Ultrasound Sonochemistry
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Improved mechanical properties of retorted carrots by ultrasonic pre-treatments
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Li Day*, Mi Xu, Sofia K. Øiseth, Raymond Mawson
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CSIRO Food and Nutritional Sciences, 671 Snedyes Road, Werribee, VIC 3030, Australia
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* Corresponding author. Tel: +61 3 9731 3233; fax: +61 3 9721 3250. Email address:
[email protected].
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Ultrasound Sonochemistry
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Abstract:
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The use of ultrasound pre-processing treatment, compared to blanching, to enhance
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mechanical properties of non-starchy cell wall materials was investigated using carrot as an
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example. The mechanical properties of carrot tissues were measured by compression and
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tensile testing after the pre-processing treatment prior to retort and after retort. Carrot samples
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ultrasound treated for 10 min at 60 °C provided a higher mechanical strength (P<0.05) to the
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cell wall structure than blanching for the same time period. With the addition of 0.5% CaCl2
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in the pre-treatment solution, both blanching and ultrasound treatment showed synergistic
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effect on enhancing the mechanical properties of retorted carrot pieces. At a relatively short
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treatment time (10 min at 60 °C) with the use of 0.5% CaCl2, ultrasound treatment achieved
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similar enhancement to the mechanical strength of retorted carrots to blanching for a much
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longer time period (i.e. 40 min). The mechanisms involved appear to relate to the stress
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responses present in all living plant matter. However, there is a need to clarify the relative
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importance of the potential stress mechanisms in order to get a better understanding of the
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processing conditions likely to be most effective. The amount of ultrasound treatment
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required is likely to involve low treatment intensities and there are indications from the
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structural characterisation and mechanical property analyses that the plant cell wall tissues
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were more elastic than that accomplished using low temperature long time blanching.
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Keywords: mechanical property, cell wall structure, carrot, retort processing, blanching,
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ultrasound, CaCl2
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Ultrasound Sonochemistry
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1.
Introduction
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The texture of fruit and vegetables is primarily determined by the mechanical
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properties of the plant cell wall and the internal pressure of the cells (i.e. the turgor pressure)
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[1-3]. Severe heat processing is known to cause the rupture of the cell membrane which leads
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to the loss of turgor pressure [4]. In addition, thermal treatment also causes chemical and
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physical changes to plant cell wall biopolymers which in turn impair the cell wall structure
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and changes the mechanical properties of cell walls [5-7]. The consequences of the loss of
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turgor pressure and changes to cell wall mechanical properties result in softening in the
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texture of thermally treated plant based foods, which is opposite to the crunchy and crispy
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texture that consumers associate with the sensory perception of fruit and vegetables [1, 8].
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Traditional technologies such as thermal processing have proven to be effective in terms of
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product safety for shelf-stable foods, however, it remains a challenge to maintain the eating
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quality of processed fruit and vegetables. The food manufacturing industry is constantly
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searching for new processing means to improve the sensory quality (e.g. texture firmness and
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taste, etc.) of vegetables in ready-made food products such as soups, sauces and yoghurts, etc.
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without compromising safety and shelf-life of the food products.
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Processes aiming to reduce the changes in the structure and texture of processed fruits
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and vegetables often focus on controlling the changes to cell wall biopolymers, in particular
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the pectins, which are most abundant polysaccharides in the plant primary cell walls and the
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middle lamella [9-11]. Depending on its molecular structure assembling, pectin bioploymers
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can be depolymerised and become water soluble at elevated processing temperature. Both
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chemical and enzymatic changes of pectin play a role in process-induced plant tissue
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softening. The loosening of pectin biopolymer network, particular in the middle lamella
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between adjacent cells upon heating, allows the cells to be readily separated and the
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weakening of the cell wall strength [1, 12]. The common approach to manipulate changes in
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pectin structure during processing to a certain point is to control pectic enzyme activities by
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inactivating the undesirable ones, e.g. polygalacturonase (PG) which induces pectin
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depolymerisation, and by accelerating the activity of those that provide positive functional
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benefits, e.g. pectinmethylesterase (PME) which removes the methyl groups from the pectin
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molecules and creates negatively charged carboxyl groups in the process [13-16]. The
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demethylesterified pectin backbonds can either interact with Ca2+ to promote the formation of
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pectin-Ca2+ network (i.e. so-called ‘egg-box’ model structure), thus strengthening the
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mechanical properties of cell walls.
