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Energy Absorption Capacity of Packaged Roma
Tomatoes under Compressive Loading
F. A. Babarinsa and M. T. Ige.
Abstract — This study focused on the energy absorption capacity of Roma tomatoes as a strength parameter that characterizes mechanical damage of the packaged fruit when subjected to compression, in multi-layers. The aim was to relate the measured strength properties of the bulk fruit to mechanical failure at the bioyield, break and peak points of deformation. Compression tests were conducted on Roma tomatoes to investigate the effects of ripeness stage, vibration level and container type on energy absorption in the packaged tomatoes under compressive loading, using a 22x3 factorial experimental design. Tomatoes of three ripeness stages: unripe (5.6 Brix%) half-ripe (3.9 Brix%) and full-ripe (3.2 Brix%), were packed in plastic crate and raffia basket. These were subjected to three levels of vibration: non-vibrated, low vibration (frequency 3.7 Hz) and high vibration (frequency 6.7 Hz), using a laboratory vibrator. The fruits were
compressed in a Universal Testing Machine and absorbed energy at bioyield, break and peak points was measured. Energy absorption decreased significantly (P=0.05) with advancing ripeness stage of tomatoes and vibration level (P=0.001). Container effects were significant at peak. Average absorbed energy ranged from 1.140Nmm to 1.875Nmm at bioyield, 13.597Nmm to 27.221Nmm at break and
15.629Nmm to 23.618Nmm at peak. The work will facilitate our understanding of the contribution of fruit ripeness and transit vibration to mechanical damage in packaged tomatoes. The results obtained can be used by designers of packaging materials, and handlers of fresh tomato fruits in Nigeria to reduce mechanical damage, especially those due to compression.
Keywords: Compressive loading, Container, Energy absorption, Packaging, Ripeness, Roma tomato, Vibration.
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ackaged tomato (Lycopersicon esculentum Mill.) is highly susceptible to mechanical damage during transportation because the fruits are more perishable and tender than other fruits and vegetables. In Nigeria, major areas of tomato production are concentrated in the northern part of the coun- try and the main variety transported is the Roma, in consider- ation of its relative firmness. Nearly all the transported toma- toes are packaged in raffia baskets in very large quantities and carried in open trucks over long distances by inter-state roads.
The greatest concentration
As noted by Rorbertson [1], requirements of modern packaging of fresh horticultural produce include the preven- tion of mechanical damage resulting from compression. The development of a promising packaging system for tomato dis- tribution will depend much on the clear understanding of the mode spoilage resulting from their resultant effects of vibra- tion and compressive forces [2]. Every mechanical damage results from the transformation of energy from one form to another as energy is persistently imparted to and absorbed by
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the bulk of packaged commodity. During transportation and handling of fruits and vegetables, the vegetative cells are sen- sitive to such external influences as energy consumption [3]. Energy transformation occurs at the different points of me- chanical deformation denoted as the bioyield, break and peak points. Energy imparted to packaged commodities during transportation originate from drops or from accelerations transmitted to the fruit through the chassis and suspension system and finally through packaging container. Hinsch et al. [4] reported that, although frequencies of 3.5, 9, 18.5 and 25 Hz were of frequent occurrence during transportation, the most significative ones are the levels of 3.5 and 18.5 Hz. The se- quence of collisions occurring between the various layers of fruit,, as a vehicle passes over a hump, are energy absorbing [5]. Distribution system can therefore be viewed as a series of discrete energy inputs for the analysis and prediction of me- chanical damage. Mechanical injury through imparted energy may either destroy the physical integrity of the produce, re- sulting in breakage, cutting, splitting, or cause bruising [6]. Wills et al. [7] categorized mechanical damage to some fruits into compression, impact and vibration injuries. O’Brien et. al. [5] associated extent of damage to level of energy imparted to and absorbed by the produce during transportation. The amount of damage to produce is thus directly related to the energy absorbed. Energy absorption is closely related to me- chanical properties and, like other terms of strength parame- ters such as load (force), deformation and stress, energy is measured at points of bioyield, break and peak. The strength
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parameters of force (load), deformation and stress in packaged Roma tomatoes have been studied in relation to the bioyield point [8] and break point [9]. The measurement and under- standing of the energy absorption capacity of packaged toma- toes under compression will facilitate prediction of failure modes and the total amount of tissue damage in the pack [10]. Villarreal [11] noted that a method of predicting damage to packaged tomatoes from packaging studies could be based on the fact that bruising in the fruits is related to the energy ab- sorbed by the individual fruit.
