Rheological properties and microstructure of tomato puree subject to continuous high pressure homogenization

Accepted Manuscript Rheological properties and microstructure of tomato puree subject to continuous high pressure homogenization J. Tan, W.L. Kerr PII: DOI: Reference:

S0260-8774(15)00237-X http://dx.doi.org/10.1016/j.jfoodeng.2015.05.025 JFOE 8183

To appear in:

Journal of Food Engineering

Received Date: Revised Date: Accepted Date:

28 January 2015 6 April 2015 18 May 2015

Please cite this article as: Tan, J., Kerr, W.L., Rheological properties and microstructure of tomato puree subject to continuous high pressure homogenization, Journal of Food Engineering (2015), doi: http://dx.doi.org/10.1016/ j.jfoodeng.2015.05.025

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Rheological Properties and Microstructure of Tomato Puree Subject to Continuous High Pressure Homogenization

J. Tan1 and W. L. Kerr1 1

Department of Food Science and Technology, University of Georgia, Athens, GA 30602

Abstract Tomato puree was processed by continuous high-pressure (CHP) homogenization at 69-276 MPa, for 1-3 passes. Laser scattering and light microscopy showed CHP reduced the pulp particles to ~10-100 µm, producing smaller and more uniform particles, with processing at 276 MPa and 2 passes producing greatest particle reduction. No differences in moisture or color were found due to different treatments. In general, G’ > G” for all samples, suggesting a soft gel network. Both the storage modulus (G’) and loss modulus (G”) decreased with CHP pressure. G’ decreased modestly with frequency between 0.1-2 Hz, and more dramatically between 2-30 Hz, with behavior characteristic of entangled polymers. In general, yield stress decreased with homogenization pressure, but increased with number of passes. CHP-treated samples had lower consistency and were less shear-thinning than the control. Repeated passes increased the consistency of CHP samples. The results suggest CHP processing produced smaller and more uniform particles, causing a reduced level of microstructure that contributes to elastic properties at small deformation.

Key words: tomato puree, high pressure, rheological properties

1. Introduction Tomatoes are one of the world’s most significant crops and are cultivated globally. In 2012, total tomato production was estimated at 161,800,000 metric tons (FAO, 2012). Tomatoes are often consumed fresh, but several varieties are processed into tomato sauce, soup, paste, puree, juice, ketchup and salsa. Tomatoes are a nutritious food with substantial amounts of vitamin C and biotin, as well as other vitamins and minerals. A 100 g serving of tomatoes typically has 1.5 g of fiber, with 90% of this as insoluble fiber. Tomatoes also contain several phytonutrients including lycopene and other carotenoids, as well as flavonols and hydrocinnamic acids. Several studies have investigated the benefits of tomatoes in reducing the risk of heart disease, improving bone health, and decreasing the risk of cancer. The role of tomato products in reducing the risk of prostate cancer has received particular attention, especially as it relates to lycopene, β-carotene and β-tomatine (Giovannucci, 1999; Etminan et al., 2004; Kirsh et al., 2006). To make flowable products, tomatoes are subject to crushing or other particle size reduction steps. The crushed tomato is typically sent through a pulper and finisher, which are rotary sieves that remove overly large pieces. The size and shape of the resulting particles, along with the final solids content of the mixture, determines the rheological properties of the fluid. In addition, tomatoes are usually thermally processed during chopping in hot- or cold- break processes. This inhibits enzymes that would cause the tomato solids to gel or aggregate, thus preventing lumpiness and promoting homogeneity. Products intended for ketchup, purees or sauces may also undergo a homogenization step to further reduce particle size, better mix components and enhance product stability. In this 3

context, high-pressure homogenization involves metering the product through a small orifice between the homogenizer valve and valve seat, at pressures between 10 and 70 MPa. Thakur et al. (1995) found that the consistency of homogenized tomato juice increased with pressures up to 3000 PSI (20.7 MPa), although the amount of the change depended on whether the juice was hotor cold- processed. The combined effects of turbulence, cavitation and shear help disrupt tissue pieces, forming smaller clusters of cells. This occurs primarily by separating the pieces along the cell walls (Stang et al., 2001). In recent years, there have been several studies on the effects of very high pressure on the particle size, physical properties and microbiological status of fruit and vegetable based liquids. In hydrostatic high pressure processing (HPP), products are sealed in flexible pouches, placed within a sealed chamber, and subject to pressures of 200-800 MPa as delivered by surrounding water. For liquid foods, continuous high pressure (CHP) processing has also been studied. This is an extension of conventional homogenization, and uses specialized pumps and valves to achieve operating pressures up to 400 MPa. Thus, fluid foods can be processed in a continuous manner. In early studies, hydrostatic pressure was used to process non-heated tomato juice contained in polyethylene pouches at pressures of 500-900 MPa (Porretta et al., 1995). Processed juice had lower counts of bacteria, yeast and molds than control. Juice viscosity increased with pressure, and the authors noted that very high pressures gave rise to a jelly-like translucent structure. While color was improved, the HPP products were found to be rancid and inedible. Hernandez and Cano (1998) found that combined HPP (50-500 MPa) and heating could reduce 32.5% of the pectin methylesterase (PME) in tomato puree, and up to 25% of the peroxidase. Overly high pressures, such as 335-500 MPa for PME, caused activation of enzymes. Others have studied the effects of HPP on the extractability of carotenoids of tomato pulp puree (Garcia 4

