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Review

Novel Energy-Saving Strategies in Apple Storage: A Review

by
Felix Büchele
1,
Kiran Hivare
1,
Kartik Khera
1,
Fabio Rodrigo Thewes
2,
Luiz Carlos Argenta
3,
Tuany Gabriela Hoffmann
4,
Pramod V. Mahajan
4,
Robert K. Prange
5,
Sunil Pareek
6 and
Daniel Alexandre Neuwald
1,*
1
Lake of Constance Research Center for Fruit Cultivation (KOB), Schuhmacherhof 6, 88213 Ravensburg, Germany
2
Department of Plant Science, Federal University of Santa Maria, Roraima Avenue 100, Camobi, Santa Maria 97105-900, RS, Brazil
3
Epagri Experimental Station of Caçador, C.P. 501, Caçador 89501-032, SC, Brazil
4
Department Systems Process Engineering, Leibniz Institute for Agricultural Engineering and Bioeconomy (ATB), Max-Eyth-Allee 100, 14469 Potsdam, Germany
5
Special Graduate Faculty, School of Environmental Sciences, University of Guelph, Guelph, ON N1G 2W1, Canada
6
Department of Agriculture and Environmental Sciences, National Institute of Food Technology Entrepreneurship and Management, Sonipat 131028, Haryana, India
*
Author to whom correspondence should be addressed.
Sustainability 2024, 16(3), 1052; https://doi.org/10.3390/su16031052
Submission received: 29 November 2023 / Revised: 22 December 2023 / Accepted: 24 January 2024 / Published: 25 January 2024
(This article belongs to the Special Issue Challenges in Sustainable Plant Cultivation and Produce Supply)
Browse Figure
Review Reports Versions Notes

Abstract

:
Storing apples for up to a year is a well-established practice aimed at providing a continuous, locally produced fruit supply to consumers and adapting to market trends for optimized profits. Temperature control is the cornerstone of postharvest conservation, and apples are typically kept at temperatures ranging from 0 to 3 °C. However, the energy-intensive process of the initial cool-down and subsequent temperature maintenance poses significant financial challenges with adverse effects on the carbon footprint. Higher storage temperatures could reduce cooling-related energy usage but also pose the risk of enhanced ripening and quality loss. This work explores different storage technologies aiming to reduce energy consumption, such as 1-methylcyclopropene, ultra-low oxygen, and a dynamically controlled atmosphere with raised temperatures. The integration of advanced monitoring and control systems, coupled with data analytics and energy management, in apple storage is also discussed. These strategies can be implemented without cost-intensive construction measures in standard storage facilities. Furthermore, beneficial side effects of higher storage temperatures in terms of a reduced occurrence of storage disorder symptoms and higher maintenance of quality attributes are also discussed for this special issue on sustainable horticultural production systems and supply chains.

