• Energy Research

Climate change is responsible for major alterations in the phenology of fruit trees in Germany. Observations over the past 70 years at 3 study sites in Germany have shown the flowering of apple trees and subsequently, fruit development to set is continuously earlier, with serious implications for fruit quality as well as storability. Calcium (Ca) plays an important role in stabilizing cell walls in all fruit including apple; sufficient calcium supply is, therefore, essential for good storability and for controlling postharvest disorders. However, as Ca is phloem immobile, the transport of the mineral into the fruit occurs solely with the transpiration stream through the xylem. Ca accumulates in the first weeks after flowering, in which the fruit shows the highest transpiration per unit (surface or mass) and subsequently the greatest transport of water. With an earlier flower onset, Ca uptake may suffer as the evapotranspiration and therefore the transport of Ca is reduced due to fewer sunlight hours in a shorter photoperiod and a smaller thermal amplitude early in the season. However, Ca deficiency and an unfavorable relation to its antagonist potassium can increase the risk of the accelerated loss of firmness or the emergence of disorders such as bitter pit or flesh browning. On account of this significant role of Ca for long-term fruit storability, a project was launched with the objective to assess the long-term effects of climate change on the Ca nutrition of apples cultivated in Germany. Climate data of the past 60 years will be analyzed and related to the recordings of apple tree phenology development stages in the respective years, in order to estimate the evapotranspiration potential during the main period of Ca uptake. Historic Ca data of ‘Golden Delicious’ apples, recorded starting in the 1970s at the Lake of Constance Research Center for Fruit Cultivation (KOB), will be analyzed to investigate trends in the Ca nutrition of apple fruit. Ultimately, the project aims to develop a multivariate model for the prediction of Ca content in apples at harvest, that incorporates factors such as the weather during the season, soil type, tree age, cultivar and cultivation practices. In the long term, this is intended to support fruit production in adaption to challenges presented by climate change.

Abstract Fruit and vegetables are stored for up to several months in large bin stacks in refrigerated rooms. Gaps between the bins and bin wall openings should allow the circulation of cooling air to remove field and respiration heat from produce. In an industrial apple store, the effects of the stacking array, fan position and number on airflow distribution in vertical gaps and inside plastic bins were investigated using stepwise reduction of fan revolution. At 100% fan power, the average air velocity in vertical gaps between bin rows in the main flow direction increased with reducing total cross section of the gaps. In contrast, the number of vertical gaps between bin rows had little effect on the average air speed between fruit inside the bins, probably due to additional flow paths in the horizontal gaps. The lowest air speed was measured when bins were stacked in one block without vertical gaps and a distance to the side walls of only 10 cm. A stacking arrangement with only one central vertical gap between four bin rows or with a large distance (> 1 m) between the wall opposite to the fans and the bins enhanced flow uniformity. Three fans situated above the vertical gaps was more effective for the ventilation of bins than four fans above the bin rows.

Abstract In refrigerated stores of fruit and vegetables, cooling air is circulated by air coolers attached to the ceiling. Uniform airflow and even temperature distribution in the product stacks is important for quality maintenance. However, the air speed close to the produce inside the bins in industrial cold stores is unknown. Airflow distribution was measured with newly developed sensors at different ventilation levels inside bins and in vertical gaps between the bins in a common apple storage room. The air speed between the fruit was low (≤0.3 m/s) compared to the average air velocity in the neighboring gap (1.15 m/s) at 100% fan power. In the bins of the upper stack area, the air speed was about 7 times higher than that in the bins at the bottom. The airflow pattern in the vertical gaps showed the formation of an air roll with relatively uniform air velocity across the height of the bin stack. Reducing the fan power immediately lowered the airflow between the fruit and in the gaps. However, airflow was detected at all measuring positions, even when the fan power was reduced to 44%.

The aim of the study was to evaluate an elevated (3.0 °C) and low (1.0 °C) storage temperature combined with dynamic controlled atmosphere monitored by respiratory quotient (DCA-RQ) and chlorophyll fluorescence (DCA-CF) on anaerobic metabolism, physiological storage disorders and overall quality of 'Nicoter' ('Kanzi®') apples after 5.5 and 8.0 months of storage plus 7d shelf-life. Fruit stored under DCA-RQ 2.0 accumulated the highest amounts of anaerobic metabolites (acetaldehyde, ethanol and ethyl acetate), regardless of storage temperature and timing of storage outturn evaluation, but it did not result in higher electrolyte leakage. Flesh breakdown, core breakdown and cavity formation were reduced at 3 °C. Storage at 3 °C combined with DCA maintained higher flesh firmness after 8.0 months storage plus 7d shelf-life. 'Nicoter' apples can be stored at 3 °C using a DCA system, based either on CF or on RQ, to save electrical energy.

The effect of a respiratory quotient dynamic controlled atmosphere (DCA - RQ), which induces ethanol production through low oxygen storage, and ethanol application on softening of Braeburn apples stored at different temperatures was investigated. DCA - RQ storage was associated with the activation of the alcohol dehydrogenase (ADH) and greater anaerobic metabolism in comparison with DCA - CF (chlorophyll fluorescence) and controlled atmosphere (CA) storage. Greater anaerobic metabolism resulted in lower ethylene production, ACC oxidase activity, membrane permeability, -galactosidase activity and, therefore, less softening after long-term storage. Ethanol application after CA storage decreased ethylene biosynthesis, respiration rate and membrane permeability. Storage at 3 °C resulted in the lowest soluble solids and acidity, but not softening. In conclusion, Braeburn apples could be stored at 3 °C under DCA - RQ1.5. Additionally, ethanol produced by the fruit, or applied externally, had a significant effect on inhibiting softening during and after storage.

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.