et al., 2001). While no effects were found on the extractability of carotenoids, water binding was enhanced after HPP and the rate of glucose diffusion through dialysis tubing was reduced. Rodrigo et al. (2007) studied the changes in color of HPP tomato puree, processed for 60 min at 65°C and between 300-700 MPa. No degradation in color was measured for the combined thermal and high pressure processing. Verlent et al. (2006) measured some rheological properties of tomato homogenates. When processed at 60°C, they found that the consistency decreased when processed at pressure up to 300 MPa. At higher pressure (~500 MPa), the homogenates had higher consistency, but did begin to develop a soft-gel translucent structure. Researchers have found that static high pressure treatments did not greatly affect the color of tomato puree or strawberry juice, implying that the lycopene or anthocyanins were not greatly lost during the process (Rodrigo et al., 2007). As with traditional homogenization, continuous high-pressure processing contributes substantial shear and turbulence to the fluid, and thus provides greater opportunity for tissue disruption. Corbo et al. (2010) used high-pressure homogenization as a means to control the growth of molds in tomato juice. They found that the levels of Fusarium oxysporum decreased somewhat with pressure between 30 and 150 MPa, and could be eliminated after three passes in the homogenizer. Colle et al. (2010) examined the effects of high-pressure homogenization (8.4132 MPa) on tomato pulp microstructure and the amount and bioaccessibility of lycopene. They found that increasing pressure caused a breakdown of tomato cell aggregate structures and had no effect on total lycopene, but enhanced the in vitro bioaccessibility of lycopene. CHP homogenization has also been investigated as a means to form very fine emulsions. Qian and McClements (2011) made emulsions of corn oil/octadecane in glycerol solutions, using a microfluidizer. They found that both higher pressure and multiple passes contributed to the

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reduction of particle size, with particles attained on the order of 116-240 nm. Yuan et al. (2008) were able to obtain particles of β-carotene in the range 113-168 nm at pressures between 60 and 140 MPa. Lopez-Sanchez et al. (2011a) studied the properties of olive oil emulsions prepared with comminuted carrot and tomato material. The rheological properties of tomato purees are critical to the product as they determine sensory properties such as mouthfeel and perceived consistency, how easily consumers can cause the puree to flow, and the design of equipment used to process the puree (Herch et al., 2000). The rheological properties are determined by the level of tomato solids (both insoluble and soluble), the particle size and shape of tomato tissue pieces, and the degree to which molecules such as pectin or hemicellulose are dissociated from the cells. In the simplest case, the consistency and flow of tomato puree under specified conditions of shear rate, shear stress and temperature are of interest (McKenna and Lyng, 2003). More in depth tests include small strain ramps to identify yield stresses at which the material begins to flow, or dynamic oscillatory tests to study the viscoelastic properties of the colloidal suspension and how these vary with temperature or frequency (Tabilo-Munizaga and Barbosa-Cánovas, 2005). A few studies have examined the effects of very high-pressure homogenization on the flow properties of food materials, although these have been mostly for formulated emulsions. For example, Floury et al. (2002) showed that CHP processing could convert soybean proteinstabilized emulsions from Newtonian liquid emulsions into shear-thinning emulsion gels. LopezSanchez et al. (2011a) studied emulsions produced with 5% olive oil and comminuted carrot/tomato blends. They found that pressures of 10 and 100 MPa, as well as multiple cycles of homogenization, had significant effects on both particle size and viscoelastic properties of the

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blends. Mert (2012) studied high pressure microfluidization up to 200 MPa and its effect on lycopene in ketchup. That work used lower pressures than presented here, and with a single pass through the system. In addition, the microfluidizer uses a substantially different valve system than in this study. There has been considerable interest in using physical processes to modify the properties of food ingredients. CHP homogenization has shown promise for altering the size of suspended solid or liquid particles, modifying product rheological properties, and affecting the sensory properties of the food material. The purpose of this study was to investigate the use of CHP for modifying the structure and rheological properties of tomato puree. This included the use of single pass homogenization at pressures not previously studied (up to 275 MPa), as well as several passes through the CHP system. The rheological properties studied included shear rate dependency of viscosity, static yield stress and dynamic measurements of storage and loss moduli as a function of frequency. In addition, color and particle size was assessed, and changes in the tomato tissue during processing were examined through light microscopy.

2. Materials and methods 2.1. Materials Roma tomatoes (Solanum lycopersicum L.) were purchased in 23 kg boxes from a local market in Athens, Georgia. The tomatoes were at Stage 6 ripeness (>90% red surface) and stored refrigerated at 2°C for less than 2 days prior to processing. Before further processing, skins were removed by first immersing 11.3 kg batches into boiling water contained in a 150 L steam

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jacketed kettle (Model M-195, Legion Utensil Inc., Long Island City, NY). The tomatoes were contained within a 35 cm x 35 cm x 35 cm mesh container for 8 min and removed once the skins became loose. 2.2. Preprocessing Softened tomatoes were loaded into a paddle pulper-finisher (Model 185S, Langsenkamp Inc., Indianapolis, IN) to remove the seeds and skin. The best recovery was attained with a mesh size of 1/16 inch (1.58 mm). The discharged pulp was then subject to a hot-break process to inactivate pectin methylesterase (PME) and polygalacturonase (PG). The pulp was pumped by a rotary pump (Model X5SS1PTYDGHLW, Roper Pump INC, Commerce, GA) through a stainless coil heat exchanger (305 cm length, 8mm ID and 10mm OD) heated by boiling water in a steam kettle. The puree reached a temperature of 94°C, as measured by a K-type thermocouple placed where the product exited the heated coil. The puree was immediately cooled in a second stainless coil heat exchanger immersed in an ice-water bath, allowing it to reach a temperature of 40°C. The pump operated at 720 rpm, causing the fluid to move through the system at 300 ml/min.