1. Introduction

The principal step in the postharvest handling of apples is the management of storage temperatures. Apples are usually kept at their lowest tolerated temperatures, which suppress the fruit metabolism, thereby slowing down the ripening and associated quality deterioration without the risk of chilling injuries (CI) development [1,2]. Cultivar-specific optimum temperature ranges exist, typically determined empirically. Most apple cultivars are recommended to be kept in a range from 0 to 3 °C [3]. It is widely assumed that these proposed optimum temperatures should be maintained throughout the full storage period, which can be extended to several months, on average 6 to 12 months [4]. This practice ensures the optimum conservation of quality attributes and avoids losses due to the possible occurrence of physiological disorder symptoms [1,5].
Apples are harvested in the orchard and exhibit the autumn temperature at that time and location before they are pre-sorted and stored. The initial cool-down process of the harvested fruit and the subsequent maintenance of these low-temperature ranges necessitate a significant energy input [6,7,8,9]. Estimating the total energy usage associated with the long-term storage of apples can prove difficult, as factors such as the design of storage facilities, installed technology and equipment, management practices, storage conditions, and the type of the stored commodity influence the total energy balance of a storage facility [7,10]. Geyer and Praeger approximated the electricity usage of fruit storage rooms for capacities from 50 to 450 t in a range from 3.0 to 5.5 kWh m−3 month−1 [11], similar results were reported by Evans et al. [12]. Interestingly, the most significant share of the total energy usage accrues in the initial cool-down phase when filling the rooms. During this phase, the cooling system operates at full capacity, mainly to cool the apples from field to storage temperature. Additionally, fruit exhibit a higher respiratory intensity, leading to increased heat production. The continuous operation of cooling and ventilation technologies constitutes a major source of heat load, and there is a greater heat input in the room during the filling process [11,13]. For the subsequent storage duration, there is a significantly smaller heat load originating from the respiratory heat generated by the commodity, running fan motors, or heat infiltration.
Long-term apple storage is therefore associated with substantial energy consumption but nonetheless crucial to providing a year-round supply of locally produced fruit. The required energy input, however, confronts fruit storage facilities with immense challenges. Sustainability efforts include a reduction in energy in the processes involved. Energy prices have risen rapidly over the past few years, while the market prices for apples have remained more or less constant and primarily responsive to the seasonal fruit supply (Figure 1) [14]. This presents an enormous financial constraint, especially as locally grown products face the competition of imported wares, often produced under cost-effective conditions [15,16]. Furthermore, if still reliant on conventional energy sources, long-term storage also contributes to a considerable carbon footprint [6,9,10,17]. These environmental and economic aspects highlight the critical need to improve the energy efficiency of the postharvest sector, ensuring long-term sustainable, environmentally friendly, and economically viable domestic fruit production.
As aforementioned, the cold room design, primarily the insulation, as well as the installed cooling and ventilation equipment, predominantly influence energy efficiency, and therefore, they also provide the highest energy-saving potential [10,19,20]. However, improvements in this aspect require cost-intensive construction and renovation measures, which cannot always be realized in existing storage facilities. It is subsequently essential to devise alternative approaches that can be implemented in the short-term and with minimal investment costs.
Previous studies have already suggested that an adjusted programming of the cooling and ventilation system has the potential to reduce energy consumption without compromising on fruit quality conservation [8]. Optimized bin stacking and vent area design of the bin are also easy to implement and have been shown to improve the airflow distribution in the cold room, thus allowing a reduced ventilation program [21,22]. Other aspects, such as watering the room floor, pre-cooling the fruit, and optimizing the filling schedule, can also contribute to higher energy efficiency [11,23]. This review focuses exclusively on three novel storage strategies to lower energy usage during the long-term storage of apples. These include elevated storage temperature together with the application of 1-methylcyclopropene (1-MCP), ultra-low oxygen/dynamically controlled atmosphere (ULO/DCA), and data-driven decision-making. It is hypothesized that elevating storage temperatures under the usage of these mentioned postharvest technologies can contribute to lower energy usage during storage.