The evaluation of packaging system for fruit and vegeta- bles in terms of mechanical damage using energy inputs has, thus, been proposed by Holt and Schoorl [12]. The authors found that the degree of resulting spoilage depends on the capacity of the fruit to absorb and withstand energy imparted by the resultant forces, particularly vibration and compression forces.
This study investigated energy absorption capacity of packaged Roma tomatoes at bioyield, break and peak points under compressive loading. It particularly studied the effects of stage of fruit ripeness, level of vibration and packaging con- tainer on the absorbed energy.
Tomatoes used in this study were of the Roma variety, hand- harvested from a local market farm in Ilorin suburb and sorted into three stages of maturity namely: unripe, half-ripe and full-ripe. Wholesome fruits were sorted for reasonable uni- formity into size range of 2.5 to 3.0cm and taken to the Engi- neering Material Testing Laboratory at the National Center for Agricultural Mechanization (NCAM), Ilorin. Stages of tomato ripeness were first judged subjectively by skin colour as 1) unripe (mature green/breaker or green pink, consisting of the first point of skin colour change from complete green to about
30% pink), 2) half-ripe, consisting of 30-70% pink to red skin and 1) using a number, 3) the full-ripe, consisting of 70-100% red skin but still firm, These were comparable to skin colour levels 2, 4 and 6 on another chart presented by McGlasson et al. [13]. The ripening stages are also equivalent to colour lev- els depicted as 1, 5 and 9 on the recommended tomato colour chart of the Organisation de Cooperation et de Development Economiques, Paris [14] .
Further objective evaluation of the ripeness stages was
done by measuring the total soluble solids (as Brix %) in the
undiluted juice of samples of the tomato fruit. The digital
hand-held refractometer (ATAGO® PAL-1 No.3810) used had
an automatic internal temperature compensation feature, a
measurement resolution of Brix 0.1% and accuracy of Brix
±0.2%. Approximately 0.3ml of the tomato samples was
blended to a uniform juice, and placed on the prism of the
digital refractometer. The total soluble solids content (in Brix
%), measured in triplicates, were 5.6, 3.9 and 3.2 for the un-
ripe, half- ripe and full ripe stages respectively.
The two packaging containers used are plastic crate (manu- factured by Shongai Packaging Industries Ltd) and raffia wo- ven basket. The plastic crate is the a nest/stack type that has been previously recommended by the Nigerian Stored Prod- ucts Research Institute for packaging tomatoes for road transportation [15]; it is similar to that described by Thomp- son [14] . The crate has external dimensions of 60cm x 40cm x
25cm high and is capable of holding 25kg of tomatoes. The
basket, which is extensively used in road transportation of
tomatoes in Nigeria, is 30cm deep and 43cm in diameter, ca-
pable of holding 20kg of tomato fruit. Both containers were
adequately ventilated and are sufficiently strong to resist fail-
ure by buckling.
A 2 x 32 factorial experiment was conducted to study the ef- fects of three ripening stages, three vibration levels and two containers on load, deformation and stress at bioyield point of Roma tomatoes under compressive loading.
The packaged tomatoes were vibrated using a mechanical vi- brator, the Gallenhamn Orbital Shaker (App. No 9B 3742 E). The respective containers were carried on the carriage plat- form vibrated by internally fitted oscillating cams, and impart- ing oscillation at the variable speed of 0-400 rev/min. Vibra- tion, designated either as low-level or high level, was applied at fixed frequencies of oscillation, 3.5 and 6.7 Hz respectively, by setting the operating speed at 200 or 400 rev/min for dura- tion of 60 minutes.