2.3. Continuous high pressure homogenization The pasteurized puree was divided into four lots (~2.5 L each) and processed in a Model nG7900 Ultra High Pressure Homogenizer (Model nG7900, Stansted Fluid Power LTD.) at 10, 20, 30 and 40 χ 103 PSI (69, 138, 207 and 276 MPa). Typical fluctuations about the set-point were no greater than ±10 MPa. The product was pumped to the systems intensifiers, raising the

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fluid to the target pressure. The fluid was then directed to a stainless steel micrometering valve (Model 60vrmm4882, Autoclave Engineers, Erie, PA). Samples were processed at the specified pressure for either 1, 2 or 3 passes. Between each run, or between cycles, the product was held at 7°C. Each treatment group had at least 400 ml to be used for subsequent analyses. Processed samples were kept at 2°C in 500 ml Nalgene centrifuge bottles. 2.4. Moisture content Moisture content of the puree, before and after processing, was determined using an automated moisture analyzer (Model HR73, Mettler Toledo, Columbus, OH)). Samples (~1 g) were loaded onto tared aluminum pans and weighed in the instrument. Moisture content was calculated based on mass before and after drying at 95°C in the halogen lamp system. As no subsequent evaporation was used, the solids in the processed samples ranged from 4.6 to 5.0 g/100g. 2.5. Particle size analysis The particle size distribution was determined by laser diffraction using a Malvern Mastersizer (Model MSS, Malvern Instruments Ltd.). Samples were introduced into a stirred tank filled with water until an obscuration of 20% was reached. Between different samples, the detector and laser were aligned and backgrounds were calibrated. The presentation was selected as “Standard-Wet” (nH2O = 1.333, nparticle = 1.5295). Particle size was displayed in terms of the volume mean diameter (d4,3). Cumulative percentiles d0.1, d0.5, and d0.9 were also determined, indicating that 10, 50 or 90% of the particles fell below the specified diameter.

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2.6. Light microscopy Microstructural features of the processed purees were assessed using light microscopy (Model DM100S, Leica Microsystems Inc., Buffalo Grove IL). Samples were diluted 1:10 with distilled water, then 250µl was placed on a microscope slide and mixed with 125µl of 0.01 M methylene blue. Most samples were studied with a 10X objective and 10X eyepiece. Digital images were attained with an 8 MP digital camera, and analyzed using the Spot Imaging software (Sterling Heights, MI). 2.7. Rheological properties Several rheological measurements were made on the processed tomatoes using a hybrid rheometer (Model Discovery HR-2, TA Instrument Inc., New Castle DE) at temperatures of 200.1℃. Yield stress was determined using a static vane method. Tomato puree samples (40 ml) were placed within the 33.7 mm diameter sample cup. A four-bladed vane (28 mm diameter x 42 mm long) was carefully lowered into the sample. The instrument was operated under stresscontrol mode, with the stress gradually increased from 0.0 to 5.0 Pa over 10 minutes. Plots of apparent viscosity versus shear stress were made. At low stresses, the material had relatively constant and high apparent viscosity (~104 Pa s). The yield point was taken as the stress at which the viscosity rapidly began to decline. Measurements of shear rate dependency were made using a conical rotor 27.98 mm in diameter and 42.06 mm in length. In this case, the sample cup was filled with 30 ml of sample. The sample was sheared at rates between 0.1 to 100 s-1 over a period of 15 minutes. Plots of

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shear stress versus shear rate were developed and fitted with a variety of models. Best fits were generally attained with a power law model: (1)

where K a consistency index, and n the power law index. Viscoelastic properties were assessed using frequency-dependent small-strain oscillatory tests. Samples were tested using a parallel plate system 20 mm in diameter. The bottom plate was loaded with ~250µl of sample and the top plate brought to a 1 mm gap. Dynamic strains were set to 1% and the samples measured in the range of 0.1 to 100 rad s-1. 2.8. Color Color was measured with a Model CR-410 chroma meter (Konica Minolta Sensing Americas, Ramsey NJ) with D65 illuminant and 10° CIE standard observer angle. Before the tests, the chroma meter was calibrated with a standard white tile provided the manufacturer. Samples were poured into 10015mm VWR petri dishes to cover all areas to a thickness of 5mm. All the petri dishes were place on white print paper when tested. The data collected by the chroma meter was displayed in L* (lightness), a*(red-green axes) and b*(yellow-blue axes) values. 2.9. Statistical methods All tests were repeated at least three times and results and the means and standard deviations calculated. Measurements were compared by two-way ANOVA using SAS 9.3 (SAS Institute Inc., Cary NC). Factors included pressure (with levels of 69, 138, 207 or 276 MPa) and

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numbers of passes (1, 2 or 3). Post hoc tests for significant differences amongst treatments was done using Tukey’s HSB test. The level of significance was set at p ≤ 0.05.

3. Results and discussion 3.1. Moisture content Moisture content of the tomato purees after different high pressure processing regimens is shown in Table 1. Neither pressure level nor number of homogenization cycles resulted in differences in moisture content amongst treated samples, or compared with the unprocessed control. The average moisture content of processed samples was 95.2 gH2O/100g. There was some concern that heat produced by pressure and shear effects might cause flashing of moisture as the product exited the homogenizer. However, the samples were rapidly cooled to ~7°C after each homogenization step to limit evaporation and heat induced degradation of lycopene and other desirable constituents. 3.2. Particle size distribution Examples of the particle size distribution (PSD) following high-pressure homogenization are shown in Figure 1. Derived measurements of average particle diameter (d4,3) and cumulative percentiles (d0.1, d0.5, d0.9) are shown in Table 2. Both pressure level and number of homogenization passes had a significant effect on particle size, as did the interaction between factors. As seen in Figure 1a, increasing pressure from 69 to 276 MPa decreased the average mean diameter. Thus, after a single pass the mean particle diameter had decreased from 318 µm to 153 µm at 69 MPa, to 83.0 µm at 138 MPa, to 66.5 µm at 207 MPa and to 64.0 µm at 276