2. Raised Temperatures with 1-MCP Application

The application of 1-MCP has become a commercial standard in the postharvest management of several horticultural products, including apples [24,25]. By blocking the ethylene receptors in the fruit, the ripening–inducing action of the hormone is inhibited, thereby slowing down ripening-associated quality deterioration in apples, such as softening, peel yellowing, reduction in acidity and sugar levels, or the occurrence of senescence disorder symptoms [24,26,27]. As 1-MCP-treated apples ripen more slowly, a variety of studies have proposed that they may be stored at higher storage temperatures without enhancing fruit metabolism intensity or compromising quality preservation. This should primarily contribute to reduced energy usage of the cooling system (Table 1) while potentially limiting the occurrence of certain physiological disorders or improving the preservation of quality attributes (Table 2). Furthermore, although respiratory heat accounts only for a minor share of the total heat load that needs to be removed from the room throughout the full season [11], 1-MCP’s effect on lowering the respiration rate of apples might also reduce heat energy produced by the stored products.
McCormick et al. tested this concept in a commercial setting by tracking the energy usage in two identical rooms with a capacity of 210 t [28]. Apples of the cultivar ‘Gala’ were kept for 5.5 months under (a) 1.5 °C and (b) 4.0 °C + 1-MCP application. The total energy balance shows a 35% reduction in energy usage with higher storage temperatures [28]. Specifically, energy consumption associated with running the cooling system, room ventilation, and defrosting was lowered. Interestingly, the energy usage of CO2 adsorbers was also lowered with elevated temperatures + 1-MCP, presumably due to a reduced fruit respiration rate. In terms of quality conservation, 1-MCP-treated fruit a higher firmness, titratable acidity, and ascorbic acid levels and experienced lower mass loss, despite storage at higher temperatures. Sensory evaluations furthermore showed a consumer preference for 1-MCP-treated fruit [28].
In a similar study published in 2012, apples of different cultivars were treated with 1-MCP and held at a temperature ~2.5 °C above the untreated control group [29]. For the ‘Gala’ cultivar, the findings of previous work were confirmed, as higher storage temperatures resulted in 35% lower total energy usage [29]. Furthermore, no adverse effects on firmness or acidity values were observed. For ‘Jonagold’ apples, energy savings of 26% could be achieved by this approach. The differential in energy savings was attributed to differences in the efficacy of the installed technology and different room ventilation settings [29]. A cultivar effect could also be suggested, as ‘Jonagold’ is known to have a higher respiration and ethylene production rate [30]. Sensory studies affirmed the conclusion that consumers rate 1-MCP-treated apples higher in terms of fruit texture quality and purchase preference [29].
Subsequent work by Kittemann et al. created an energy comparison in identical rooms (11 t apple capacity) between ULO at 1 °C and ULO at 5 °C + 1-MCP [31]. Apples of the cultivars ‘Golden Delicious’, ‘Jonagold’, and ‘Pinova’ were kept together in each room for 7 months. The results highlight an even greater energy-saving potential than the previously described studies. A 70% lower energy usage due to the 4 °C temperature differential was demonstrated [31]. Refrigeration compressors and ventilation fans accounted for the majority of the energy savings. The study also emphasized that at higher storage temperatures, no electrical defrosting of the evaporators was necessary after the initial cool-down period [31]. 1-MCP was confirmed to compensate for the elevated temperatures, as no adverse effects on firmness conservation were described. It was further shown that the combination of higher storage temperatures + 1-MCP can reduce the mass loss of the fruit. Both ‘Jonagold’ and ‘Pinova’ apples, furthermore, had higher scores for texture in sensory studies [31].
A recent 4-year study conducted in a commercial storage facility (room capacity of 640 t apples) in Brazil showed that temperatures can be increased from 0.7 to 2.0 °C for ‘Gala’ apples if treated with 1-MCP [32]. The energy consumption for operating the evaporator fans and compressors was reduced by 21 ± 1% and 20 ± 4%, respectively. Furthermore, it could also be demonstrated that an increase in storage temperature limits the development of fungal decay, particularly for apple fruit produced in warmer regions [32].
