A multi-parameter approach to optimise the design and performance of ventilated fruit packaging

Abstract

Ventilated packaging is a critical component in the fruit cold chain and influences both the preserva-tion of fresh produce quality, as well as overall cold chain efficiency. Many package design types are currently used in the SA export industry, however, very few have been fully optimised to function effectively within the cold chain. This article, therefore, presents some of the findings from a recently completed PhD project, which investigated the effectiveness of three new vent hole designs for cartons packed with pome fruit on trays. Specifi-cally, the work made use of a novel multi-parameter analytical approach in conjunction with computa-tional fluid dynamics, which sub-categorised carton performance into mechanical strength, fruit cooling rate and fan-related power consumption. The work showed that substantial improvements in current carton designs are still possible using this approach.

1. Introduction

The process of exporting fruit from packhouse to retail entails a complex system of procedures and treatments across multiple locations. Packaging
is thus a critical design component of the fresh produce cold chain, as it facilitates desirable inter-actions between the produce and the surrounding conditions (e.g. gas exchange, heat transfer and mechanical forces). However, a consequence of the many fruit types in production and the various unique cold chain requirements is the presence of a plethora of different carton designs. To illustrate, a survey of packaging used for pome fruit export between 2010 and 2011 in SA, identified over 11 unique ventilated packaging designs (Berry et
al., 2015). Interestingly, the survey (summary in Figure 1) showed that pear fruit were primarily exported in Mk6 and Mk7 cartons, whereas apple fruit were exported in mainly Mk4 and Mk9 cartons. This preference for a specific package type can be largely attributed to direct requests from the export markets, who specify explicit packaging require-ments. A contribution to the plethora of packag-ing types is that each type of carton is packed in combination with various internal packaging types, such as trays, liner bags and carry bags (thrift bags). Despite this diversity, a long-term goal of the horti-cultural industry has been to optimise vent hole designs for improved cold chain efficiency and fruit quality preservation. However, a package design can only be considered optimal if it caters for the various functionalities of the respective design space (i.e. cold chain). To illustrate, after packaging, pallet stacks are often first forced-air cooled (FAC; horizontal airflow) using relatively high air speeds (0.2 and 0.6 m s-1). This pre-cooling step is critical to extending fruit quality preservation, since fruit respiration rates increase considerably with increas-ing temperature. Inadequately cooled fruit entering the subsequent storage and transport phases may, therefore, respire more rapidly and, ultimately have a reduced shelf life at the retailer. The primary method to increase cooling performance during FAC is to improve vent hole designs on the carton since the alternative option of increasing the airflow rates will substantially increase fan-related energy consumption. Nevertheless, the disadvantage of vent holes is a significant reduction in carton strength, which in turn exposes fruit to mechanical damage, particularly during transit (loading with forklifts, truck, rail and ship). Two additional factors to consider during carton design are the practicality (e.g. ergonomics and compatibility) of the design to the cold chain conditions, as well as the cost of a design. Specifically, the cost of the package with respect to materials, manufacturing and energy consumption during cooling should always be consid-ered (Figure 2 provides a visual depiction of these functionalities, demonstrating the need for a compromise between the contradic-tory design features).

Figure 1: Percentage of carton types used for export between 2010 and 2011, other designs not included. The Mk4, Mk6 and bushel cartons are examples of telescopic designs, whereas the Mk7 and Mk9 are retail display carton designs.

Figure 2: Competing factors influencing carton design, where the circle area denotes the general level of design importance.