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MPa. There was no further reduction of particle size at pressures higher than 207 MPa. Higher processing pressures also resulted in a narrower PSD. In the CHP system, many factors influence the change in particle size. A needle-seat micrometering valve was used in which the pressure is determined by the force exerted on the needle blocking the flow pathway. This creates tremendous shear stress as particles speed through narrow pathways. In addition, particles impinge on hard surfaces at relatively high velocities. Rapid changes in pressure may also create cavitation, creating bubbles in the fluid. Fluctuating collapse of these voids create shock waves that may further disrupt particles. Finally, the high pressure and shear conditions cause heating of the fluid, which may further promote breakdown of particles. The diminishing effects of increased pressure may be due to several reasons. First, it takes more than proportionally higher pressures to force the suspension through a narrower gap. Also, it is likely easier to disrupt clusters of many tomato cells than to break apart small clusters and individual cells. In addition, at sizes less than ~250 µm cell wall fragments dominate, and these are inherently more difficult to break apart than adjacent cells. Reduction in particle size with increasing pressure has also been reported for other tomato products, although at lower operating pressures. Augusto et al. (2012) investigated the highpressure homogenization of tomato juice at pressures up to 150 MPa. They found that unhomogenized control (4.5° Brix juice) had particles in the size range 100-1000µm. At 150 MPa, the particle size range was reduced to 10-300µm. They also observed a plateau at which increasing pressure resulted in only small further changes in particle size. Homogenization pressures up to 9 MPa decreased the particle size of ketchup (Bayod and Tornberg, 2011). For unprocessed paste, ~50% of the particles were larger than 100µm in diameter (d3,2). After homogenization, a multimodal distribution of particle sizes was observed, and only 20% of the 13

particles were larger than 100µm. Mert (2012) used microfludization at up to 200 MPa to affect the properties of ketchup. With a conventional valve homogenizer, they measured d3,4 values of 328 µm, while microfluidization reduced d3,4 to between 81 and 29 µm, for pressures between 120 and 200 MPa. Similar effects on particle size have been noted for high pressure processing of carrot dispersion, apple and citrus juice (Lopez-Sanches et al., 2011b; Donsì et al., 2009; Lacroix et al., 2005). Subsequent passes through the high-pressure homogenizer, at the same pressure, resulted in even further reduction in particle size (Figure 1b). This is reflected in changes in the cumulative diameters in Table 2. Thus, for control 10% of the particles had diameters less than 164 µm, while 90% had diameters less than 488 µm. However at 207 MPa, for example, 10% had diameters less than 14.9 µm and 90% had diameters less than 138 µm. Hence, there was a narrowing of the range (d0.9 – d0.1) from 324 µm to 123 µm. Most of the subsequent reduction in size was attained on the second pass, with only a modest reduction occurring with the third pass. For example, d4,3 decreased from 318 to 64.0 µm after one pass at 276 MPa, to 36.7 µm after two passes, and to 31.1 µm after the third pass. The greater number of passes also led to narrower PSD. As both pressure and number of passes were relevant factors, similar results might be attained by trading greater number of passes for higher pressure. Thus, processing tomato puree at 138 MPa resulted in d4,3 of 56.5 µm after 2 passes, slightly smaller than a d4,3 of 64.0 µm realized after 1 pass at 276 MPa. 3.3. Microstructure Fig. 2 shows microphotographs of the tomato puree processed at different pressures, and number of passes through the CHP homogenizer. Most of the particulate structure of the puree

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consists of cell fragments, and some intact cells (Bayod et al., 2007). Most of these are relatively soft parenchymal cells associated with the mesocarp, as much of the skin and seeds were removed in processing. In general, changes in the microstructural features of the puree mirrored the results of particle size analyses. Generally, high homogenization pressures caused a reduction in particle size. Most notably, larger elongated tissue fragments were reduced to pieces with more uniform dimensions in all directions. At lower pressures (69 and 138 MPa), particles were reduced in size as compared to control, but there were still evidence of stained clusters up to 337 µm in length. At greater pressure levels (207 and 276 MPa), much of these clusters were reduced in size. In addition, it became more difficult to see distinct outlines of the cellular material, suggesting that the cells became more disrupted and the stained polysaccharides were less restricted. This suggests at very high pressure, more of the cellular array was disrupted, releasing the cell wall material into the surrounding medium. Increasing the number of passes through the CHP system also helped reduce overall particle size. In addition, greater number of passes created a more uniform distribution of particle sizes. Greater pressure and number of passes also created particles that were both stained and had areas of translucency. As the methylene blue stains acidic regions of the cell, and particularly nuclear material, the appearance of more transparent matter indicates cell rupture and the release of cytoplasmic or cell wall material. In addition, for purees processed at 207 or 276 MPa, there was a greater tendency for particles to re-associate, presumably due to the released pectin and cell wall materials. Similar results have been reported for tomato juice by Augusto et al. (2012) and Kubo et al. (2013), as well as for ketchup by Mert (2012). 3.4. Rheological properties

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Figure 3 shows a plot of shear stress versus shear rate for the unhomogenized and CHP treated samples. Data between 1 and 45 s-1 were fit to a power law equation, and fitted parameters K and n are summarized in Table 3. At shear rates below 1 s-1 the samples showed evidence of elastic behavior. That is, there was an initial precipitous increase in shear stress, followed by a decrease. As this was evidence of a static yield stress, this was examined by a different method. Interestingly, including a yield stress term (σo) in the power law model did not improve the fits. Thus, once the elastic network was broken and flow was initiated, the apparent viscosity was relatively low. The consistency index (K) can be taken as the apparent viscosity at low-shear rate (1 s-1). The control had the greatest K (3474 mPa.s), and all CHP homogenized samples had significantly lower values (538-902 mPa.s). Two-way ANOVA showed that the number of cycles was a significant factor for K in the CHP-treated samples. After three passes in the homogenizer, the samples were always more viscous than after one or two passes. Amongst the CHP samples, those processed at the highest pressure had the lowest K value after 1 cycle (538 mPa.s), and the highest K value after 3 cycles (902 mPa.s). Augusto et al. (2012) found viscosity in the range of ~500-900 mPa.s for tomato juice processed for one pass at up to 150 MPa. However, in their case viscosity tended to be greater for samples processed at higher pressure, as did Mert (2012). For conventionally homogenized ketchup, the zero-shear viscosity was on the order of 20,000-50,000 mPa.s. For tomato paste, the viscosity depended on solids concentration, ranging from as low as 3000 mPa.s to as high as 600,000 mPa.s. The power law index n was least for the control puree (0.12), indicating greater shearthinning behavior than for the CHP-treated samples. While n was greater for all CHP samples