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Various pre-processing treatments have been explored in attempt to improve the
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quality, especially textural properties, of processed fruit and vegetables. For example, it has
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been shown that low temperature blanching (e.g. 60 °C) for a relatively long time (e.g. 30–40
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min) prior to further thermal processing could improve the firmness of carrot tissues [17-20].
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This low temperature long time blanching (LTB) provided a condition that helps to elevate
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the PME activity (prior to its being inactivated by high temperature) [21], thereby resulting in
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a lower degree of pectin depolymerisation, better adhesion of the cell walls and the overall
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stronger mechanical properties of thermally processed plant materials compared to those that
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have not been pre-treated (Ng & Waldron, 1997; Sila, Smout, Vu, & Hendrickx, 2004).
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Another common practice is to soak vegetables in CaCl2 for a period of several hours to
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overnight. The presence of additional Ca ions increases the interaction between pectins and
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strengthens polymer networks within the cell wall as well as between the cell walls. The Ca
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crosslinked pectin biopolymer network within the cell wall structure enhances the pectin’s
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resistance to heating degradation, and thus comparatively higher mechanical properties of
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plant materials after thermal processing. Soaking in CaCl2 in combination with LTB
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blanching has showed synergistic effect on the improved firmness of thermal processed
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vegetables [17, 22].
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Recently, emerging innovative processing technologies such as high hydrostatic
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pressure processing (HPP) have shown potential in minimising softening of plant tissues [23-
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26]. HPP treatment in combination with high temperature (~100 °C) has shown to be effective
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to enhance the firmness of vegetable tissues [27-29]. This is believed due to the shift of the
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optimal temperature for PME action towards higher temperatures at pressure levels beyond
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atmospheric pressure, therefore having positive effects to reduce the degree of methyl
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esterification of pectin and to minimise changes in the pectin fractions during treatment,
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consequently causing negligible changes in intercellular adhesion [16, 30, 31].
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The use of low frequency ultrasound as a processing aid to improve quality and safety
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of processed foods has also attracted considerable interest. To date, the technology has been
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largely explored for inactivating enzymes such as polyphenoloxidases and peroxidises [32].
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Denaturation of protein is thought to be the main reason for inactivation of enzymes either by
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free radicals in sonolysis of water molecules or shear forces resulting from the formation or
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collapse of cavitating bubbles [33]. Ultrasound has also been reported to enhance processes
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such as bioactive extraction, crystallisation, freezing, emulsification, filtration and drying
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through its highly efficient heat and mass transport mechanism [34, 35]. A few studies have
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also immerged on the use of ultrasound to modify the viscosity of particulate systems (e.g.
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cell wall particles and/or starch granules). Ultrasound treatment enables a higher penetration
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of moisture into the cell wall fibre network which causes swelling of cell wall particles
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resulting in an increase in the viscosity of tomato puree [36]. Curulli et al. [37] found that
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ultrasound at a selected frequency, energy and time profile can be used to modify the surface
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structure of plant tissue which has a cellular structure with substantial starch content (e.g.
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potatoes). The modified surface structure acts as a moisture barrier to retain the moisture
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within the core structure in a high temperature subsequent cooking process.
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However there have been few reports on the use of ultrasonic technologies to improve
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the mechanical properties of primarily polysaccharides based cell wall materials (e.g.
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containing no or little starch). So the objective of this study was to evaluate whether
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ultrasound technology can be used as a pre-processing treatment alternative to improve
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mechanical properties of plant materials that subject to severe heating processing such as
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retorting.
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2.
Materials and Methods
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2.1.
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Material
Carrots (c.v. Kuroda) were purchased from a local supermarket. The skin, top and
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bottom ends of the carrots were removed. Carrots were then cut into discs of ~15 mm
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thickness with a diameter of ~15–25 mm.
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For tensile testing, carrot strips (2 mm thickness) were prepared by slicing the carrots
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lengthwise using a kitchen vegetable slicer (Swisstar V-slicer). The carrot scrips consisted of
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phloem tissue only. The strips were then cut into 7 mm wide bands using a razor blade.
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Reverse Osmosis water (RO water) was used throughout the trial.
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2.2.
Pre-processing treatment
The pre-processing treatment conditions are summarised in Table 1. A total of 9
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treatments including controls were applied to the carrots. A Branson Ultrasonic Bath
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(Branson Ultrasonics Corporation, USA) was used for all treatments except the controls.