Absorbed energy was measured while compressing the toma- toes using the Testometric Universal Testing Machine (UTM), (manufactured by Testometric Co. Ltd. UK), with a force ex- erting capacity of 50kN (Fig. 1). The machine was installed in the Engineering Material Testing Laboratory of the National Center for Agricultural Mechanization (NCAM), Ilorin. The functional components of the testing machine include the load frame, load cell, crosshead, control console and a printer.
The compression test was conducted in triplicates by mounting and compressing the tomatoes in the loading space of the UTM. A pair of rigid plates of 1.27cm thick plywood was used as the force-transmitting devices, one as bottom support and the other as top loading device for the fruit. Load- ing rate (crosshead speed) of 2.50 mm/min was applied as recommended by Mohsenin [3]. The electronic computing unit of the UTM was set to measure selected strength parameters (load, deformation and stress) at the break point of the com- pressed tomatoes. Data sheets of measured values and load- deformation plots were obtained directly as produced with the aid of a PC.
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Data collected from compression test runs were subjected to statistical analysis using randomized complete block design based on a 32x2 factorial experiment. Statistical analysis was carried out using SPSS 110 software package. Treatment means were compared using Duncan’s Multiple Range Test (P< 0.05).
A typical compression load-deformation curve generated for a strength parameter at break of the compressed tomatoes is shown in Fig. 2. The curves generated for the fruit compres- sion generally had sharp peaks following the elastic defor- mation at the end of each compression, just as observed in our previous works [8], [9]. rather than rounded peaks. The point of maximum force or rupture coincided with the peak point on the force-deformation curves The observed behavior in compression has been attributed to soft, weak brittle materials by Fellows [16]. He particularly remarked that the point of maximum force or rupture could also occur at bioyield point. This, thus, explains why break and peak points may not be distinguishable from bioyield point in curves such as that in Fig. 1. The energy required to deform tomato fruit from the bioyield point is referred to as the modulus of resilence. The energy required to deform the fruit to its maximum maximum strength is referred to as the modulus of toughness. These two properties, however, could not be clearly differentiated on the force-deformation curves for the stated reason.
Table 1 gives the result of the statistical Analysis of Variance (ANOVA) of the data showing the effects of ripeness stage, vibration level and container type on the energy absorbed at the bioyield point. The statistical analysis indicates that the effects of level of vibration and those of ripeness stage were both significant (at P=0.001) on energy at bioyield. The effects of vibration, with F-value 20,661, were however stronger than the effects of ripeness with lower F-value of 8,835. The effects of container type were not significant on energy at bioyield. Vibration*Container interaction was the only two-factor inter- action observed to be significant (at P=0.05).
The statistical Analysis of Variance (ANOVA) of the data on the energy at break of compressed tomatoes is presented in Table 2. The effects of level of vibration and those of ripeness stage were significant (at P=0.001 and P=0.005, respectively) on energy at break. The effects of container type, however, were not significant on energy at break. None of the two-factor interactions showed significant effect. This implies that con- tainer type was not as important as vibration level and stage of ripeness in determining energy at break during compres- sion of packaged Roma tomatoes.
Table 3 gives the result of the statistical Analysis of Vari- ance (ANOVA) of the data for energy at peak of compressed
tomatoes. The effects of level of vibration and those of con- tainer type were both significant at P=0.001 on energy at peak while those of ripeness stage were significant at P=0.05. The effects of container type, with F-value 4,759, were not as strong as the effect of ripeness and vibration with higher F- values of 10,032. None of the two-factor interactions had sig- nificant effects on energy at peak.
The statistical analysis of variance means and differences among the three stages of ripeness tested were presented in Table 4. Energy absorption capacity of the compressed fruit is influenced by the stage of ripeness at the three points of de- formation tested namely: bioyield, break and peak points. The energy absorbed decreased with advancing stage of ripeness. Energy at bioyield reduced by 11.6% and 28.5%in the half-ripe and full-ripe stages respectively, as compared to values ob- tained for the unripe fruit. Following a similar trend, energy at break reduced from by 21.8% and 36.5% in the half-ripe and full-ripe stages respectively. Values of absorbed energy rec- orded for both break and peak points show just marginal dif- ferences. This can be attributed to the observation that the point of maximum force or rupture coincided with the peak point on the force-deformation curves as noted ealier.