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(0.28-0.39), there was no particular trend associated with pressure or number of passes. Thixotropic properties generally occur in fluids with some degree of continuous microstructure, which is disrupted by a finite stress and subsequent flow field. The associations may be due to flocculation, alignment of fibers, entanglements and molecular associations. Shear-thinning occurs in the viscous flow region due to alignment of elongated particles, disruption of junction zones and break-up of flocculated particles (Barnes, 1997). For the CHP-treated samples, the particles are smaller and likely more spherical in nature, factors which generally decrease thixotropy. As noted by Augusto et al. (2011), tomato products are a two-phase system with complicated rheology. The aqueous serum phase contains sugars, salts, acids and some soluble polysaccharides, but is by and large a Newtonian fluid. The dispersed phase contains clusters of cells, single cells, fragmented cells or cell remnants, as well as colloidally dispersed polysaccharides. The system is best thought of as a non-colloidal dispersion, as many of the particles are above 10 µm in size. As such, the flow properties of the material are dominated by hydrodynamic forces, and less by interparticle forces and Brownian motion (Genovese et al., 2007). Other factors such as particle size distribution and shape are also important in this regime. Of course, fruit and vegetable juice and purees may contain particles in the noncollodial range (>10µm) as well as the colloidal range (1nm-10µm). For the former case, the simplest description of the relative viscosity was postulated by Einstein as (Hiemenz and Rajagopalan, 1997):

ηr =

η = 1+[η ]φ ηs

(2)

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where ηs is the viscosity of the solvent and ηthe viscosity of the dispersion. According to this viewpoint, the viscosity increases as a function of φ, the relative volume fraction of particles dispersed in the total volume. The Einstein equation holds primarily in dilute suspensions, however. For concentrated dispersions, the Krieger-Dougherty equation is often applied: ⎛

ηr = ⎜1− ⎝

−[ η ]φ m

φ ⎞ ⎟ φm ⎠

(3)

Where φm is the maximum packing fraction of solids, and typically varies from 0.60-0.74, depending on the type of packing (Genovese et al., 2007). It can be determined experimentally through sedimentation experiments. This equation may be modified further to account for various particle shapes and deformability, as well as interaction amongst particles. Both microscopic and particle size analyses showed that the suspended tomato particles were reduced in size with higher pressure, or greater number of passes through the CHP system. Concomitantly, the viscosity at low shear rates was reduced. Two factors may be at play. First, in addition to volume fraction the actual particle size and distribution also determines the viscosity. While Equations 2 and 3 do not implicitly involve particle size, particle size is known to influence the relative viscosity in some cases. For example, Pavlik (2011) measured the viscosity of dispersions with glass particles ranging from 26 to 71 µm in diameter. The viscosity of dispersions containing particles between 36 and 71 µm varied only slightly with φeven at relatively high volume fractions. Dispersions with 26 µm particles, however, had distinctly lower viscosity, particularly at high volume fractions. They believed that suspensions with smaller particles generated more open space between neighboring particles, allowing the continuous phase to flow, and producing a fluid that behaved more like a dilute suspension that could be

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modeled by Einstein’s equation. There may also be differences in the maximum packing fraction

φm due to changes in particle size and distribution, shape or even deformability. In addition, the CHP processing may also reduce the actual volume fraction of particles, particularly if cells are disrupted and disperse their contents, creating suspensions that are more appropriately characterized as colloidal. Some studies have reported an increase in shear-thinning properties after processing of fruit and vegetable products. Silva et al. (2010) found an increase in shear-thinning for pineapple pulp processed at up to 30 MPa. When processed at greater than 40 MPa, the pulp exhibited phase separation due to increased aggregation. Shijvens et al. (1998) used screening procedures to produce applesauce with various size fractions of particles. Those with particles in the range 0.63-1.0 mm were less shear-thinning than those with particles in the range 0.35-0.63 mm or <0.35 mm. Lopez-Sanchez et al. (2011a) studied emulsions made from vegetable oil, and either carrot or tomato, and subject to homogenization pressures of 10 or 100 MPa. They found that samples processed at the higher pressure were more shear-thinning than those processed at lower pressure. In our case, processed samples were less shear-thinning than unprocessed pulp, but there were no clear differences amongst samples processed at different pressures and number of passes. Any differences between this study and other researches may be due to the greater homogenization pressures used here, different microstructural features (e.g. dispersed pulp versus emulsions), and lower particle sizes attained in this study. The results from yield stress measurements are shown in Table 4. The control puree had the highest yield stress at 2.62 Pa. All treated samples had lower yield stress, although the differences were not large. After one pass, samples processed at 276 MPa had the lowest yield

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stress (1.09 Pa), those at 138 or 207 MPa slightly higher yield stress (1.70-1.85 Pa), and those at 69 MPa even higher yield stress (2.26 Pa). In several cases, however, increasing the number of passes in the CHP system led to greater yield stress than found after 1 pass. Thus at 276 MPa, the yield stress was 1.09 Pa after one pass, 1.69 Pa after two passes, and 2.12 Pa after three passes. Although the samples did have a measureable yield stress, they were not large compared to some other food products. For example, pancake batter has a yield stress of 10 Pa, ketchup 15 Pa, and mayonnaise 100 Pa. Abu-Jayil et al. (2004) studied the yield stress of tomato paste with solids content ranging from 5.57% to 25% by weight, and found the yield stress ranged from 1.6 to 101.9 Pa. Mert (2012) found that the yield stress of ketchup ranged from 15.1 to 20.9 Pa for samples processed by microfluidization at pressures between 20 and 200 MPa. For samples processed by a conventional valve homogenizer, the yield stress was only 9.4 Pa. This highlights that the type of particle size reduction is important to the final product properties. With microfluidization, two fluid microstreams collide at high speed. The streams are subjected to high shear and substantial impact forces. With the valve homogenizer, the fluid is forced through an orifice created by a valve and valve seat. This creates high turbulence and shear, as well as cavitation and impact. The role of dispersed phase properties on yield stress (σo) is not completely understood. In fact, the term yield stress may be just a practical designation. Certainly, some viscoelastic materials behave elastically at stresses below the yield value (σ< σo), and flow at higher stresses (σ> σo). However, most of these would likely yield at σ< σo given sufficient time. However, Bayod et al. (2007) showed that tomato products deformed at σ< σo maintained elastic behavior for up to 30 min. In general, yield stress is greater for systems with greater volume fraction of