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Water (12 L) with or without 0.5 wt% (45 mM) CaCl2 at required treatment temperature was
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added to the bath, followed by the addition of carrot discs (1.2 kg) and the strips (about 50).
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For blanching only, the temperature of the bath was maintained by heating, but without the
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application of ultrasound for the required time. For ultrasound treatment, the ultrasound (US)
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power of the bath was applied (40 kHz/850W, calculated in-bath power allowing for
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transducer and acoustic inductance losses 0.046 W/cc) together with the use of a 400 KHz
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probe (400KHz/520W, calculated 0.021 W/cc,, SONOSYS Ultraschall systeme GmbH,
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Germany) immersed in the bath to give approximate in-bath total power of 0.067 W/cc.
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All treatments were repeated in two consecutive days with freshly prepared carrots.
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2.3.
Retorting
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The carrots were removed from the bath after the treatment, placed in 400 mL aluminium
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cans (75mmW×110mmH, VISYPAK, Australia) with equal amount of water by weight and
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cooled down to room temperature (~30 °C) in an ice bath. One can of treated carrot discs and
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strips from each treatment was reserved for mechanical and tensile testing. Thermocouples
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were attached to the centre of a representative carrot disc in one can for each treatment to
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record the temperature profile during retorting. The cans were then sealed and retorted in a
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FMC retort (Model Surdry AR-171, USA) overpressure rotary retort. Processing time was
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established based on an equivalent accumulated lethality (F) of 10 min at 121 °C. All
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treatments were retorted in one batch and two batches of retorting were carried out at
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consecutive days for repeated treatments.
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2.4.
Mechanical testing – compression
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Carrot cylinders were prepared according to the method of Singh et al. [38] to a final
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size of 15 mm in diameter and 10 mm in length. Compression tests were performed using an
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Instron 5564 instrument (Instron Pty Ltd., Melbourne, Australia) loaded with a 500 N cell and
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Ultrasound Sonochemistry
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a 35 mm aluminium cylinder at the speed of 60 mm/min to 60% strain. The true stress (σe)
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and true strain (εe) were calculated using the equations:
FH
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σe =
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
ε e = ln
 Ho 
(1)
Ao H o
 H 
(2)
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where F is the force measured during compression, Ao is the initial cross-sectional area of the
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sample, H o is the initial sample height and H is the actual height after deformation.
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Stress/strain plots were used to represent the typical mechanical behaviour of carrot
tissues and average strengths (maximum stresses at fracture) were calculated.
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2.5.
Mechanical testing – tensile
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Carrot strips (40 mm long, 7 mm wide, 2 mm thick) were clamped into a Linkham TST350
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tensile stage (Linkham Scientific Instruments, Surrey, UK) with both ends secured by
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superglue and double-sided sticky tape. A notch was made in the middle of the strip to initiate
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and guide the fracture. The tensile forces were recorded as the strip was pulled horizontally
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from both ends at a speed of 6 mm/min until the strip fractured completed. The tensile
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strength results were presented as an average of six strips for each processing treatment.
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2.6.
Microstructure characterisation by CLSM
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Flat subsamples of the phloem tissue from retorted carrot cylinders were cut by razor
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blade and stained by addition of a few drops of Congo red solution (0.1% in distilled water).
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Images of the microstructure was captured by a Leica TCS SP5 (Leica Microsystems GmbH,
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Wetzlar, Germany) using a HC PL APO 20× immersion fluid objective.
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Fractured carrot strips from the tensile tests were dismounted from the tensile stage.
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The fractured ends were stained with Congo red (0.1% in distilled water) for 5 minutes. The
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microstructure of the fractured samples was captured using an N PLAN, L 20× air objective.
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2.7.
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Statistical analysis
One-way Analysis of Variance (ANOVA, MINITAB®14) was used to analyse data
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with a confidence interval of 95.0 based on pooled standard deviations. Treatment
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comparisons with P-values smaller than 0.05, were considered significantly different.
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3.
Result and discussions
Thermal processing of carrots is known to result in plant tissue softening leading to
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poor sensory textural perception. The loss of tissue hardness is initially caused by damage to
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the cell membrane and associated loss of turgor pressure, which is then followed by the break
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down of cell wall biopolymers, namely pectins, through β-elimination reaction. Blanching at
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low temperature (e.g. 60 °C) for a relatively long time (e.g. 40 min) has been shown to
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improve mechanical strength of cell walls by stabilising the pectin structure [39, 40].