Table 1. Energy-saving potential through the combination of ULO/DCA, 1-MCP, and elevated temperatures.
Table 1. Energy-saving potential through the combination of ULO/DCA, 1-MCP, and elevated temperatures.
Room Size [t]CultivarsControlHigh TemperatureEnergy Savings (%)Ref.
210Gala1.4 °C ULO4.0 °C ULO + 1-MCP35[28]
210Gala1.3 °C ULO3.7 °C ULO + 1-MCP35[29]
640Gala0.7 °C CA2.0 °C CA + 1-MCP20[32]
100Elstar1.5 °C ULO4.0 °C ULO + 1-MCP25[29]
250Jonagold1.5 °C ULO4.0 °C ULO + 1-MCP26[33]
11Jonagold, Pinova,
Golden Delicious		1.0 °C CA/DCA5.0 °C ULO + 1-MCP63[31]
11Jonagold, Pinova, Topaz, Golden Delicious1.0 °C CA/DCA4.0 °C ULO + 1-MCP26[33]
11Jonagold, Pinova1.0 °C ULO/DCA4.0 °C ULO + 1-MCP51[33]
11Jonagold, Pinova,
Golden Delicious		1 °C ULO3 °C ULO/DCA12[33]
11Pinova1 °C ULO3 °C ULO15–50[33]
-Royal Gala, Pink Lady3 °CDCA 5 °C35% cool-down
15% storage		[34]
11Red Prince1 °C DCADCA and variable temp.35%[35]
11Red Prince, Jonagold, Pinova1 ° C DCADCA and variable temp.20%[36]
A substantial advantage of 1-MCP is that the ethylene-inhibiting effect persists for a certain time period after the fruit is removed from storage [24,27]. Studies have accordingly shown that applications before storage prolong the shelf life of apples and also provide benefits in long-distance transportation and subsequent marketing [26,37,38]. 1-MCP applications have been demonstrated to improve the quality conservation post-storage during a 7-day simulated shelf life period for multiple apple cultivars, including Braeburn’, ‘Red Prince’, ‘Jonagold’, ‘Pinova’, ‘Santana’, ‘Fuji, ‘Galaxy’, and ‘Pink Lady®’ [26,36,38,39,40,41]. Furthermore, Toivonen et al. showed in a multi-year study that 1-MCP permits the short-term storage (4 weeks) of ‘Sunrise’ summer apples at temperatures of up to 22 °C [42].
Table 2. Interactive effects of raised temperature + ULO/DCA and/or 1-MCP on stored fruit.
Table 2. Interactive effects of raised temperature + ULO/DCA and/or 1-MCP on stored fruit.
Beneficial EffectReferences
Higher firmness, higher TSS, and higher TA[26,28,29,31,35,36,41,43,44]
Lower mass loss[28,29,31,35]
Higher scores in sensory consumer panels (texture, taste, and buying preference)[28,29,31]
Reduced chilling injuries, flesh breakdown, core breakdown, water core, cavities, and mealiness [26,34,35,40,43,44,45,46,47]
Reduced rot incidences[31,32,33,48]
Increased abundance of volatile organic compounds[38]
1-MCP application may subsequently also render the fruit less sensitive to possible temperature variations in the post-storage sector, and maintaining the cool chain may consequently be of lower importance. So far, no studies on apples have tested under practical conditions whether 1-MCP applications pre-storage can directly translate to energy savings in the post-storage and marketing sectors. However, the idea of delaying the 1-MCP application until after storage to improve quality conservation and potentially reduce energy usage during the marketing period can be ruled out as a viable strategy, as the timing of application majorly effect the efficacy of the ethylene inhibitor [24,49].
While these findings indicate the great potential of 1-MCP to cut energy usage during storage without requiring extensive renovations or installation measures, it is nonetheless important to acknowledge that the usage of this compound is not suitable or recommended for all apple cultivars. There are some great differences between the cultivars in their responsiveness to the ripening–enhancing action of ethylene as well as their sensitivity to 1-MCP [24,50]. It is, therefore, crucial to confirm the effectiveness of 1-MCP for any cultivar, especially as, in some instances, the application of this product may also play a role in increasing the occurrence of disorders [51]. As 1-MCP inhibits ripening, it also maintains apples in an unripe state, in which they are more susceptible to physiological injuries caused by elevated CO2 values [24], as could be shown for ‘Braeburn’ or ‘Empire’, in which 1-MCP increased the risk of internal browning or skin necrosis symptoms [39,52].
Furthermore, it must be ensured that the energy savings and the additional value in fruit conservation cover the cost of the application. Another great limitation is that, as a synthetic product, the usage of 1-MCP is not permitted in organic fruit production. It is consequently essential to implement other ripening-suppressing measures that allow higher temperatures in apple storage.