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particles and stronger interaction forces amongst the particles (Genovese et al., 2007). As noted, yield stress values increased with solids concentration, and therefore φ, in tomato pastes (Mert, 2012). The influence of particle size on yield stress is less well understood, contradictory in some cases, and depends on the specific particle range of interest. Qui and Rao (1988) reported that yield stress of applesauce was greatest for particle diameter of 0.5 mm and 100% pulp content, and least for 1.0 mm particles and 80% pulp content. This may be an indication that increased surface area enhanced interparticle attractive forces. Espinosa et al. (2011) found that yield stress of apple puree increased with pulp content and decreased with decreasing particle size. For tomato pulp, Yoo and Rao (1994) showed that yield stress increased with solids concentration, but was generally greater for purees with particles sizes ~0.34 mm as compared to those ~0.71 mm. However, at relatively low concentrations (11-12% by weight) purees with smaller particles had lower or no different yield stress than those with larger particles. Bayod et al. (2007) observed that homogenized tomato paste had greater yield stress than nonhomogenized paste. They attributed this to a change in the nature of the network structure. Homogenized samples have fewer intact cells and cell clusters, and are more fibrous in nature. In addition, homogenization creates more surface area that may enhance interparticle forces of attraction. Fig. 4 and Table 5 show how the complex shear moduli vary with frequency. In all cases, the storage modulus G’ was greater than the loss modulus G”. Thus, at 1% strain (σ~0.16Pa), the system behaves as an elastic solid, and might be characterized as a soft gel network (Rao et al., 1992). For the unprocessed control, the G’ at low frequency (0.1 rad/s) was 148.7 PA, while that

21

for all processed samples ranged from 10.9 to 21.0 Pa. A measurable value of G’ in the viscous region is typical for dispersions and concentrated polymer solutions. It has been suggested that this is due to the relative dense packing of particles, interparticle forces, and entanglements of cells, fibers, and large colloidal molecules (Mert, 2012). These are the same factors, of course, that contribute to yield stress. It is worth noting that G’ at both 0.1 and 1.0 rad/s were well correlated and decreased with particle size (d4,3). That is:    0.1     ′  0.0023 · ,  0.315 · ,  25.9

   0.993

(3)

   1.0     ′  0.0031 · ,  0.486 · ,  37.1

   0.993

(4)

All samples also became notably stiffer at ω>3 rad/s. Below ~3 rad/s, G’ ∝ω
; at ω>3 rad/s, G’ ∝ω
. True gels are more likely to have chemical bonds that hold the network, and show little frequency dependence. The frequency dependency shown by the tomato samples is more typical of “weak gels” that are held together by physical associations. At low frequencies, some of the structure behaves elastically, but there are still relaxation or flow processes that can occur. At higher frequencies, the time scale is too short for these elements to flow, and the structure as a whole behaves more elastically. Similar behavior has been observed for tomato homogenates subject to static high pressure (Verlent et al., 2006). However, tomato juice showed little viscoelastic behavior, with G’ varying little with frequency over the range 0.01-10 rad/s. For tomato paste, Sanchez et al. (2002) found that G’ was dependent on ~ω0.12 over the range of 0.01100 rad/s. G’ values shown in Table 5 are in general lower than those reported by other researchers working on related products. Bayod et al. (2008) studied tomato paste and ketchup with different 22

levels of total solids (23.42-38.75% TS). For tomato paste, G’ at 1 Hz ranged from 7124 to 10,411 Pa while for ketchup G’ ranged from 562 to 732 Pa. In our study, however, the total solids level was much lower (~5%), as were the size of particles. For microfluidized tomato ketchup, Mert (2012) reported an increase in G’ for pressures between 20 and 120 MPa, and a decrease for pressures from 120 to 200 MPa. For samples processed at 69 MPa, additional passes through the CHP system led to a lower G’, while at 207 or 276 MPa it led to a greater G’. However, at 69 MPa, additional passes reduced the particle size (d4,3) of relatively big particles from 153 to 91 and 83 µm. At 276 MPa, after 1 pass the particle size was already 64 µm; additional passes reduced it to 37 and 31 µm. One theory is that in the former case reducing particle size was more important to reducing G’ by allowing these large particles to better slip pass each other when under stress. In the latter case, the further reduction of relatively small particles allowed more surface area and interaction amongst particles, but in this case colloidal forces rather than hydrodynamic forces are more dominant. Another consideration is that subsequent homogenizer passes allow greater swelling of the cell fragments, increasing their effective phase volume in the suspension (Lopez-Sanchez et al., 2011b; Bayod and Tornberg, 2011). Table 5 also lists tan(δ) at 1 rad/s, which is the ratio of loss to storage modulus (G”/G’). For the unprocessed control, tan(δ) = 0.31. For the CHP treated samples, tan(δ) was smaller with values ranging from 0.16 to 0.18 (with the exception of tan(δ) = 0.22 for samples treated for 2 passes at 276 MPa). Similar results have been reported for tomato juice (Lopez-Sanchez et al. 2011a) and other plant products (Pickardt et al., 2004; Romana and Taylor, 1992). This means that while the control had higher values of G’, the viscous component G” was proportionally

23

greater than for the processed samples. One postulate is that while smaller particles and less entanglements led to a reduced storage modulus, it also resulted in a lower viscosity of the lossy component. 3.5. 4. Color Measured color parameters are shown in Table 1 for the differently processed tomato purees. Lightness value (L*) for the control was 56.3. CHP processed samples were somewhat darker (L* = 51.9 to 53.2), but there were no differences in lightness amongst the CHP treated samples. While there were no differences in b* amongst any samples, the control had slightly lower a* (32.3) as compared to any of the CHP treated samples (a* = 37.1 to 38.1). Thus, CHP treated samples were slightly more red in color. A previous study by Rodrigo et al. (2006) showed that no color degradation of tomato puree occurred during combined thermal and high hydrostatic pressure treatments. This implies that lycopene, the most abundant carotenoid in tomato and responsible for the red color (Colle et al., 2011), was not lost during processing. However, some studies have shown that high-pressure treatments might reduce the storage stability of lycopene. For example, Kudo et al. (2013) found that tomato juice treated at 75 and 100 MPa had significant color degradation after 60 days storage.