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In this study, this specific low temperature long time blanching (LTB) pre-treatment
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was used to compare the effectiveness of ultrasound treatment. Compression test of carrot
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cylinders prepared from the carrot pieces and tensile test of carrot strips that were treated at
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selected conditions were used to assess the mechanical properties of the carrots from various
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pre-treatment before and after the retorting process. It has been suggested that a combination
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of two or more frequencies of ultrasound can produce a significant increase in cavitations
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compared with single frequency [41], therefore a combination of 400 kHz and 40 kHz was
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selected for this study.
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3.1.
The effect of pre-treatments on the mechanical properties of carrots prior to
retorting
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Fig. 1 shows the maximum stress to fracture values for pre-treated samples compared to non-
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treated control (C1). Duplicate pre-treatment runs for each treatment conditions were carried
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out in two separate days. No significant differences were found between the two days for all
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treatment except the ultrasound only treatment at room temperature (USrt) (P=0.015). The
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control (C1) which had not been exposed to any pre-treatment, had the lowest maximum
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stress to fracture values of 2.9 N/mm2 than all the treated samples, whereas the carrot tissues
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treated with blanching at 60 °C for 40 min (LTB) showed the highest mechanical resistance
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(maximum stress = 4.0 N/mm2) to fracture. No significant differences were found for the
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carrot tissues treated by blanching for 10 min (LTBs), ultrasound at room temperature (USrt),
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or ultrasound at the blanching temperature. However they all had a slightly higher maximum
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stress to fracture values in the range of 3.3-3.5 N/mm2 than the control C1, but lower than the
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carrot tissues treated by LTB (P<0.005). Close examination of the compression curves for
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non-treated carrots (C1), and those pre-treated by ultrasound at 60 °C (US) and blanching
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(LTB) (Fig. 2) indicate that there is a significant difference in the strain-stress relationship
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between the raw carrots (C1) and those pre-treated carrots (LTB and US). A one-step steep
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increase in the stress as a function of strain was observed for raw carrots, whereas the pre-
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treated carrots underwent a two-stage stress response to applied strain: a much slower stress
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response initially up to the applied strain of 20%, followed by a similar rate of stress increase
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to that of raw carrots up to the point of its fracture. Not only were the maximum stresses in
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which the carrot tissues fractured higher for those pre-treated than raw, the fractures also
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occurred at a much higher strains than for the raw carrots.
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The differences in the mechanical behaviour of carrot tissues between non-treated and
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those treated by LTB and US were also observed for the strip samples that underwent tensile
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test (Fig. 3). An average maximum force of 3.9 N to fracture carrot materials (LTB and US)
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was observed, although a slight decrease compared to the control C1, the difference was not
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statistically significant (P=0.213). However, one major difference found between the control
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and treated samples in the tensile testing was the distance that the carrot strips had been
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stretched apart until they fractured. The pre-treated samples (both LTB and US) took
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approximately twice as long the distance to fracture compared to the control. The CLSM
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images taken of the fracture edge of the fractured strips showed that the tensile force had
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caused the cell walls to break leaving cell fragments along the fracture path in all of the
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examined cases regardless whether the carrot strips have been pre-treated or not (Fig. 3). The
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similarity of microstructure and comparable forces for the pre-treated carrot strips indicate
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that adhesion between the cell walls were much greater than the strength of the cell wall after
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the LTB and US treatment. The effect on the cell wall fracturing behaviour by pre-treatments
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(LTB or US) of carrot tissues was not obvious compared to the raw carrot tissues.
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As expected, there was a significant reduction of the apparent Young’s modulus for
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the raw carrots to those pre-treated carrots (prior to retort) due to the loss of turgor pressure
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caused by pre-treatments (Table 2). Both cell turgor pressure and the integrity of cell walls
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are important in determining the rigidity or firmness of plant materials. The mechanical
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failure of raw carrots is primarily caused by cell rupture initiated by the abrupt breakage of
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cell membranes and cell walls through increased internal pressure [38, 42]. Plant tissues
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losses turgor pressure rapidly when the tissue internal temperature reached 50 °C due to heat
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damage of cell membrane [4]. The LTB and US treated carrots are likely to have no or very
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low turgor pressure, thus the plant materials are more deformable and showing less resistance
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to compression (e.g. lower stress at early stage of applied strain) and elongation (e.g. lower
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force required to the same elongation distance). It has also been suggested that a more
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deformable material will require a higher force to fracture than a rigid one [43]. The
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maximum stress results to fracture the pre-treated carrots are in agreement this, no significant
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differences were found in the tensile forced required to fracture the carrot strips. The results
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provided similar evidence to that demonstrated by Trejo Araya et al. [44] who showed that
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when the turgor was lost after high pressure treatment, the maximum force required to cut
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through the sample increased and also at a at a long displacement. Both compression and
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tensile tests suggest that pre-treatment of carrot materials using a long time blanching (40
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min) or using ultrasound at a much shorter time (10 min) had similar effect on the mechanical
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properties of the carrot cell walls that became more elastic and deformable compared to the
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carrot tissues that had not been subjected to any treatment (C1).
