile of the apple juice samples was investigated by headspace solid-phase micro-extraction (HS-SPME) combined with gas chromatography-mass spectrometry (GC-MS). Samples of 5g were weighed into 18mL SPME vials and placed onto a CTC CombiPal auto-sampler (CTC Analytics AG, Zwingen, Switzerland), attached to a Varian C3800 gas chromatograph coupled to a Saturn 2000 mass spectrometer (Varian Ltd., Palo Alto, CA). HS-SPME of the preheated samples (40°C for 15 min) was conducted under agitation for 2min using a 65µm PDMS/DVB fibre (Supelco, Bellefonte, PA). The SPME fibres were then introduced into the injector port of the GC for 10min at 250C, where the volatiles extracted by the fibres were desorbed thermally and introduced into the capillary column. The GC was performed on a DB-WAXetr capillary column (60m, 0.25mm i.d., 0.25µm film thickness), which was programmed to have an initial oven temperature set at 40°C, held for 5min, and increased to 200°C at 8°C/ min and finally to 250°C at 10°C/min, with a constant helium flow of 1.0mL/min. Blank runs were conducted between each sample analysis to eliminate any remaining sample in the system.
Analysis was carried out in duplicate and selected ions were used for quantification of the individual components. Aroma compounds were identified using the US National Institute of Standards and Technology (NIST) 08 library of mass spectra and gas chromatographic retention indicates reported of standard compounds.
Results and Discussion
Physicochemical characterization
The physicochemical characteristics including the pH, acidity (expressed as malic acid), and the soluble solids (°Brix) of the pure-pressed apple juice, the SDR-made concentrate, SDR-made reconstituted juice and the commercial reconstituted sample are shown in Table 1. The results indicate that the pH of the apple juice decreases in the juice concentrates due to an increase in the relative amount of acid in samples. It was also found that the decrease in the pH of the concentrates is dependent on the processing temperature, which could be explained as a result of evaporative effect during processing. Meanwhile, there is no significant difference in the pH between the SDR-made reconstituted juices and the pure-pressed sample (original). All measured pH values presented in this study have a recorded standard deviation of ± 0.07.
The tartness of pure fruit juices is largely due to organic acids: in apple juice malic acid is the principal acid. The results show that the acid content of the SDR-made apple juice increases with increased concentration of the juice. The slight increase of the acidity with the processing temperature may be caused by the release of the bound forms of acid in the juice. Organic acids are normally present in fruits in free and bound forms. The higher the processing temperature, the higher amount of bound acid is released. However, due to the short residence time of the SDR in the concentration process (0.4s for single pass over the disc), this thermal effect on the acidity change of the sample is not significant. Regarding the reconstituted apple juices, the acidity values were similar to those of a pure-pressed sample. The relatively high acidity of the commercial reconstituted juice is likely due to the presence of the ascorbic acid added by the manufacturers, although the exact amount added is not known. In general, despite a small variation, there was a correlation between the higher acidity and the lower pH values amongst the SDR-made apple juice concentrates.
Soluble solids, sometimes called total soluble solids (TSS), in fruit juice comprise not only the sugars but also include many other soluble substances such as salts, acids and tannins. However, sugars (mostly sucrose) are the principal solids constituents and are usually measured for all practical purposes. The soluble solids, expressed in degrees Brix (ºBrix), affect the physical properties such as density, viscosity, and boiling point elevation, of the juice products. Similar to the acidity results, it was found that the °Brix value of the concentrates increased with concentration. The increased °Brix value with the processing temperature could be attributed to the evaporative effect [16]. In terms of the reconstituted apple juice, there was slight decrease in the soluble solids compared to the pure-pressed apple juice but the change was not significant.
Particle-size distribution
The particle-size distribution is an important factor determining the stability of a juice concentrate. Particles in a cloudy juice can adhere together and form aggregates of increasing size (flocculation) which may settle because of gravity. Flocs might undergo coagulation and produce a much denser form, which is an irreversible process. The particle-size distributions of the apple juice samples are shown in Figure 2.
Both the pure-pressed apple juice and the SDR-made apple juice concentrates (concentrate II) show fairly narrow distributions with the average particle size d43 in the range 4 to 12µm. Thus, the concentration process using the SDR does not have detrimental effect on the particle size of the product.