Conclusions Continuous high pressure processing of tomato puree resulted in substantial changes in particle size and uniformity of particle size distribution. The average particle size was reduced by up to ten-fold, with sizes on the order of 30-80 µm made possible. Although not studied in this

24

work, this is in the size range in which consumers no longer perceive a particulate structure. These changes were made possible in a continuous manner, and would not be realized with static high pressure alone. The change in size and shape of particles in the dispersed phase also altered the rheological properties of the puree. The material was less viscous at low shear and with a lower yield stress, but had less shear thinning behavior. This has implications for developing products with greater solids, as well as for pumping requirements during processing. This may also make it easier to produce spray-dried and similar products.

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Bayod, E., Månsson, P., Innings, F., Bergenståhl, B., & Tornberg, E. (2007). Low shear rheology of concentrated tomato products. Effect of particle size and time. Food Biophysics, 2(4), 146-157. Bayod, E., Willers, E.P., & Tornberg, E. (2008). Rheological and structural characterization of tomato paste and its influence on the quality of ketchup. LWT - Food Science and Technology, 41(7), 1289-1300. Bayod, E., & Tornberg, E. (2011). Microstructure of highly concentrated tomato suspensions on homogenisation and subsequent shearing. Food Research International, 44(3), 755-764. Colle, I., van Buggenhout, S., van Loey, A., & Hendrickx, M. (2010). High pressure homogenization followed by thermal processing of tomato pulp: Influence on microstructure and lycopene in vitro bioaccessibility. Food Research International, 43(8), 2193-2200. Colle, I.J., Andrys, A., Grundelius, A., Lemmens, L., Lofgren, A., van Buggenhout, S., van Loey, A., & Hendrickx, M. (2011). Effect of pilot-scale aseptic processing on tomato soup quality parameters. Journal of Food Science, 76(5), C714-C723. Corbo, M.R., Bevilacqua, A., Campaniello, D., Ciccarone, C., & Sinigaglia, M. (2010). Use of high pressure homogenization as a mean to control the growth of foodborne moulds in tomato juice. Food Control, 21(11), 1507-1511. Donsì, F., Esposito, L., Lenza, E., Senatore, B., & Ferrari, G. (2009). Production of shelf-stable annurca apple juice with pulp by high pressure homogenization. International Journal of Food Engineering, 5(4), 1556-3758.

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Lopez-Sanchez, P., Nijsse, J., Blonk, H. C., Bialek, L., Schumm, S., & Langton, M. (2011b). Effect of mechanical and thermal treatments on the microstructure and rheological properties of carrot, broccoli and tomato dispersions. Journal of the Science of Food and Agriculture, 91(2), 207-217. McCrae, C.H. (1994). Homogenization of milk emulsions: use of microfluidizer. International Journal of Dairy Technology, 47(1), 28-31. McKenna, B.M. (2003). Introduction to food rheology and its measurement. In Texture in Food, (B.M. Mckenna and J.G. Lyng, eds.) pp. 130-132, University College Dublin, Ireland. Mert, B. (2012). Using high pressure microfluidization to improve physical properties and lycopene content of ketchup type products. Journal of Food Engineering, 109(3), 579-587. Nielsen, H.B., Sonne, A.M., Grunert, K.G., Banati, D., Pollák-Tóth, A., Lakner, Z., Olsen, N.V., Žontar, T.P., & Peterman, M. 2009. Consumer perception of the use of high-pressure processing and pulsed electric field technologies in food production. Appetite, 52(1), 115-126. Pavlik, M. (2011). The dependence of suspension viscosity on particle size, shear rate, and solvent viscosity. College of Liberal Arts & Social Sciences Theses and Dissertations. Paper 71.
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30

Schijvens, E.P.H.M., Vliet, T.V., & Dijk, C.V. (1998). Effect of processing conditions on the composition and rheological properties of applesauce. Journal of Texture Studies, 29(2), 123-143. Silva, V.M., Sato, A.C.K., Barbosa, G., Dacanal, G., Ciro-Velásquez, H.J., & Cunha, R.L. (2010). The effect of homogenisation on the stability of pineapple pulp. International Journal of Food Science and Technology, 45(10), 2127-2133. Stang, M., Schuchmann, H., & Schubert, H. (2001). Emulsification in high‐pressure homogenizers. Journal of Life Science Engineering, 1(4), 151-157. Tabilo-Munizaga, G., & Barbosa-Cánovas, G.V. (2005). Rheology for the food industry. Journal of Food Engineering, 67(1), 147-156. Thakur, B.R., Singh, R.K., & Handa, A.K. (1995). Effect of homogenization pressure on consistency of tomato juice 1. Journal of Food Quality, 18(5), 389-396. Verlent, I., Hendrickx, M., Rovere, P., Moldenaers, P., & Loey, A.V. (2006). Rheological Properties of Tomato-based Products after Thermal and High-pressure Treatment. Journal of Food Science, 71(3), S243-S248. Yoo, B., & Rao, M.A. (1994). Effect of unimodal particle size and pulp content on rheological properties of tomato puree. Journal of Texture Studies, 25(4), 421-436. Yuan, Y., Gao, Y., Zhao, J., & Mao, L. (2008). Characterization and stability evaluation of βcarotene nanoemulsions prepared by high pressure homogenization under various emulsifying conditions. Food Research International, 41(1), 61-68.

31

Figure Legends

Fig.1: Particle size distribution of tomato puree processed by continuous high-pressure homogenization (CHP) (a) after 2 passes at 69-276 MPa, (b) after 1-3 passes at 207 MPa.