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3.2.
The effect of pre-treatments on the mechanical properties of carrots after retorting
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Fig. 4 shows the maximum stress to fracture values of the carrot samples after
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retorting tested by compression. When comparing different pre-treatments, the control C1 and
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ultrasound treated at room temperature (USrt) had highest impact on carrot tissues softening
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demonstrated by their low maximum stress to fracture values. All the other pre-treated
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samples displayed a significantly increase in the maximum stress to fracture values compared
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to the C1 and USrt, however the extent of the increase was treatment dependent (P<0.05). In
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particular, the carrots pre-treated with LTB (60 °C, 40 min) and US (60 °C for 10 min)
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showed an increase of ~2.5–3 times in the maximum stress values to that of C1 or USrt.
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Interestingly carrot samples ultrasound treated for 10 min at 60 °C (US) exhibited a higher
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mechanical strength (P<0.05) than blanching only for the same time period (LTBs) (Fig. 4).
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Microstructural examination of the carrot cylinders after retorting showed some differences in
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the extent of cell separation and cell deformation dependent on the specific pre-treatment
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(Fig. 5). The C1 sample (no pre-treatment) displayed substantial gaps between the cells and a
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high degree of cell deformation, whereas the LTB and US treated carrots showed much less
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tight cell connection.
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Fig. 6 shows typical tensile profiles of the carrots strips from the control, LTB and US
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treatment respectively, after the retorting. The tensile strength of the carrot strips after
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retorting was overall much lower than those obtained prior to retorting, at ~10% of the stress
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values before retorting. Similar maximum force to failure values were found for the carrot
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strips which had been treated by LTB or US, but at ~1.5–2 times higher than the control C1
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(P<0.05). The CLSM images showed that the fracture of the carrot strips after retorting had
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propagated between the cells and cell walls were still intact along the fractured edge (Fig. 6,
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CLSM images). The distance that the carrot strips were stretched to before failure is shorter
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after retorting compared to them before retorting. Although there appears to be some small
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but significant differences (P = 0.013) depending on the various pre-treatments where the
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LTB can withstand a slightly higher strain than the US but neither of the pre-treatments
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showed a significant difference to the control (P = 0.25 and P = 0.16).
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In carrots, β-elimination reaction is pronounced in the highly methoxylated pectins
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and is the main contributor to thermal texture degradation during thermal processing [45].
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Severe thermal processing such as retorting is likely to cause significant changes to the
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pectins through β-elimination. Substantial cell separation observed after retorting, particularly
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in the non-treated carrots (C1) (Fig. 5) indicates that the middle lamella had been severely
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affected by the retorting process and the adhesion between the cells had become much weaker
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than the strength of the cell wall. The fact that LTB and US treated samples show similar
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fracture behaviour, e.g. through the middle lamella/intercellular joints with minimal cell
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rupture (Fig. 6, CLSM images), suggests that the higher values obtained for the maximum
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stresses to fracture in compression and tensile forces to failure are associated with the strength
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of the middle lamella prior to its failure in which the pre-treatment did give some protection
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to the breakdown of pectins. Improved mechanical properties due to low-temperature
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preheating were found to correlate with strengthened intercellular adhesion and with
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significant modification in matrix-bond pectins [45, 46]. Pre-treatment such as blanching at
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60 °C (LTB) provided a condition where PME activity was encouraged and resulted in a
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reduction in pectin methylester groups. This change to the pectin molecules provided a greater
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opportunity for the pectin polymers to be ionically cross-linked by divalent ions such calcium.
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In addition, the reduction in the degree of methylation in the pectins also reduces the rate and
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the extent of β-elimination reaction at high temperatures [47, 48]
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Interestingly, in this study, the carrot discs that had been pre-treated using ultrasound
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at room temperature had minimum effect on the improvement of carrot tissue mechanical
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properties after retorting. However, even with a very short time (e.g. 5 min, USs), application
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of ultrasound at 60 °C enhanced the mechanical properties of retorted carrots, more than
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blanching alone for 10 min (LTBs) (Fig. 4). Increasing the time of the ultrasound treatment to
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10 min (US), a further increase in the mechanical properties of retorted carrots was found,
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significantly higher than blanching alone for the same time period, though slightly lower than
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the long time blanching (LTB).
