FIELD PROCESSING OF TOMATOES 2. Product Quality and Composition

SUMMARY— Mechanically harvested tomatoes were macerated, heated by steam injection to several constant temperatures from 170–240°F and held in a treatment pot from 0-4.6 min at pH levels (adjusted with cont. HCI) ranging from natural down to pH 2.0. Treated macerates were neutralized to original pH with cone NaOH, put through a finisher screen and pumped through a deaerator and chiller. The optimum processing conditions based on highest consistency (efflux-pipet flow time) in seven series of samples were 212°F breaking temperature and a macerate treatment time of 3.3 min at pH 2.75. With the optimum conditions, juice solids yield from VF-145 tomatoes was 4.3% greater and consistency 90% higher than from the natural pH juice. The neutralized acid products could contain 13% less tomato solids and still have a consistency equal to that of the products extracted at natural pH. Dry waste decreased 33% and total wet waste 47%. A 1-day delay before processing caused 9.5% loss in recovered juice solids. Ascorbic acid, reflectance and lycopene data showed no trends due to acid extraction. Water soluble color increased slightly in the acid extractions.     

FLOW BEHAVIOUR OF TOMATO KETCHUPS

The flow behaviour of 13 tomato ketchups was characterized using a coaxial cylinder viscosimeter. Power law values for the flow behaviour index n averaged 0.35 ± 0.016 which indicated the pseudoplastic nature of ketchups. The consistency coefficient K values for ketchups depended more on temperature, pectin and pulp contents than on total solids. Activation energy (Ea) values for flow of ketchups varied from 1.99 to 4.88 kcal/mole and were significantly related with pectin contents. The Casson's yield values for ketchups showed a significant correlation with pectin contents. Michaels and Bolger model parameters showed that much greater forces were involved in the breaking of network structure and aggregates in ketchups than needed for flow. Multiple regressions of K and apparent viscosity on pectin and pulp contents of ketchups were highly significant.     

Yield Stress and Relative Viscosity of Tomato Concentrates: Effect of Total Solids and Finisher Screen Size

Yield stresses of tomato concentrates, prepared using two finisher screens (0.69 mm and 0.84 mm diam) were determined using the vane method, Casson model, and dynamic rheological data. Magnitudes of vane yield stress were higher than those using other methods. They increased with increase in total solids and were about the same in concentrates from both screen sizes. Magnitudes of Casson viscosities of concentrates and serum viscosities from 0.84 mm screen were slightly higher than those from 0.69 mm finisher screen.     

EFFECT OF UNIMODAL PARTICLE SIZE AND PULP CONTENT ON RHEOLOGICAL PROPERTIES OF TOMATO PUREE

The vane method in controlled shear stress mode was used to determine the yield stress and the shear rate—shear stress data of tomato purees containing 10–35% pulp of two different average particle sizes: 0.34 and 0.71 mm. Consistency index and apparent viscosity increased significantly with pulp content and decreased with average particle size. The effect of pulp weight fraction (P) on relative viscosity (η_r) could be described by the single parameter equation: η_r= [1 – (P/A)]⁻², while the effect of particle diameter on η_r could be described using Peclet number. Magnitudes of yield stress determined directly by the vane method were higher than those obtained by using the Casson model, and were proportional to the square of pulp content. Reduced Casson yield stress—P data on purees of both particle sizes followd a single curve. Effects of pulp content and particle size on vane yield stress and apparent viscosity were evident from the correlation forms with high values of R².     

CREEP AND DYNAMIC RHEOLOGICAL BEHAVIOR OF TOMATO CONCENTRATES: EFFECT OF CONCENTRATION AND FINISHER SCREEN SIZE

Based on laser diffraction, pulp particles > 900 μm were detected in tomato concentrates; particles greater than 100–200 μm were found in serum from a concentrate. Creep-compliance and dynamic rheological data showed that viscoelastic properties of 21 °Brix tomato concentrates using a 0.84 mm finisher screen were higher than those from a 0.69 mm screen. Both types of viscoelastic properties were sensitive to increase in concentration in that they increased with increase in Brix of the concentrates. Magnitudes of instantaneous modulus (G₀= 1/J₀) of the tomato concentrates were of the same order of magnitude as the storage modulus (G') at 1 rad s^-1.     