2. Recommendations in scientific literature

A common observation in past studies is that current packaging systems still have considerable potential for improvement, which could lead to increased energy efficiency and effectiveness of the cold chain with respect to fruit quality preservation. Prior work in this regard has mainly focussed on identifying an improved vent hole design (size, position, shape and configuration), typically for cartons packed with loose fruit (e.g. citrus). Various recommendations for improved vent hole designs have been made in literature and several general guidelines are thus available (Pathare et al., 2012). Principally, it has been shown that it is the vent hole area and not the shape, that has the largest effect on cooling performance. However, vent holes should be positioned to evenly distribute airflow to all the fruit of the package, particu-larly when the interior is compartmentalised by internal packaging (Zou et al., 2006a, 2006b). A general recommended compromise between corrugated fibreboard carton strength and adequate air penetration is 5-6% total ventilation area (TVA) across all carton wall surfaces (Mitchell, 1992). However, plastic crates, which are substan-tially stronger than cartons can generally have TVAs of up to 25% without compromising structural integrity (Vigneault & Goyette, 2002). A critical factor to consider is the presence of internal packaging in the carton, which can interfere with the effectiveness of the placed vent holes. For example, Ngcobo et al. (2012) showed that the use of liner bags in table grape cartons can increase airflow resistance between 50-83% as a result of the vents being directly obstructed. Past work has also shown that the use of multiple vents versus smaller ones can distribute airflow to packed fruit more evenly, and is a useful approach to preserving mechanical strength (de Castro et al., 2005; Mitchell, 1992). Additionally, Singh et al. (2008) and Han & Park (2007) observed a linear relationship between vent hole TVA and carton strength. The authors also showed that the use of extended (oblong) circular vent holes, parallel to the carton height, preserved carton strength better. The strength of a carton is closely related to the material properties of the board used. The interaction between board grade and vent hole design has not been well addressed in past literature. This can be attributed to the large variability in board types, for example, each manufacturer produces several unique fibreboard types in various grades (g m-2), the combinations of which can be used to manufacture numerous corrugated fibreboard types, each with different fluting and liners. Furthermore, the actual cost of fibreboard is generally dependant on location and supplier, which makes providing a specific recommendation challenging.

3. A multi-parameter approach

Most of the recommendations on vent hole design discussed above have been determined based only on one or two performance functionalities (Figure 2). Contradicting parameters are, therefore, not emphasised, making the recommendations difficult to apply in commercial practice. A multi-parameter evaluation is a useful approach that incorporates the most pertinent functionalities for the respective design space and identifies appropriate design trade-offs. The application of a multi-parameter evaluation was thus the premise of a recent PhD study in agricultural engineering by Dr Tarl Berry, which was completed at the department of mechani-cal engineering (March 2017). Several of the most relevant performance parameters used to investigate the various packaging functionalities are discussed below, followed by a case study example.

3.1. Packaging functionalities and performance parameters

3.1.1. Cooling and airflow related parameters

The main priority during a cooling operation is to facilitate rapid and uniform fruit cooling rates, to quickly remove undesirable heat (e.g. field heat, respiration heat or heat gain after a cold chain break). However, an important consideration is the temperature tolerances of the fruit, which if exceeded, could cause either chilling or freez-ing injury. In practice, the cooling period can be quantified using the seven-eighths cooling time (SECT; hours), which represents the time period needed for the fruit to decrease by seven-eighths of the intended temperature change. The other factor to consider is the uniformity of cooling in a pallet stack. Practically, it is not possible to measure every location in a pallet stack and probed locations must, therefore be selected with care, since measure-ments taken from a well cooled location can give a false indication of the overall cooling rate (volume-averaged temperature). A complementary performance parameter, is the convective heat transfer coefficient (CHTC), which represents the heat transfer rate across the fruit surfaces with respect to the temperature difference (W m-2 K-1). The CHTC value, however, can only be effectively predicted using computational methods. Full spatial predictions of the respective airflow patterns, airflow rates and air temperature inside the packaging is needed. Therefore the CHTC is also a valuable indicator of cooling uniformity, which can be determined by the variation in high or low heat transfer rates. The package ventilation design has a substantial influence on the airflow patterns in a pallet stack and thus on the various cooling performance parameters. Knowledge of the airflow distribution and cooling uniformity in a pallet stack is also criti-cal, as it informs practitioners about ideal measure-ment locations and also guides package designers towards improved ventilation configurations. The ventilation in a pallet stack also has an impor-tant effect on the airflow resistance through the system. Typically, this parameter can be quantified by determining the quadratic relationship between the airflow rate (m3 s-1) and the pressure drop (Pa) across the system. The fan-related energy consump-tion (J, kW h) can further be calculated using a combination of the resistance to airflow (m3 s-1 and Pa) and cooling duration. It is also important to note, that the fan setup in FAC equipment should ideally be calibrated so that the working point of the system (power usage) operates at an efficient rate. This is determined by the intersection of the system curve (packaging airflow resistance) and the fan performance curve. Where the fan performance curve is dependent on the respective fan and motor efficiencies (Baird et al., 1988). In the past, the parameters listed above were investigated individually using experimental methods. The recently introduced computational fluid dynamics (CFD) predictive modelling allows a user to rapidly evaluate cooling, airflow and fan-related energy consumption parameters at a high resolution of detail and at a fraction of the cost.