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The mechanism of how ultrasound alters cell wall structure and its related mechanical
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properties is still unclear. One theory is that it may penetrate the membrane in such a way that
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allows the diffusion out of inner content at a much earlier stage of heating, thus cause a
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gradual decease of tugor pressure, therefore lowing the impact on cell wall structure.
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Secondly, ultrasound waves may aid to a more efficient mass transfer, by doing so, it
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enhances the interaction of enzyme with the substrate and higher diffusion rate of ions
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through cell walls and more efficiently through bound cells across entire parenchyma tissue.
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Increased water diffusion through cell wall materials by ultrasound treatment have been
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reported to shortening the soaking time of chickpeas allowing.
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Alternatively, from plant physiology point of view, living plant tissues respond to
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physical stressors through stress response biochemical pathways that typically involve the
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rapid synthesis H2O2 which could enhance phenolic polymer cross bridging between cells and
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within cell walls, the suppression of pathways that would break down these structures,
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synthesis of cell wall carbohydrates, activation of specific PMEs and, if appropriate, the very
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rapid synthesis of waxy compounds to seal wounds against microbial invasion. It has recently
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been established through enzyme kinetic studies that phenolic synthesis is a mechanism
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involved in ultrasonic blanching at temperatures below those lethal to the plant’s enzyme
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pathways [49]. While it has also been demonstrated through gene expression analysis that
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carbohydrate modification and synthesis pathways are stimulated by a range of stressors [14,
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50-53]. Our recent study also showed the stresses to the plant during growth by limiting the
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supply of minerals could induce accelerated production of phenolic compounds and changes
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to the mechanical properties of the cell wall structure [38, 54]. Furthermore when plant tissues
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have been previously stressed they are more rapidly responsive when exposed to stress again
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[55]. Whether similar mechanisms are likely to be involved in LTB has not been definitively
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established.
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3.3.
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Synergistic effect of pre-processing treatments in the presence of CaCl2
The synergistic effect of LTB or US pre-treatment in the presence of CaCl2 was also
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investigated. Although a slight increase in the maximum stress to fracture of LTB and US
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treated carrots was observed compared to the control (C2) before retorting (Fig. 7(a)), they
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weren’t significantly different to those found for LTB and US treated carrots without CaCl2
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(Fig. 1). However both LTB and US treated carrots showed a significantly increase in the
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resistance to fracture, i.e. a 3–4 fold increase in the measured maximum stress, compared to
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the control (C2) (Fig. 7(b)). As expected, LTB treatment in the presence of Ca had synergistic
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effect on plant cell wall mechanical properties, a higher maximum stress to fracture value was
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found for the carrot treated in the presence of Ca (Fig. 7(b)) than LTB alone (Fig. 1).
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Similarly, a higher maximum stress was also required to fracture the carrots that have been
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treated by US with Ca than those treated by US without Ca, indicating also a synergistic
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effect.
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The presence of Ca during pre-treatment of carrots may serve two purposes: 1)
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promoting cell wall biopolymer cross-links, specifically pectin-Ca interaction, 2) accelerating
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PME catalysis. The amplified PME activity in the presence of cation is thought due to the
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shift of the pH optimum for the enzyme activity and competitive nature of cations with the
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enzyme which releases enzyme molecules that were initially bound to 'blocks' of carboxy
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groups on pectin [13, 16, 56]. The crosslink via divalent ion bridges on the resulting pectins
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(with a lower degree of methyl esterification) could form intermolecular networks within the
377
cell wall matrix, thus reinforce the cell wall strength and increase its firmness. It is interesting
378
that in the present study the addition of Ca2+ ions also had synergistic impact of the structural
379
parameters of the ultrasound treated retorted carrot resulting in an increase in the maximum
380
stress to fracture. Fig. 8 presented typical compression curves of carrots (after retorting) that
381
had been pre-process treated by LTB or US with or without Ca, showing the enhancement of
- 16 of 32 -
Ultrasound Sonochemistry
382
the mechanical properties of both pre-treatment and synergistic effects with the addition of Ca
383
ions. Interestingly, with a much shorter treatment time, US treatment in the presence of Ca
384
achieved similar effects to enhance the mechanical properties of carrot tissues to that of LTB
385
alone.