3. Raised Temperatures in ULO and DCA Storage

Controlled atmosphere (CA) settings, meaning the adjustment of oxygen (pO2) and carbon dioxide partial pressures (pCO2) in the room atmosphere, slow down fruit respiration and suppress ethylene production in apples, thus greatly extending their storability [5]. ULO refers to storage conditions with a lower pO2 than in traditional CA, in ranges around 1.0 kPa [53]. A further development of CA storage is the principle of DCA technologies, which were introduced more than two decades ago and find application in many apple-producing countries for a great variety of apple cultivars [34]. DCA systems, regardless of the mode of action, aim to monitor in real-time the respiration dynamics of stored apples in order to identify stress signals by the fruit caused by low-oxygen stress and accordingly define the lowest possible pO2 in the storage atmosphere, which is hypothesized to slow down fruit ripening to a minimum and, therefore, provide optimum quality conservation [54]. Thus far, DCA systems based on chlorophyll fluorescence (DCA-CF), ethanol (DCS or DCA-Eth), respiratory quotient (DCA-RQ), and carbon dioxide release rate (DCA-CD) have been introduced [54]. Further DCA system improvement can be achieved by integrating its control with real-time respiration data [55]. These improvements enable the system to dynamically adjust temperature, pO2, and pCO2 based on the unique requirements of the stored apples, thereby reducing unnecessary energy consumption.
It must be acknowledged at this point that DCA technologies necessitate a higher standard for the gas-tightness of the room [54]. Leakages can potentially distort the identification of low oxygen stress signals and prevent precise pO2 control, with potentially adverse consequences such as fermentation damage to the fruit material [56,57]. Gas-tightness must consequently be tested and ensured before the usage of a DCA system.
The idea behind energy savings with ULO or DCA technologies mirrors the principle discussed for 1-MCP applications. It is assumed that these storage conditions suppress the metabolism intensity of the fruit and thus allow higher storage temperatures. In addition, as mentioned before for 1-MCP, the combination of low pO2 and elevated pCO2 in ULO/DCA storage works as a ripening brake, reducing the fruit respiration intensity and thereby limiting respiratory heat production [58].
Neuwald et al. tested the potential of elevating storage temperatures from 1 to 5 °C in ULO and DCA storage for multiple apple cultivars and determined total energy savings of ~15 to 50% during a 5- to 7-month storage period [33]. The highest saving potential was shown for the cooling compressors and the evaporator fans. Interestingly, higher temperatures provided an unsuspected beneficial side effect as they were associated with lower rot incidences in ‘Pinova’ apples. However, higher temperatures were also found in this cultivar to increase peel yellowing [33].
For ‘Nicoter’ apples, both DCA-CF and DC-RQ were demonstrated to allow raising storage temperatures by 2 °C [59]. While this study did not attempt to create an energy balance, it could nonetheless be shown that higher storage temperatures may provide secondary beneficial effects, as lower flesh breakdown, core breakdown, and cavity incidences were found. This effect could also be demonstrated for the ‘Braeburn’ variety, as the storage at 3 °C in DCA-RQ storage resulted in reduced cell damage and a slowed-down fruit softening [45].
Both et al. made similar conclusions, arguing that it is possible in DCA-RQ to store ‘Galaxy’ apples at higher temperatures of 2 °C while maintaining high flesh firmness and limiting the occurrence of physiological disorders [26]. This could be confirmed in a later study, which showed that ‘Galaxy’ apples can be stored at 2.5 °C in DCA-RQ, and compared to 1.5 °C, they showed higher firmness and reduced mealiness incidences [46]. Ludwig et al. proposed a similar potential for ‘Royal Gala’ apples, arguing that by lowering the pO2 in the storage atmosphere from 1.2 to 0.8 kPa, temperatures can be increased by 0.5 °C [47]. The combination of higher temperatures and extremely low oxygen levels resulted in higher flesh firmness and total soluble solids and reduced flesh breakdown and mealiness incidences. This potential could also be confirmed in the same study for ‘Galaxy’ apples, as lower pO2 of 0.4 kPa allowed to raise storage temperatures by 0.5 °C without adverse consequences for fruit conservation. Furthermore, this was also associated with lower mealiness symptoms and ethylene production [47].
While it can be concluded based on these previously listed studies that elevated and static-kept storage temperatures in DCA systems have the potential to save on energy usage, this approach does not necessarily consider the stored product’s physiology, meaning its sensitivity to the storage temperature. It could be suggested that in the early stages of DCA storage, fruit have a higher metabolism intensity than later on in the storage season and, respectively, may be more responsive to the atmosphere and temperature settings. Based on this hypothesis, a recent study investigated a novel temperature variation program: In DCA-RQ storage, temperatures were kept for the initial 30 days at 0.5 °C until the fruit adapted to the low pO2 levels, and were then raised to 2 °C to 3 °C for the subsequent 8 months of storage [43]. Although this study was conducted on a smaller scale and, therefore, did not attempt to create a comparison in terms of energy efficiency, it could be demonstrated that this temperature management in DCA-RQ 1.3 results in higher quality maintenance than storage at 0.5 °C or under CA/ULO + 1-MCP conditions [43]. Based on the above-described studies, a significant potential for energy savings could subsequently be assumed as well.
Recently, a new approach for temperature management in DCA storage has been introduced, which aims to monitor the fruit respiration dynamics in storage as a function of the established temperature. This storage system is referred to as DCA-CD Plus [60]. Its principle hypothesizes that, similarly to an optimum pO2, an optimum storage temperature can also be defined, which fluctuates during the storage period and can be estimated by monitoring the CO2 release rate of the stored apples [36,39,60]. Thus far, limited research has been published on the efficacy of this new concept in reducing the energy consumption of the cooling system while maintaining high fruit quality. Nonetheless, a preliminary study on ‘Red Prince’ apples stored for 9 months showed the DCA-CD Plus system can dynamically adjust the storage temperature setpoints from the baseline of 1 °C to temperature ranges up to 4.5 °C without adverse consequences to the conservation of quality parameters such as firmness, titratable acidity, or peel color [35]. Subsequent studies on rooms filled with ‘Red Prince’, ‘Pinova’, ‘Jonagold’, or ‘Braeburn’ confirm the efficacy of DCA-CD Plus in dynamically adjusting and elevating storage temperatures without accelerating fruit ripening and the associated quality deterioration [36,39].
In terms of energy efficiency, DCA-CD Plus was shown to reduce energy usage by the evaporators, compressors, and electrical defrosting, resulting in a total energy saving of more than 35% [35]. Subsequent work found 40% less energy usage of the evaporators due to dynamic temperatures in DCA-CD Plus. Energy usage for evaporator defrosting was cut by 30% as well [36]. Although defrosting of ventilators accounts for a relatively small proportion of the total energy consumption of the cooling system, these findings nonetheless suggest that dynamic temperature adjustments limit the risk of ice buildup on the evaporator. Ice accumulation on the evaporators reduces the heat transfer, increases the pressure drop, and accordingly negatively affects the cooling capacity of the refrigeration system. In addition, this might also dehumidify the air cycling inside the rooms and thereby promote fruit transpiration and mass loss [61]. According to these initial findings, dynamic temperatures may allow running the refrigeration air coolers throughout the storage duration in a defrost-free modus, with possible benefits in terms of energy efficiency and fruit quality conservation [35].
When considering the discussed findings on 1-MCP and DCA technologies, it could be argued that the combination of both approaches may ultimately provide the highest energy-saving potential during storage as well as in the subsequent marketing period. Köpcke et al. tested this idea by storing ‘Elstar’, ‘Jonagold’, and ‘Gloster’ apples for 5 months under DCA-CF and 1-MCP under higher temperature ranges of 3.5 to 10 °C [44]. The results highlighted that elevated storage temperatures reduced chilling injuries, watercore, and internal browning. The combination of 1-MCP and DCA was found to maintain higher fruit quality than either treatment alone. Furthermore, it was argued in this work that using DCA technologies could limit risks if the 1-MCP application had an insufficient effect due to incorrect application timing or the advanced ripeness of the treated fruit material [44,62]. Findings by Büchele et al. support these arguments, as they showed that storage temperatures can be increased by 3 °C for ‘Santana’ apples if kept under DCA-CD + 1-MCP [38]. No adverse effects of the higher storage temperature on firmness, titratable acidity, or peel color were observed, even after an additional 7 d shelf-life period at 20 °C. This simultaneously underlines the earlier proposed energy-saving potential in the post-storage sector. As these two listed studies did not include a comparison of energy usage, the energy-saving potential can naturally only be hypothesized based on similar studies that showcased increased temperatures to reduce the energy consumption of the cooling system.