APPLICABILITY OF FLOW MODELS WITH YIELD FOR TOMATO CONCENTRATES

Shear rate-shear stress data on about fifty tomato concentrates were employed to study the applicability of three flow models: Herschel-Bulkley (H-B), Mizrahi-Berk (M-B), and Vocadlo. The magnitudes of the three parameters of each model were determined by means of nonlinear regression analysis. The H-B and M-B models described very well the flow data. The logarithm of the apparent yield stress values predicted by each model and total solids of the concentrates were related by a quadratic equation. The magnitudes of the flow behavior index of each model did not change significantly over the total solids range studied: 6–36%. The consistency index of the Vocadlo and H-B model, and the concentration of the tomato concentrates were related by a power relationship. Over the temperature range 15–55°C, the flow behavior index of each model did not change significantly. The yield stress decreased with increase in temperature reaching a nearly constant value at 55°C. The consistency index of each model and temperature could be correlated by the Arrhenius equation.     

番茄汁液流变性质的关联

对经45、32、24和16目筛网过筛所得番茄汁液的流变性质进行了研究。采用旋转式粘度计测定了番茄汁的剪切速率-剪应力数据。通过剪切速率-剪应力实验数据的关联发现,番茄汁液的流变行为可用Bingham模型表征。确定了不同温度和不同网目下番茄汁液的稠度指数和屈服应力。同时也建立了不同网目下番茄汁液的表观粘度与温度函数关系的关联式。     

Effect of Particle Size,Temperature, and Total Soluble Solids on The Rheological Properties of Watermelon Juice: A Response Surface Approach

Central composite design was used to analyze the effect of particle size (0.075, 0.15, 0.25, 0.355, 0.425 mm), temperature (1.6, 5, 10, 15, 18.4°C) and total soluble solids (14.77, 25, 40, 50, 65.23°Brix) on the rheological properties of watermelon juice. Experimental values of consistency coefficient k, varied from 0.178–0.628 Pa sⁿ and flow behavior index n from 0.281 to 0.949. Regression equation was computed and used to predict the values of k and n. Results revealed that coefficient of determination (R²) and standard error for consistency coefficient k were 0.84 and 0.043 and for flow behavior index n were 0.42 and 0.102 respectively. Surface graphs showed that k value increased with increase in total soluble solids and particle size while decreased with temperature.     

RHEOLOGICAL PROPERTIES OF NONCOHESIVE APPLE DISPERSION WITH HELICAL AND VANE IMPELLERS: EFFECT OF CONCENTRATION AND PARTICLE SIZE

The proportionality constant, k_s, between shear rate, γ, and agitation velocity, N, for a helical ribbon‐screw (HRS) agitator was 17.8. Using the HRS agitator, values of consistency index K and the flow behavior index n of 14 apple pulp suspensions at seven different solids concentrations and two average particle diameters 0.71 mm and 1.21 mm were determined; in addition, values of the Casson viscosity η_c and yield stress σ_OC were also calculated. The magnitudes of K increased and of n decreased with increase in pulp concentration. Experimental values of the vane yield stress, σ_Oν, measured with a six‐blade vane increased with increase in pulp content. The values of σ_OC obtained using the Casson model were close to the experimental values σ_Oν. The effect of particle size on the relative viscosity, η_r, was correlated with Peclet number.     

VISCOSITY AND QUALITY OF TOMATO JUICE AS AFFECTED BY PROCESSING METHODS

ABSTRACT

Quality and viscosity of tomato juice is strongly governed by mechanical and thermal abuse during processing. The effect of processing and storage duration on the viscosity and quality parameters of tomato juice was evaluated in the present work. Tomato juice was obtained by two different methods, normal method (NM) and variable method (VM), and was heated by both conventional and microwave hot break treatments. Tomato juice was evaluated for various quality characteristics including precipitate weight ratio, degree of serum separation (DOSS), Brookfield viscosity, lycopene levels, vitamin C, phenols and antioxidant activity. Changes observed in several quality parameters during storage were statistically insignificant. The methods and processing had the greatest contribution toward viscosity, whereas storage duration had no significant effects. Viscosity parameters viz precipitate weight, DOSS and Bostwick consistency revealed that juice processed by VM was more viscous than that of NM, irrespective of the hot break processing used. Ascorbic acid and lycopene content varied significantly with mechanical methods and processing conditions. However, phenolic content and antioxidant activity remained stable. The variable process represents an improvement over the conventional by enabling tomato processors to improve the consistency of the tomato product.

PRACTICAL APPLICATIONS

Quartering or slicing tomatoes followed by hot break processing techniques can significantly improve the quality of tomato juice in terms of reduced degree of serum separation and precipitate weight ratio. The modified technology could be considered as a viable and economical method for small‐ and medium‐scale tomato processors with limited resources and facilities at their disposal.