386
Without doing a whole of cycle analysis on a hypothetical processing situation there is
387
little that can be said about the energy inputs of calcium treatment vs LTB vs ultrasonic
388
processing? There is energy input into the manufacture of the calcium salt used vs. vessel
389
heat loss during holding the product in the LTB and the thermal and transmission
390
inefficiencies in providing electrical power to the ultrasonic transducers. The energy
391
requirement for the ultrasonic treatment is likely to improve by more than an order of
392
magnitude, depending on the processing vessel configuration, on scale up to production scale.
393
Suffice it to say that the electrical power involved is likely to be in the 10s of kW for an
394
industrial process and that the treatment can likely be applied during the hydraulic conveying
395
of the carrot pieces. Each processing plant differs in processing and energy management
396
practices and the significance of using a small amount of electrical power vs. recycled heat
397
will vary widely between plants. The greatest benefit in using the ultrasonic process vs.
398
calcium salt addition is likely to be in the market with clean labelling and possibly preferred
399
texture. The drawbacks of LTB are the size of treatment vessels required and the leaching of
400
water soluble nutrients.
401
402
403
4.
Conclusions
The potential for using ultrasound to improve the mechanical properties of canned
404
non-starchy vegetables has been demonstrated using carrot as an example. The mechanisms
405
involved appear to relate to the stress responses present in all living plant matter. However,
406
there is a need to clarify the relative importance of the potential stress mechanisms in order to
- 17 of 32 -
Ultrasound Sonochemistry
407
get a better understanding of the processing conditions likely to be most effective. The
408
amount of ultrasound treatment required is likely to involve low treatment intensities and
409
there are indications from the textural analysis that the texture induced is less tough and
410
rubbery than that accomplished using calcium ion addition or low temperature long time
411
blanching. Proper sensory evaluation is required to confirm this.
412
413
414
Acknowledgement
415
The authors would like to acknowledgement Ms Jenny Favaro, Mr Piotr Swiergon and Mr
416
Andrew Lawrence for their technical assistance during the experimental trials.
- 18 of 32 -
Ultrasound Sonochemistry
417
References:
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
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439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
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- 22 of 32 -
Ultrasound Sonochemistry
575
Table 1
576
Pre-treatment experiment conditions for carrot pieces with various thermal heating time with
577
or without ultrasound in the absence or presence of CaCl2.
578
Sample No.
Pre-treatment
Temperature, time
1
Control 1 (C1)
____
2
Low temperature blanching short time (LTBs)
60 °C, 10 min
3
Low temperature blanching long time (LTB)
60 °C, 40 min
*1
34 °C*2, 10 min
4
Ultrasound without heating (USrt)
5
Short time ultrasound at blanching temperature (USs)*1
60 °C, 5 min US*3
6
Long time ultrasound at blanching temperature (US)*1
60 °C, 10 min US*3
7
Control 2 (0.5wt% CaCl2, C2)
____
8
Low temperature long time blanching in 0.5% CaCl2 (LTB + Ca)
9
Long time ultrasound at blanching temperature in CaCl2 (US + Ca)
60 °C, 40 min
*1
60 °C, 10 min US*3
579
580
*1
581
Ultrasonics Corporation, USA) and a 400 KHz/520W probe (SONOSYS Ultraschall systeme
582
GmbH, Germany).
583
*2
Temperature of the water used.
584
*3
3-5 °C increase in temperature was recorded after ultrasound treatment.
Ultrasound was applied using a combination of an Ultrasonic Bath (40KHz/850W, Branson
585
586
587
588
589
590
591
592
593
594
595
596
- 23 of 32 -
Ultrasound Sonochemistry
597
Table 2
598
Elastic properties of raw and treated carrots before and after retorting.
Sample no.
Sample Treatment
Young’s modulus (compression)
Young’s modulus (tensile)
(N/mm2)
(N/mm2)
Prior to retorting
After retorting
Prior to retorting
After retorting
1
Control 1 (C1)
9.1
0.22
12.0 ± 1.1
0.7 ± 0.3
3
Low temperature
7.1
0.56
4.0 ± 2.6
1.4 ±1.1
6.4
0.57
5.3 ± 3.1
1.8 ± 0.6
blanching (LTB)
6
ultrasound (US)
8
Low temperature
0.70
blanching (LTB) + Ca
9
ultrasound (US) + Ca
0.57
599
- 24 of 32 -
Ultrasound Sonochemistry
600
c
4
b
b
2
Maximum stress (N/mm )
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
b
b
a
3
2
1
0
C1
LTBs
60 °C
10 min
LTB
60 °C
40 min
USrt
10 min
USs
60 °C
5 min
US
60 °C
10 min
626
Fig. 1. Maximum stress to fracture carrot pieces after blanching with or without ultrasound
627
pre-treatments. The compression test was carried out prior to retort.