4. Advanced Monitoring and Data Analytics

In the evolving landscape of apple storage practices, the incorporation of advanced monitoring and control systems, coupled with data analytics, represents a revolutionary approach to precision energy management. Such systems, equipped with air flow [63] and heat-flux sensors [64], enable real-time data collection, providing a comprehensive understanding of the storage environment. Such advanced monitoring technology ensures meticulous tracking of temperature dynamics and airflow patterns within storage facilities. Paired with real-time control systems, this technology allows for dynamic adjustments tailored to the specific needs of stored apples, reducing unnecessary energy consumption. For instance, by adapting temperature settings and regulating airflow with precision, these systems contribute not only to the preservation of fruit quality but also to significant energy efficiency gains.
Furthermore, the integration of data analytics enhances decision-making processes. Analyzing the wealth of data collected by monitoring systems provides insights into trends and patterns, empowering stakeholders to make informed decisions about storage conditions. This proactive approach, rooted in data-driven insights, minimizes energy wastage by identifying areas where energy may be expended unnecessarily. The continuous improvement cycle facilitated by data analytics ensures that apple storage facilities evolve over time, aligning with the most efficient and sustainable energy usage practices. By strategically combining technology and data-driven insights, the industry not only ensures optimal fruit preservation but also lays the groundwork for a future where energy-intensive processes are managed with unparalleled efficiency and sustainability.
One promising avenue for future exploration is the application of digital twin technology in the realm of apple storage. Digital twins, which replicate physical systems in a virtual environment, offer the potential for real-time simulations and data-driven decision-making [65,66]. In the context of energy conservation, a digital twin of an apple storage facility could enable precise monitoring and adjustment of conditions, optimizing energy usage.