     

Rheological Properties of Tomato Concentrates as Affected by Particle Size and Methods of Concentration

Shear rate-shear stress data were obtained on tomato concentrates made from juices that were produced using finisher screen openings (FSO): 0.020, 0.027, 0.033, and 0.045 in. In general, the apparent viscosity of the concentrates at a shear rate of 100 set^-1 increased with increase in FSO. However, concentrates made from juice using a 0.027 in FSO had the highest apparent viscosity. Magnitudes of yield stress of concentrates increased in direct proportion to FSO. Apparent viscosities of concentrates made by evaporating tomato juice were lower than those obtained either by evaporating the serum or by reverse osmosis concentration of the serum.     

IN-LINE MONITORING OF TOMATO CONCENTRATE PHYSICAL PROPERTIES DURING EVAPORATION

This work continues the development of an ultrasonic sensor system for in‐line measurement of the physical properties of fluid foods. The test system was a semi‐batch evaporator in which juice from three varieties of processing tomatoes was concentrated from 5 to 24 Brix. The physical properties of the tomato concentrates were measured and correlated to ultrasound properties. Speed of sound was linearly correlated with density, soluble solids content and total solids content of the tomato concentrates. Shear viscosity of the tomato concentrates flowing in viscometric pipe flow was evaluated using ultrasonic Doppler velocimetry (UDV). The UDV technique combines a fluid velocity profile and a simultaneous pressure drop to create a rheogram. Simple shear viscosity was modeled as power law behavior for tomato concentrates as a function of solids content for the three processing varieties. Off‐line measurements of apparent viscosity were well correlated to the in‐line measurements.     

EFFECT OF CONCENTRATION ON THE RHEOLOGY AND SERUM SEPARATION OF TOMATO SUSPENSIONS

The °Brix value of the tomato concentrate, from which tomato suspensions were prepared, was shown to have a large effect on the resulting apparent viscosity and storage modulus. The apparent viscosity of a tomato suspension prepared from a 30 °Brix tomato concentrate was only 35% of that of a suspension prepared from a 4.9 °Brix juice, both standardized on the same water unextractable solids (WUS) (0.65%) and total tomato solids (TS) level. After homogenizing the difference in apparent viscosity was only about 10–15%. A similar trend was found for the dynamic moduli. The decrease in rheological parameters of the nonhomogenized suspensions is probably partly related to the decrease in diameter of the tomato particles due to the concentration process. Moreover, microscopic fracture of the cellulosic, microfibrillar network of the cell wall plays a part. Serum separation of diluted nonhomogenized tomato suspensions was higher if made from a concentrate that was concentrated to a higher °Brix value. This phenomenon is discussed in terms of variations in uniaxial compression of the network in the diluted tomato suspensions caused by gravitational force.     

TREATMENT of TOMATO LYE PEELING WASTEWATER BY FERRIC CHLORIDE COAGULATION and FILTRATION

The suspended solids, soluble and insoluble COD, and particle size were measured for lye peeling wastewater from the tomato industry. the wastewater was treated by conventional coagulation techniques using ferric chloride as a coagulant. Acidification and carbonation were compared as methods to lower the pH. Jar testing was done using a pH level of 9–13 and coagulant levels of 8.6–51 mg/L. the most effective coagulation was achieved at a coagulant dose of 26 mg/L at a pH level of 9 resulting in an average residual turbidity reduction of 92%. Coagulation significantly reduced the insoluble solids (60%), but had no significant effect on dissolved organic matter, thus total COD was only decreased by 14.5%. Filtration had no effect on the suspended or dissolved solids in wastewater.     

THE ROLE of PULP INTERPARTICLE INTERACTION IN DETERMINING TOMATO JUICE VISCOSITY

Pulp interparticle interaction plays a major role in determining the viscosity of tomato juice. Indices of second order and of compounded high order interparticle interaction have been evaluated by diluting tomato juice samples with their own serum, and determining the slope of the reduced viscosity curves at the very dilute and at the original concentration regions, respectively. the second order interaction index, reflecting the behavior of a pair of colliding particles, has practically a similar value for samples having the same process history. On the other hand, tomato juice samples differ markedly in their value of the interaction index describing the effect of high order interparticle interaction on the viscosity at the original juice concentration. the higher that value, the higher the viscosity. the value of this index decreases by concentrating the tomato juice and increases by homogenization. It has a maximum value at pH 4–4.5 and most importantly it is greatly dependent on the mechanical abuse history of the juice. Enzymatic depolymerization of the soluble and especially the insoluble pectin markedly reduces the value of the interaction index, indicating that pectin plays a major role in determining the level of that interparticle interaction.     

