Pomegranate arils (‘Wonderful’) tolerance to low O 2 stress during active modified atmosphere storage: based on real time respiration rate Z.A. Belay 1 , O.J. Caleb 2,3 , P.V. Mahajan 3 and U.L. Opara 1,4,a 1 Postharvest Technology Research Laboratory, South African Research Chair in Postharvest Technology, Department of Food Science, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa; 2 Postharvest and Wine Technology, Agricultural Research Council, Infruitec-Nietvoorbij, Stellenbosch 7599, South Africa; 3 Department of Horticultural Engineering, Leibniz-Institute for Agricultural Engineering and Bioeconomy (ATB), Potsdam-Bornim, D-14469 Potsdam, Germany; 4 Postharvest Technology Research Laboratory, South African Research Chair in Postharvest Technology, Department of Horticultural Science, Faculty of AgriSciences, Stellenbosch University, Private Bag X1, Stellenbosch 7602, South Africa. Abstract Successful characterization of the relationship between the respiration rate (RR) and low O2 limit (LOL) is critical for optimizing the modified atmosphere packaging (MAP). It is well documented that low O2 atmosphere helps to reduce RR; however, it could lead to stress from the accumulation of end products from glycolysis. Therefore, the study investigated the response of pomegranate arils exposed to low O2 atmosphere (2.5 kPa O2 + 18 kPa CO2 + 79.5 kPa NO2) and identified the low O2 limit at 5 and 10°C. The aim was achieved by using real-time RR data, respiration quotient (RQ) and tentatively identified volatile organic compounds (VOCs). Changes in VOCs were considered to determine either fermentative volatiles emitted due to the shift from aerobic to anaerobic respiration. The headspace gas evolution showed that, changes in gas composition were significantly affected by the storage temperature and storage duration, however, minimum change was observed at 5°C. Respiration quotient was 1.0 at 5°C, and a sharp increase of 3.0 was observed at 10°C. Response of the arils to low O2 stress was observed with shifts in VOCs composition, especially those involved in anaerobiosis. Ethanol, acetaldehyde, and hydrazine, methyl- complexes were emitted under low O2. Low O2 tolerance limit at the studied temperatures was then estimated as 2.18-2.28 kPa at 5 and 10°C, respectively. Keywords: respiration quotient, fermentative metabolites, enriched CO2 INTRODUCTION Pomegranate arils are rich in antioxidants and phenolic compounds which are known to be essential for preventing chronic diseases (Ersan et al., 2009). Storing arils under low O2 concentration during modified atmosphere packaging slows down the physiological response and maintains the quality (Banda et al., 2015; Ló pez-Rubira et al., 2005). A minimum RR of 1.5 mL O2 kg -1 h -1 and 0.5 mL CO2 kg -1 h -1 was reported for pomegranate arils (‘Hicaz’) under low O2 atmosphere (2 kPa O2 and 10 kPa CO2) at 4°C by Ersan et al. (2009). Similarly, the advantage of low O2 (1-3 kPa) to extend the shelf life of pomegranate arils (‘Mollar de Elche’) stored at 5°C for 15 d was reported by Ló pez-Rubira et al. (2005). However, under low O2 condition, the metabolism can switch from aerobic respiration to fermentation (Ampofo-Asiama et al., 2014). Therefore, a critically low O2 concentration should be avoided during modified atmosphere storage in order to maintain the fruit quality. According to Beaudry et al. (1992), the O2 concentration at which tissue fermentation is induced may be taken as the lowest O2 limit. In addition, anaerobic RR can be detected by the upswing in the RQ associated with the synthesis of acetaldehyde, ethanol and CO2 (Beaudry et al., 1992). a E-mail: opara@sun.ac.za Acta Hortic. 1201. ISHS 2018. DOI 10.17660/ActaHortic.2018.1201.29 VII Int. Conf. on Managing Quality in Chains & II Int. Symp. on Ornamentals in association with XIII Int. Protea Research Symp. Eds.: U.L. Opara and E.W. Hoffman 213