Fruit at full-ripe stage consistently had the least energy
absorption, indicating that they are most susceptible to com-
pression damage at all points. The observed decrease in ab-
sorbed energy with advancing stage of ripeness implies that
tomato fruit at an early stage of ripeness is capable of absorb-
ing greater amount of imparted energy before yielding under compression. Thus, fruits at full-ripe stage had the least ener-
gy absorption capacity, and are likely to be most susceptible to compression damage at the respective points of deformation. On the other hand, fruits at unripe stage, with the highest en- ergy absorption capacity, are considered to be least susceptible to compression damage. This explains the observation of Vil- larreal [11] that unripe tomatoes form an energy-absorbing load. The author noted that these unripe tomatoes are more affected in energy dissipation such that all the imparted ener- gy could be dissipated in compaction rather than bruising. The energy available for deformation, hence damage, is related to the energy absorbed into the system as a function of strength properties of fruits.
Although the absorbed energy at peak point increased steadily with increasing ripeness stage, the mean values had no significant differences. The differences were significant only between the half-ripe and full-ripe stages, but not be- tween the unripe and half-ripe stages. In absolute terms, the decrease of energy absorption was greater below half-ripe than towards full-ripe stages. The observed reduction in ener- gy absorption capacity of compressed tomatoes as ripening advanced can be attributed to a variety of bio-chemical and textural changes which are known to contribute to the deterio- ration of the commodity texture and finally leading to disrup- tion of the cellular structure [17]. Upon the absorption of ener- gy, the tissues of the fruit undergo yielding, whereby its abil-
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ity to resist the applied load is drastically reduced and exhibits a continuous rupture plane where it has weakened substan- tially [18]. Rupture will also vary with cell turgidity, cell wall stiffness, and cell wall (plasmalemma) permeability, which all change as the tissue ripens. Schouten et al. [19]. thus reported that mechanical damage, perhaps, results in immediate loss in cell wall only in the riper fruit of tomato.
The statistical analysis of variance means and differences among the three levels of vibration is given in Table 5. Energy at bioyield, break and peak of the compressed tomato all re- duced with increase in level of vibration. Average absorbed energy, at bioyield, for example, reduced by 21% when low vibration was applied to the fruit and by 39% when vibration was applied at high level. Similar trends of response to vibra- tion application occurred at the break and peak levels of de- formation. This indicates that the resistance of the fruit to me- chanical damage (by yielding, breaking or rupture) of the tissues reduces according to the level of vibration imparted to them before compression. Wills et al. [7] noted that bruising of tissue damage results from strain energy being dissipated in the tissue. The amount of damage (measured as bruise vol- ume) depends on the amount of energy that is absorbed and the nature of the tissue. In line with the classification of fail- ure in some fruit and vegetables by Schoorl & Holt [10] from energy consideration, damage at bioyield can be associated with bruising. The energy absorption by the tomato fruit greatly determines the quality of the fruit during handling because yielding of the tissues, which results from compres- sion, accelerates subsequent deterioration of the fruit. Hence, if quality is to be ensured, the vibration resulting to such damage must be minimized.
The higher absorbed energy obtained for non-vibrated tomato at break point implies higher resistance to mechanical damage of cracking than vibrated fruit imparted with the same compression energy. Damage resulting from the break point in tomato is classified, from energy consideration, as cracking and is also associated with the amount of strain ener- gy being dissipated in the tissue [10]. Hence, while it is con- venient to model failure conditions in terms of stress, the basic failure mechanisms are defined in terms of energy required for cleavage to occur. For example, there must be sufficient ener- gy stored in the material to provide for the creation of new surface.
The results (Table 2) show that subjection of packaged to- matoes to vibration reduced their ability to withstand stress at break during compression. During vehicular vibration the tomatoes first get compacted as vibration force relocates the individual fruits relative to other fruits in the bulk and the contact surfaces move down. This resulting initial compaction of fruits thereby reduces interstitial space with little or no bruising. The compression effect on individual tomatoes is then determined by the relative motion of the upper com- pressing fruit and the lower compressed fruit at the contact surfaces. This modifies the deformation and changes the total
amount and distribution of energy dissipated into the fruit layers. The change in energy is thus affected by vibration at the various interfaces of each layer with increased number of contact points for contact compression. This eventually gov- erns the distribution of energy dissipation, during subsequent inter-layer compression when external load is applied.