3.1.2. Mechanical strength

Packages need to be sufficiently strong to protect produce from compression, impact and vibrational forces. Compression forces are typically consid-ered as the most important mechanical factor, since cartons are stacked into tall (~2 m) pallet stack structures, the total mass of which must be supported by cartons at the pallet bottom. Additionally, the carton also loses mechanical strength over time, as a result of the high humid-ity conditions, which gradually break the various hydrogen bonds between the individual cellulose fibres. Box compression strength is thus assessed using box compression tests (BCTs), which evaluate the compressive force (N) versus the cross-head displacement (m) in a load-deflection curve (Frank, 2014). The actual box strength is then often defined as the peak compression force or the peak force up till a pre-specified deformation distance, however, a detailed analysis of the data is always necessary. Packaging should provide cushioning from shocks and vibrations as a result of handling and transport. A package’s resilience against impact forces can be quantified by evaluating the peak acceleration as a function of the static load (Pa) for different drop heights (dynamic cushioning curves) (Guo et al., 2011; Wang, 2009). Generally, however, it is the internal packaging that protects fruit from impact damage. For instance, trays in cartons can reduce both incidence and susceptibility of bruising by 50%, compared to apples packed in carry bags
(Fadiji et al., 2016).

3.2. Multi-parameter assessment of vent hole design – A case study

3.2.1. Material and methods

Mechanical strength evaluations were performed experimentally using box compression strength tests. Conversely, cooling rate, cooling uniformity, airflow and fan-related power consumption performance parameters were evaluated using
a validated computational fluid dynamics model (Defraeye et al., 2013). The full details regarding the experimental and numerical setups can be found in Berry et al. (2016, 2017).

Figure 4: Fan-related power usage to achieve a seven-eighths cooling time of five hours for a single carton packed with apple fruit (Berry et al., 2016).

Figure 5: CFD prediction of the CHTC and airflow distribution in a Mk4 carton, when using the four proposed vent hole designs at 1.0 L kg s-1 flow rate.

3.2.3. Role of vent hole configuration

The airflow distributions and CHTC are shown in Figure 5 and the results demonstrate a clear interaction between vent hole design and the internal packaging. After including trays, both the EV and ST configurations required 700% and 31% more energy (higher flow rates), respectively, to cool the produce to set temperature. This can be attributed to the influence of the trays, which caused airflow to bypass the central regions of the carton (Figure 5).

Small modifications to the EV design, for instance, extending the vent holes horizontally would, there-fore be a potential solution to this challenge.

The MV vent hole configuration enabled the most energy efficient cooling for both cartons packed with and without trays. Specifically, the MV configuration reduces energy consumption compared to the ST configuration by 58% and 25% for cartons packed with and without trays, respectively. Similarly, the AV configuration also improved energy consumption with trays (38%) and without trays (11%). This observation is consistent with literature, which recommended centrally positioned vent holes for loose fruit. However, this study showed that the vent hole configuration strategy must be substantially adjusted when using internal packaging.

Figure 6 shows carton mechanical strength for each of the four vent hole configurations and three board types examined. Results showed vent hole configuration had a significant effect on carton strength. However, the type of board used to manufacture the carton has a significant interaction with respect to the effect of configurations. When using B-flute, the MV and EV configurations had a somewhat larger peak compression force than the AV and ST configurations. For C-flute, the ST vent hole configuration was significantly stronger than the rest and for BC-flute board, the EV configuration showed the larger resistance to compression force. It should be noted, however, that the type of fibreboard used in the liners and fluting is an important factor and influences the vent hole configurations effect on strength.

Figure 6: Peak compression force for an unventilated carton (control), ST, AV, EV and ST vent hole configurations. Error bars indicate standard error of the mean. B, C and BC fluting structures are illustrated at the top of the graph (Berry et al., 2017).