- 25 of 32 -
Ultrasound Sonochemistry
628
629
Blanching (LTB)
2
Stress (N/mm )
4
3
Ultrasound
(US)
No treatment
(C1)
2
1
0
0.0
0.1
0.2
0.3
0.4
Strain
0.5
0.6
630
631
632
633
634
Fig. 2. Typical compression curves of carrot piece from the control (no pre-treatment), or
635
treated with low temperature low time blanching (LTB, 60°C, 40 min) or low temperature
636
with the application of ultrasound (US, 60°C, 10 min).
637
638
- 26 of 32 -
Ultrasound Sonochemistry
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
Fig. 3. Typical CLSM tensile profiles of carrot strips from the control (no pre-treatment), or
670
treated with low temperature low time blanching (LTB, 60°C, 40 min) or low temperature
671
with the application of ultrasound (US, 60°C, 10 min), and CLSM images of fractured edges
672
corresponding to each pre-treatment.
C1
LTB
US
5
Blanching
(LTB)
Force (N)
4
No treatment
(C1)
Ultrasound
(US)
3
2
1
0
0
1
2
3
Distance (mm)
- 27 of 32 -
4
5
Ultrasound Sonochemistry
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
Fig. 4. Maximum stress to fracture of carrot pieces pre-treated with various blanching and
699
ultrasound. The compression test was carried out after retorting.
2
Maximum stress (N/mm )
0.25
0.20
f
e
0.15
d
c
0.10
a
a
0.05
0.00
NT
LTBs
60 °C
10 min
LTB
60 °C
40 min
USrt
10 min
- 28 of 32 -
USs
60 °C
5 min
US
60 °C
10 min
Ultrasound Sonochemistry
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
Fig. 5. Micrographs of the carrot cylinders after retorting showing differences in cell structure
716
depending on the specific pre-treatment.
C1
LTB
US
25 µm
- 29 of 32 -
Ultrasound Sonochemistry
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
Fig. 6. Typical CLSM tensile profiles of carrot strips after retorting from the control (no pre-
745
treatment), pre-treated with low temperature low time blanching (LTB, 60°C, 40 min) or low
746
temperature with the application of ultrasound (US, 60°C, 10 min), and CLSM images of
747
fractured edges corresponding to each pre-treatment.
LTB
0.3
Blanching
(LTB)
Force (N)
Ultrasound
(US)
US
0.2
0.1
C1
No treatment
(C1)
0.0
0.0
0.5
1.0
Distance (mm)
- 30 of 32 -
1.5
2.0
Ultrasound Sonochemistry
748
0.25
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
Fig. 7. Maximum stress to fracture of carrot pieces pre-treated with various blanching and
773
ultrasound in the presence of 0.5% CaCl2. (a) prior to retorting; (b) after retorting.
(b)
(a)
2
2
Maximum stress (N/mm )
b
Maximum stress (N/mm )
b
4
a
3
2
1
0
g
h
0.20
0.15
0.10
b
0.05
0.00
CaCl2
C2
CaCl2
LTB
60 °C
40 min
CaCl2
US
60 °C
10 min
CaCl2
C2
- 31 of 32 -
CaCl2
LTB
60 °C
40 min
CaCl2
US
60 °C
10 min
Ultrasound Sonochemistry
0.25
Blanching (LTB) + Ca
0.20
2
Stress (N/mm )
Ultrasound (US) + Ca
0.15
Blanching (LTB)
0.10
Ultrasound (US)
0.05
No treatment (C1)
0.00
0.0
0.1
0.2
0.3
Strain
0.4
0.5
774
775
776
777
778
Fig. 8. Typical compression curves of carrot pieces after retorting. Carrot pieces were
779
prepared from the control (C1), pre-treated with low temperature low time blanching (LTB)
780
with or without 0.5% CaCl2, and low temperature blanching in combination with the
781
application of ultrasound (US) with or without 0.5% CaCl2.
782
783
784
785
786
- 32 of 32 -