5. Conclusions

There is an evident need to enhance the energy efficiency of the postharvest sector in pome fruit storage, striving for SDG 12, ‘Sustainable consumption and production’. The high energy usage associated with cooling the product jeopardizes the sustainable and economic viability of the storage of locally produced fruit and stands as a major contributor to the carbon footprint across the entire fruit production chain. This review explored three strategies aimed at reducing energy consumption in refrigeration systems. Research shows that, depending on the apple cultivar, storage temperatures can be increased by 2 to 4 °C with the application of the ethylene inhibitor 1-MCP or the establishment of extremely low pO2 levels (<1.0 kPa) in ULO/DCA storage. Furthermore, positive benefits of increased temperatures on fruit preservation can be concluded as well, as research shows lower incidences of internal browning, watercore, mealiness, and storage rot and that 1-MCP and/or ULO/DCA improved firmness and maintained titratable acidity. The incorporation of advanced monitoring and control systems, coupled with insightful data analytics and digital twin, can also serve as a crucial strategy for effective energy management in apple storage. In this context, it can be asserted that these technologies contribute to more sustainable fruit preservation and economic efficiency by limiting food loss and waste in the postharvest sector. Importantly, the broad commercial applicability of these technologies is suggested, as they may be implemented in standard CA storage facilities without the need for cost-intensive construction measures.

Author Contributions

Conceptualization, F.B., K.H. and D.A.N.; writing—original draft preparation, F.B., T.G.H. and P.V.M.; writing—review and editing, F.B., K.H., K.K., F.R.T., L.C.A., T.G.H., P.V.M., R.K.P., D.A.N. and S.P.; visualization, F.B.; supervision, D.A.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

We are grateful to the Landwirtschaftliche Rentenbank, Germany, for supporting the research project ‘DyNatCool’ and financing the position of the first author (F. Büchele). We are grateful to Michael Blanke for the invitation to publish in the journal Sustainability and also for editorial guidance.

Conflicts of Interest

All authors declare that they have no conflicts of interest.

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Sustainability 16 01052 g001
Figure 1. Average apple market prices and electricity costs in the Lake Constance Region in Southern Germany (with permission [18]).
Figure 1. Average apple market prices and electricity costs in the Lake Constance Region in Southern Germany (with permission [18]).
Sustainability 16 01052 g001
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Büchele, F.; Hivare, K.; Khera, K.; Thewes, F.R.; Argenta, L.C.; Hoffmann, T.G.; Mahajan, P.V.; Prange, R.K.; Pareek, S.; Neuwald, D.A. Novel Energy-Saving Strategies in Apple Storage: A Review. Sustainability 2024, 16, 1052. https://doi.org/10.3390/su16031052

AMA Style

Büchele F, Hivare K, Khera K, Thewes FR, Argenta LC, Hoffmann TG, Mahajan PV, Prange RK, Pareek S, Neuwald DA. Novel Energy-Saving Strategies in Apple Storage: A Review. Sustainability. 2024; 16(3):1052. https://doi.org/10.3390/su16031052

Chicago/Turabian Style

Büchele, Felix, Kiran Hivare, Kartik Khera, Fabio Rodrigo Thewes, Luiz Carlos Argenta, Tuany Gabriela Hoffmann, Pramod V. Mahajan, Robert K. Prange, Sunil Pareek, and Daniel Alexandre Neuwald. 2024. "Novel Energy-Saving Strategies in Apple Storage: A Review" Sustainability 16, no. 3: 1052. https://doi.org/10.3390/su16031052

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