Figure 7: Effect of TVA on (a) Average peak compression force for the three board types and (b) average power usage to cool one carton packed with fruit to set temperature in five hours. Error bars indicate standard error of the mean.

3.2.4. Role of board material properties

According to the discussion above, the identification of an optimal carton vent hole configuration is highly dependent on the type (fluting and fibreboard combinations) of board material used. Largely, this can be attributed to the unique mode of failure that is specific to the interacting board properties and carton design. The implications of this interaction indicate that mechanical related recommendations are only relevant to the specific board and conditions tested and cannot be reliably extended to other board types or conditions (e.g. high humidity conditions).

3.2.5. Role of carton vent size

Figure 7 shows the average peak compression force with respect to vent hole size for the three board types investigated. Results show a negative linear correlation between the TVA and peak compression strength. Interestingly, the effect on strength of using different vent hole configurations was equivalent to about a 2% change in TVA. Figure 7 further shows the effect of TVA on cooling performance with respect to power usage and cool-ing period. The results show a negative non-linear relationship between the TVA and power usage. The improvement in power usage by increasing TVA thus has diminishing returns when considering the linear loss in compression resistance.

4. Practical applications

The discussions above highlight the value of using a multi-parameter approach to identify trade-offs between various package functionalities. The use of CFD models thus allow users to rapidly quantify the various performance parameters relating to flow and cooling. Mechanical aspects can further be evaluated through rapid prototyping and experimental BCT methods. The multi-parameter approach can be applied to new package design proposals, before experimental implementation. Additionally, innovative and high risk conceptual packaging systems can also be evaluated without the need for specialised facilities or expending costly resources.

The proposed Multivent was shown to be a promising vent hole design for improved forced-air cooling efficiency, which also achieves similar or greater mechanical strength compared to other vent hole designs, if matched with an appropriate board type. An additional factor to consider is that the cooling process continues in-transit, using a vertical airflow system. In this regard, the Edgevent design, which has vent holes located along the top and bottom carton edges can easily be modified to facilitate an uninterrupted passage of vertical airflow through a pallet stack.

5. Future prospects

Our goal for future work in this area is to extend the recommendations discussed above to other types of fruit and more complex packaging systems, such as cartons packed with other internal packaging types and alternative cooling processes. A common cold chain challenge, for example, is to effectively cool produce packed using either carry bags or liner bags with a vertical airflow system (i.e. refrigerated freight container). The ultimate goal is to develop novel approaches to the design and performance optimisation of future fruit packaging.

The flexibility of CFD models also invites the potential for many other performance parameters to be included during a cold chain evaluation. For instance, predicting the distribution and adsorption properties of 1-MCP in various applications in storage/packaging configurations (Ambaw et al., 2014). Similarly, the practicality of other postharvest chemical treatments can also be investigated. Ultimately, these approaches and tools will enable the fresh fruit industry to optimise both the performance of the cold chain and corresponding packaging systems so that fruit quality is optimally maintained, with minimum expenditure of energy and utilisation of natural resources.

Acknowledgments

This work was based upon research supported by the South African Research Chairs Initiative of the Department of Science and Technology and National Research Foundation.

References

AMBAW, A., VERBOVEN, P., DELELE, M. A., DEFRAEYE, T., TIJSKENS, E., SCHENK, A., VERLINDEN, B. E., OPARA, U. L., & NICOLAI¨, B. M. (2014). CFD-based analysis of 1-MCP distribution in commercial cool store rooms: Porous medium model application. Food and Bioprocess Technol-ogy, 7(7), 1903–1916.

BAIRD, C. D., GAFFNEY, J. J., & TALBOT, M. T. (1988). Design criteria for efficient and cost effective forced air cooling systems for fruit and vegetables. ASHRAE Transactions, 94(1), 1434–1454.

BERRY, T. M., DEFRAEYE, T., NICOLAI¨, B. M., & OPARA, U. L. (2016). Multipa-rameter analysis of cooling efficiency of ventilated fruit cartons using CFD: Impact of vent hole design and internal packaging. Food and Bioprocess Technology, 9(9), 1481–1493.