EFFECT OF PROCESSING CONDITIONS ON THE COMPOSITION AND RHEOLOGICAL PROPERTIES OF APPLESAUCE

The relation between the composition and rheological properties of applesauce was studied in two ways. For artificial applesauces, reconstituted from isolated applesauce particles and applesauce serum, the residual shear stress increases exponentially with pulp content. Below a particle size of 0.625 mm the values of residual shear stress and apparent viscosity are lower for smaller particles. The effect of the processing conditions (finisher screen openings and cooking time) and raw material (apple variety and ripeness) on particle size and pulp content was studied on sauce made in a pilot plant. The particle size and pulp content of these applesauces affect the rheological properties in the same way as was shown for the artificial applesauces.     

Linear viscoelasticity of tomato sauce products: influence of previous tomato paste processing

Tomato paste is used as an ingredient for the manufacture of tomato sauce products. This work deals with the influence that some tomato paste processing variables (finisher screen opening and breaking temperature) and tomato variety exert on the linear viscoelastic properties of tomato sauce products. Four tomato varieties (two commercial varieties, one with a lower content of polygalacturonase enzyme, and one with a low content in pectin methylesterase enzyme) have therefore been used. Dynamic viscoelastic tests and particle size distribution (PSD) measurements have been carried out. From the experimental results obtained, we may conclude that the dependence of the linear viscoelastic properties of tomato sauce on the above-mentioned variables is very complex. Nevertheless, the evolution of the viscoelastic characteristics of these products is mainly related to their mean volume diameter and tomato paste water insoluble solids content (WIS). This solid content is mainly controlled by tomato variety and tomato paste processing conditions.     

EFFECTS OF pH ON QUALITY OF STORED TOMATO JUICE

Tomato juice was prepared by the hot-break method and filled hot into cans containing sufficient concentrated HCl to adjust pH to 4.0, 3.5, 3.0, 2.5 and 2.0. Controls were also prepared with no HCl added. Product was stored at ambient (18–22°C) and 40°C up to 3 months. As pH decreased below 4.0, consistency also showed a significant decline through pH 2.0. At the same time there was no apparent pH effect on serum viscosity. Further studies indicated that this effect was noticeable after 4 hr, at pH 3.0 to 5.0. Addition of as much as 1% NaCl to tomato juice had no effect on consistency of tomato juice. Lightness (Hunter color L ) decreased with pH after 3.5, redness (a_L) decreased after pH 4.0, and yellowness (b_L) decreased after pH 3.0. All storage effects were more profound at the higher temperature. The storage of tomato juices at pH less than 4.0 does not appear to be advantageous to the processor.     

EFFECT OF HOMOGENIZATION PRESSURE ON CONSISTENCY OF TOMATO JUICE 1

Pressurized homogenization of tomato juice at room temperature (28C) leads to an increased consistency and reduced serum separation in the juice. Percentage increase in consistency (as measured by Libby's tube) of hot processed juice from different cultivars ranged from 95 ± 3 to 282 ± 13 s. The magnitudes of change in consistency and serum spearation of cold processed juice due to homogenization were smaller than those for hot processed juice. Increases in consistency of cold processed juice ranged from 42 ± 4 to 213 ± 10 s. Consistency in all the samples increased with increasing pressure of homogenization up to a pressure of 3000 psi, above which it became constant. Homogenization of juice at higher temperature resulted in a very small increase in consistency. However, the magnitude of increase in consistency decreased with increasing temperature. Serum separation slightly increased at higher pressure of homogenization, after reaching a minimum with increasing pressure initially. In general, the juice with higher initial efflux and serum viscosities showed a larger increase in consistency and greater reduction in serum separation.     

IRREVERSIBLE SHEAR THINNING AND THICKENING OF TOMATO JUICE

Tomato juice behaves as a weak gel of pulp network. Shearing of the juice disrupts that pulp network causing an irreversible loss of viscosity, that follows a first order kinetics with a time constant that is shorter the higher the shear rate (e.g. 900, 80 and 15 s for rates of 10, 100 and 1000 s⁻¹, respectively). However, at a high enough shear rate (e.g., 1000 s⁻¹) the disruption of the pulp structure is immediately followed by a counteracting process that partially restores the capability of the suspended particulates to reform a new network and in turn recover a large part of the lost viscosity. Unfortunately, that recovery process requires high enough shear rate and long time, a combination which is rare in common industrial practice. Normally, the mechanical shearing during processing of tomato juice might be a major cause of an irreversible loss of viscosity.