Table 6 shows the statistical analysis of variance means and differences among the two types of container, plastic crate and raffia basket, tested during the compression testing of toma- toes. Energy at bioyield, break and peak points was all mar- ginally higher in crate than in basket, with the container ef- fects being significant (P=0.001) only at the peak point.
Results obtained on the effects of ripeness stage and vibra- tion level on energy absorption capacity of Roma tomato are useful to designers of packaging materials and handlers of fresh tomatoes. The results will facilitate reduction of mechan- ical damage in packaged tomatoes, especially those due to compression, thereby preserving the fruit quality. It has been noted that damage inflicted on fruits is related to energy available for bruising and the characteristics of particular fruits [20]. The energy available is in turn related to the energy input to the system, suspension characteristic of the vehicle and the properties and the packaging of the fruits. As can be clearly seen from the results, the fruits vibrated at high level absorbed the greatest energy.
Results of the present work investigated the resistance of packaged Roma tomatoes to compression damage based on the measurement of energy absorption capacity of the fruit as a strength parameter in multi-layers. The results revealed that advance in fruit ripeness stage as well as increase in level of applied vibration lower energy absorbing capacity of Roma tomato fruit. This implies that the compressed fruit, when handled at an early stage of ripeness can absorb greater amount of imparted energy before mechanical (compression) damage by yielding or breaking. The findings also point to the need to minimize the magnitude of forced vibration transmitted from the road during transportation of the pack- aged tomatoes. This prevents tissue failure, classified as cracking, slip or bruising that result from strain energy being dissipated in the tissue. The information obtained from this study can be of great help to designers of packaging contain- ers and handlers of tomatoes at various stages of distribution. This will minimize the mechanical damage especially those due to compression and ensure deliverance of good quality fruit to consumers and processors.
The authors sincerely acknowledge the cooperation and sup- port provided by the Director, National Center for Agricultur- al Mechanization (NCAM), Ilorin, Nigeria, for allowing the use of the Universal Testing Laboratory for the experimental
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work. Engr. O. A. Ogunjirin is particularly appreciated for his skillful technical assistance.
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TABLE 1.
STATISTICAL ANALYSIS OF VARIANCE OF RESULTS FOR ENERGY AT BIOYIELD OF ROMA TOMATO FRUIT
Source Type III Sum
df Mean
F Sig.
a
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TABLE 2.
STATISTICAL ANALYSIS OF VARIANCE OF RESULTS FOR ENERGY AT BREAK OF ROMA TOMATO FRUIT
a. R Squared =.576 (Adjusted R Squared = .465)
TABLE 3.
STATISTICAL ANALYSIS OF VARIANCE OF RESULTS FOR ENERGY AT PEAK OF ROMA TOMATO FRUIT
a. R Squared =.628 (Adjusted R Squared = .530)
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TABLE 4.
STATISTICAL ANALYSIS OF VARIANCE MEANS AND RIPENESS STAGES.
Means with the same letter were not significant- ly (p = 0.05) different.
TABLE 5.
STATISTICAL ANALYSIS OF VARIANCE MEANS AND VIBRIATION LEVEL.
Means with the same letter were not significantly (p =
TABLE 6.
STATISTICAL ANALYSIS OF VARIANCE MEANS AND CONTAINER TYPE.
Type of container
Fig. 1. Compression of tomatoes in plastic crate using
Testometric Universal Testing Machine. (UTM).
Fig. 2. Load-deformation curve for vibrated unripe tomatoes packaged in basket
Strength parameter
Crate Basket
Energy at yield (N.m) 1.543a 1.455a Energy at break (N.m) 20.607a 19.200a Energy at peak (N.m) 20.378a 18.136b
Means with the same letter were not significantly
(p = 0.05) different.
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