BERRY, T. M., DELELE, M. A., GRIESSEL, H., & OPARA, U. L. (2015). Geometric design characterisation of ventilated multi-scale packaging used in the South African pome fruit industry. Agricultural Mechaniza-tion in Asia, Africa, and Latin America, 46(3), 1–19.

BERRY, T. M., FADIJI, T. S., DEFRAEYE, T., & OPARA, U. L. (2017). The role of horticultural carton vent hole design on cooling efficiency and compression strength: A multi-parameter approach. Postharvest Biology and Technology, 124, 62–74.

DE CASTRO, L. R., VIGNEAULT, C., & CORTEZ, L. A. B. (2005). Effect of container openings and airflow rate on energy required for forced-air cooling of horticultural produce. Canadian Biosystems Engineering, 47, 1–9.

DEFRAEYE, T., LAMBRECHT, R., TSIGE, A. A., DELELE, M. A., OPARA, U. L., CRONJÉ, P., VERBOVEN, P., & NICOLAI¨, B. M. (2013). Forced-convective cooling of citrus fruit: Package design. Journal of Food Engineering, 118(1), 8–18.

FADIJI, T. S., COETZEE, C., PATHARE, P. B., & OPARA, U. L. (2016). Suscep-tibility to impact damage of apples inside ventilated corrugated paperboard packages: Effects of package design. Postharvest Biology and Technology, 111, 286–296.

FRANK, B. (2014). Corrugated box compression—a literature survey. Packaging Technology and Science, 27(2), 105–128.

GUO, Y., XU, W., FU, Y., & WANG, H. (2011). Dynamic shock cushioning characteristics and vibration transmissibility of X-PLY corrugated paperboard. Shock and Vibration, 18, 525–535.

HAN, J., & PARK, J. M. (2007). Finite element analysis of vent/hand hole designs for corrugated fibreboard boxes. Packaging Technol-ogy and Science, 20(1), 39–47.

HO, Q. T., CARMELIET, J., DATTA, A. K., DEFRAEYE, T., DELELE, M. A.,

HERREMANS, E., OPARA, U. L., RAMON, H., TIJSKENS, E., VAN DER SMAN, R. G. M., LIEDEKERKE, P. VAN, VERBOVEN, P., & NICOLAI¨, B. M. (2013). Multiscale modeling in food engineering. Journal of Food Engineering, 114(3), 279–291.

MITCHELL, F. G. (1992). Cooling Methods. In A. A. Kader (Ed.), Post-harvest technology of horticultural crops (2nd ed.). USA: University of California: Davis, California.

NGCOBO, M. E. K., DELELE, M. A., OPARA, U. L., ZIETSMAN, C. J., & MEYER,

452. J. (2012). Resistance to airflow and cooling patterns through multi-scale packaging of table grapes. International Journal of Refrigeration, 35(2), 445–452.

PATHARE, P. B., OPARA, U. L., VIGNEAULT, C., DELELE, M. A., & AL-SAID, F. A. J. (2012). Design of packaging vents for cooling fresh horticultural produce. Food and Bioprocess Technology, 5(6), 2031–2045.

SINGH, J., OLSEN, E., SINGH, S. P., MANLEY, J., & WALLANCE, F. (2008). The Effect of ventilation and hand holes on loss of compression strength in corrugated boxes. Journal of Applied Packaging Research, 2(4), 227–238.

VIGNEAULT, C., & GOYETTE, B. (2002). Design of plastic container open-ing to optimize forced-air precooling of fruits and vegetables. Applied Engineering in Agriculture, 18(1), 73–76.

WANG, D. (2009). Impact behavior and energy absorption of paper honeycomb sandwich panels. International Journal of Impact Engineering, 36(1), 110–114.

ZOU, Q., OPARA, U. L., & MCKIBBIN, R. (2006A). A CFD modeling system for airflow and heat transfer in ventilated packaging for fresh foods: II. Computational solution, software develop-ment, and model testing. Journal of Food Engineering, 77(4), 1048–1058.

ZOU, Q., OPARA, U. L., & MCKIBBIN, R. (2006B). A CFD modeling system for airflow and heat transfer in ventilated packaging for fresh foods: I. Initial analysis and development of mathematical models. Journal of Food Engineering, 77(4), 1037–1047.