Fig. 2: Photomicrographs of tomato puree after CHP processing. From top to bottom: increasing pressure from 69MPa to 276 MPa; from left to right: increasing number of passes (1-3).

Fig. 3: Shear stress versus shear rate for control () and CHP samples processed two times at ( ) 69, ( ) 138, () 207 and () 276 MPa.

Fig. 4: Storage (G’) and loss modulus (G”) (a) as a function of frequency for tomato puree processed at 207 MPa for 1-3 passes; (b) puree processed in 1 pass at 69-276 MPa.

32

Table 1.

Moisture content and color values for tomato puree processed at 69-276 MPa for 1-3 passes.

Pressure (MPa)

Number Passes

Moisture Content (%)

L*

a*

b*

95.44a

56.29b

32.26b

35.56a

1

95.26a

52.82a

37.01a

35.31a

2

95.39a

52.36a

37.84a

35.05a

3

95.26a

51.97a

37.91a

34.33a

Control

69

33

138

207

276

a

1

95.24a

52.84a

37.52a

35.67a

2

95.31a

52.82a

37.67a

35.55a

3

95.06a

52.28a

37.99a

35.52a

1

95.19a

52.59a

37.96a

35.39a

2

95.18a

52.74a

38.09a

35.39a

3

95.10a

53.17a

37.64a

35.42a

1

95.19a

52.82a

37.70a

34.98a

2

95.00a

53.45a

37.07a

36.23a

3

95.01a

52.69a

37.93a

35.03a

Values in a column not followed by the same letter are significantly different at p < 0.05

34

L* - lightness on a scale from 0 (black) to 100 (white); a* - values on the color axes ranging

from green (-100) to red (100); b* – values on the color axes ranging from blue (-100) to yellow

(100)

Table 2.

35

Mean and cumulative diameter of tomato puree as a function of CHP homogenization pressure

and number of passes.

Pressure (MPa)

No. Passes

d 4,3 (µm)

d 2,3 (µm)

d 0.1 (µm)

d 0.5 (µm)

d 0.9 (µm)

318h

216h

164i

311i

488j

1

153a

51.2a

31.0a

113a

337a

2

91.2b

26.5b

21.8b

70.5b

190b

3

81.2c

26.1b

21.1b

65.5c

164c

1

83.0c

23.9c

19.2c

63.5d

175bc

2

56.5e

17.5de

14.2d

46.5g

112ef

3

50.41ef

16.4ef

13.1e

42.7h

98.1fg

1

66.5d

18.4d

14.9d

51.8e

138d

Control

69

138

207

36

276

a

2

46.2f

14.8f

12.6e

39.5i

88.9gh

3

34.3g

10.7g

9.05g

30.4j

65.0i

1

64.0d

17.7de

14.3d

49.7f

133de

2

36.7g

11.2g

10.4f

32.1j

69.6hi

3

31.1g

9.7g

7.82h

27.3k

60.6i

Values in a column not followed by the same letter are significantly different at p < 0.05

37

Table 3.

Rheological parameters for CHP processed tomato puree at 1 to 45 s-1 as fit by the powerlaw

model (Eqn 1).

Pressure (MPa)

Number Passes

K (s) (mPa.sn)

Rate Index n

3474a

0.12c

1

771bc

0.28ab

2

652cd

0.37ab

3

750bc

0.34ab

1

623cd

0.39ab

Control

69

138

38

207

276

a

2

649cd

0.36a

3

868b

0.34b

1

584cd

0.39a

2

869b

0.36ab

3

878b

0.36ab

1

538d

0.41a

2

771bc

0.38ab

3

902b

0.36ab

Values in a column not followed by the same letter are significantly different at p < 0.05

39

Table 4.

Yield stress for tomato puree CHP processed at 69-276 MPa for 1 to 3 passes.

Pressure (MPa) Control

Number Passes

Yield Stress (Pa) 2.62e

40

69

138

207

276

1

2.26ab

2

2.07b

3

2.10b

1

1.85c

2

2.14b

3

2.27ab

1

1.70c

2

2.12b

3

2.45a

1

1.09d

41

a

2

1.69c

3

2.12b

Values in a column not followed by the same letter are significantly different at p < 0.05

42

Table 5.

Storage (G’) and loss (G”) moduli for CHP treated tomato puree.

Pressure

No.

  at 0.1s-1

  at 1 s-1

  at 0.1s-1

  at 1 s-1

(MPa)

Passes

(Pa)

rad/s (Pa)

(Pa)

(Pa)

148.74g

199.52g

46.03e

1

20.98a

27.97a

2

17.12b

3

1

Control

69

138

tan  

tan  

30.21g

0.31a

0.15a

3.48ab

2.59a

0.17c

0.092b

23.20bc

2.80bc

2.19abc

0.17c

0.091b

16.86b

23.26bc

2.85bc

2.17abc

0.17c

0.094b

16.06bc

22.03cd

2.75bc

2.00cd

0.17c

0.096b

43

207

276

a

2

16.82b

23.11bc

2.85bc

2.11bcd

0.16c

0.094b

3

15.98bc

22.33bcd

2.87bc

1.90cde

0.17c

0.091b

1

12.79ef

17.76ef

2.13cd

1.66de

0.18c

0.092b

2

15.36bcd

21.52cde

2.81bc

1.99cd

0.18c

0.094b

3

17.08b

26.12ab

3.86a

2.54ab

0.17c

0.093b

1

10.94f

15.39f

1.87d

1.48e

0.18c

0.085b

2

13.34def

18.94def

2.42cd

1.77cde

0.22b

0.097b

3

14.10cde

19.90cde

2.60cd

1.80cde

0.18c

0.091b

Values in a column not followed by the same letter are significantly different at p < 0.05

44

Highlights

• • • •

Tomato puree was homogenized at pressures up to 276 MPa High pressures and number of passes produced a less viscous product with smaller and more even suspended particles Samples processed at high pressures were less shear-thinning and had lower yield stress Greater number of passes slightly reduced the yield stress

45