Rheological properties
Figures 3a and 3b show that all the apple juice samples exhibited shear-thinning behaviour over the entire range of shear rate tested (0.1 - 200s-1). The results are consistent with the general observations reported in the literature [17,18]. Shear-thinning behaviour is a common type of flow behaviour in food systems due to a breakdown of structure under the influence of the shear forces [19,20]. The viscosity profiles of the pure-pressed apple juice and the commercial sample are very similar. The viscosity of the concentrates increases with the concentration of the apple juice as expected. It was observed that the apple juice concentrates II (70% volume reduction) have a higher viscosity than the apple juice concentrates I (30% volume reduction) due to the higher sugar content of the latter. The results also showed that the viscosity increases slightly with the processing temperature, possibly due to evaporative effect. Generally, there was no significant dependence of the viscosity profile on the processing temperature in agreement with no significant effects of processing temperature on the particle size distributions (Figure 2).
Figure 3c presents the viscosity profiles of the reconstituted apple juice made from concentrate II processed at different temperatures. The viscosities of the pure-pressed apple juice and the commercial reconstituted juice sample are also presented as references. The results show that the viscosity curves of all these samples tended to be the same. Thus, the SDR can also produce concentrates that do not significantly alter the rheological properties of the reconstituted products.
Aroma component analysis with gas chromatography-mass spectrometry (GC-MS)
Aroma in fruit juice comprises a mixture of hundreds of different organic compounds present at very low amounts, ranging from ppm (ormL/L) to ppb (or µg/L) levels. However, these aroma compounds are highly volatile, and traditional thermal concentration methods using either plate or tubular heat exchange units cause massive losses of the aroma compounds to the vapour phase.
Figure 4 shows GC-MS chromatograms of samples recovered from the SDR and the pure-pressed apple juice. Compounds of the ester, aldehyde and alcohol groups that contribute to the fruity flavour of the apple juice were identified in both the volatile component and the concentrate (Table 2). It is noteworthy that these recovered volatile aroma components can be added back to the reconstituted juice or to enhance the sensory quality of other food products [3,4]. Along with the identified aroma components, new peaks were also found in both the GC chromatograms of the volatile and the concentrate, indicating the possible development of new aroma compounds via the SDR processing. Although these compounds have not yet been identified and are awaiting further investigation, there are reports in the literature verifying the production of new aromas or the changes in flavours of foods and beverages with novel heat treatments/thermal processing [10,21-24].
Visual observations
Colour is a major factor determining the acceptability of processed juice products. Concentration using thermal treatment can induce darkening, which affects the quality of product that leads to consumer dissatisfaction. Due to the increase in solid concentration and the reduction in water activity, non-enzymatic browning reactions (e.g. Maillard reaction) and pigment destruction have been found to be major causes of such problems in apple juice [25,26]. These chemical changes are dependent upon the processing time and temperature as well as the storage time [8,27].
The advantage of using the SDR for the concentration of apple juice is the efficient heat and mass transfer of the system, allowing the concentration process to be achieved with a much shorter residence time, reducing the detrimental thermal effects on the colour of the final product. Visual observations of the SDR-made apple juice concentrates are illustrated in Figure 5. The more concentrated samples have a longer residence time on the SDR disc, so the colour of the apple juice concentrates II appears to be slightly darker than that of the apple juice concentrates I. This colour change could also be related to the low pH and high malic acid content which is associated to the kinetics of Maillard reaction [28]. Overall, however, all the samples have a very similar colour compared to the pure-pressed apple juice, which once again verifies that the SDR can be used for the concentration of apple juice without compromising the appeal of the product.
Conclusion
The spinning disc reactor (SDR) technology has been used for the concentration of apple juice, and the physicochemical properties of the concentrates were investigated. It can be concluded that this novel SDR technology is capable of producing apple juice concentrates efficiently without compromising the quality of the juice products. Compared to traditional concentration methods using thermal treatment, the SDR allows more efficient heat and mass transfer that facilitates the concentration process. In addition, the short residence time of the juice sample on the SDR disc minimises the heat damage to the product. The experimental results indicate that the processing temperature has no significant detrimental effects on the physicochemical properties and the quality of the apple juice concentrates. The SDR-made reconstituted apple juices are comparable to both the original pure-pressed apple juice sample and the commercial reconstituted product. The new peaks found in the GC chromatograms of both the volatile component and the concentrate suggest the development of new aroma compounds during the concentration process, which can be recovered and added back to the reconstituted apple juice to enhance the flavour of the product.
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