Respiratory quotient based dynamic controlled atmosphere storage of apple (Malus x domestica Borkh.) fruit

To ensure year-round availability, high quality pome fruit is commonly stored at low temperature and under controlled atmosphere (CA) conditions to slow down the biochemical process of respiration, which leads to quality loss and finally decay of the stored fruit. In order to meet the increasing demands of the consumers of today and to meet the market competition, post-storage quality needs to be preserved to a higher extent than with conventional storage and preferably in a nonchemical way. One way to achieve this, is storing fruit at lower O2 levels, closer to and preferably at the anaerobic compensation point (ACP) – the O2 level at which fruit respiration is minimal. This can be achieved by storing the fruit with dynamic controlled atmosphere (DCA) storage. With DCA storage, the O2 level in the storage room is gradually reduced until low O2 stress is detected, based on measurement of a bioresponse signal of the stored fruit. Subsequently, the O2 level is slightly increased until relief of low O2 stress, and the cycle restarts. The main objective of this work was to develop engineering solutions contributing to effective DCA storage based on the respiratory quotient (RQ) for pome fruit and to evaluate how DCA based on RQ affects post-storage quality of apple fruit. The research hypothesis was that RQ-DCA storage would lead to better firmness and color retention compared to regular CA storage and comparable to DCA based on chlorophyll fluorescence, without inducing storage related disorders or off-flavor. The technical hypothesis was that pressure driven leakage of the cool rooms used for storage of apple fruit may corrupt RQ measurement and that the effect of leak on measured RQ values could be taken into account by the development of a leakage model of the storage system. RQ is specific, biochemically well understood and shows a steep increase upon low O2 stress, making it a good indicator bioresponse signal. The principle of using RQ as a bioresponse to detect low O2 stress has been known for many years, yet until recently, RQ has only been measured off-line as leakage of storage rooms can corrupt its measurement. Lately, few commercial RQ based systems have been introduced to the market. These systems measure either gas composition changes in an entire cool room or in small containers to be positioned in a storage room in attempts to circumvent issues with room gas leakage. Besides increased cost and operational burdens, this implies the risk of storing a whole cool room at undesirable gas conditions that are only suitable for the sample of fruit in the measurement container. The idea of using leak corrected RQ estimates for automatic control of the CA gas composition in storage rooms has been patented by the MeBioS postharvest group, yet it had not been used for DCA control so far at the start of this PhD research. To evaluate the effect of DCA based on RQ (RQ-DCA) on post-storage quality of apple fruit, a pilot infrastructure consisting of lab scale storage containers and accompanying control units for automatic control of CA gas composition was built. Software algorithms for automatic RQ measurement based on measured changes in gas composition and dynamic control of the CA gas composition based on measured RQ values were developed. ‘Granny Smith’ apples were stored under RQ-DCA and the effect on fruit quality immediately after storage as well as during subsequent shelf life was investigated and compared to the effect of regular CA and regular CA storage with 1-methylcyclopropene (1-MCP) treatment. Superficial scald, one of the major storage disorders in ‘Granny Smith’ leading to significant losses, was found to be controlled almost completely during storage with RQ-DCA and subsequent shelf life. Magness-Taylor firmness and skin background color (hue angle) were found to be significantly higher after RQ-DCA storage compared to conventional CA storage and were comparable to these of fruit stored with regular CA after 1-MCP treatment up to 7 d of shelf life at 18 °C. In-fruit ethanol concentrations were found to be very low (< 0.028 g L-1), indicating the RQ-DCA system managed to store the fruit close to the ACP, but fruit metabolism did not shift from aerobic respiration to fermentation. Although the RQ-DCA system managed to control O2 and CO2 partial pressures in the storage container in an autonomous way, difficulties to obtain reliable RQ measurements were observed at very low O2 levels due to reduced fruit respiration and when the pressure of the storage container increased due to leakiness. To improve the reliability of correct RQ measurements for the effects of gas exchange between storage room and environment, a leak correction method was developed to correct real-time measurements of the respiratory quotient (RQ) for gas leakage of a cool room. The method is based on a mathematical model of leakage of the room that was simplified by performing sensitivity analyses of the contribution of the pressure driven and concentration driven gas transport terms. It was shown that pressure driven leakage (time constant 1.17 s) was dominant over diffusive leakage (time constant 7.14 x 1013 s). The model was validated using an empty storage container at typical DCA conditions. The validated model was successfully implemented in the automated RQ-DCA control system for storage of ‘Braeburn’ apple fruit. RQ estimates without leakage correction were heavily biased when leakage of atmospheric air into the storage container occurred and would lead to erroneous control of the storage atmosphere composition. With the developed leakage correction, RQ measurements were found to be accurate under all circumstances. Finally, the developed RQ-DCA technology was compared to alternative common practice and current state of the art storage technologies. ‘Golden Delicious’ apples from two European orchards located in two different climate types, were stored under RQ-DCA, RQ-DCA + 1-MCP, DCA based on chlorophyll fluorescence (DCA-CF), regular CA and regular CA + 1-MCP during two subsequent storage seasons. The RQ-DCA technology was partially scaled-up to a semi-commercial cool room. Both RQ-DCA and DCA-CF technology had a similar and significantly positive effect on post-storage firmness and skin background color compared to regular CA storage, without inducing storage disorders. Fruit treatment with 1-MCP, prior to storage, influenced post-storage quality even more and was found to be the dominant factor above storage technology. Fruit stored under RQ-DCA and DCA-CF maintained similar firmness and color up to 7 d of shelf life, but were able to ripen further afterwards. No synergetic nor additive effect of RQ-DCA storage and 1-MCP treatment was observed. Fruit stored under RQ-DCA after 1-MCP treatment in the pilot room were found to have similar firmness and color to fruit stored under RQ-DCA after 1-MCP treatment in a small storage container. Based on the obtained results, it was concluded that RQ-DCA significantly improved the post-storage quality of apple fruit compared to regular CA storage and to the same extent as chlorophyll based DCA. RQ-DCA completely controlled superficial scald, a major post-storage skin disorder, leading to significant economic losses. Post-storage quality after RQ-DCA storage was similar to post-storage quality after storage under regular CA after treatment with 1-MCP up to seven days of shelf life. Therefore RQ-DCA may be regarded as a substitute for 1-MCP and diphenylamine (DPA) in the organically grown apple fruit market. Although confined storage spaces are constructed as airtight as possible, it was concluded gas exchange between storage room and environment biased RQ measurement. Leak correction of gas concentration measurements for leaking of the storage facility was crucial to obtain reliable RQ measurements. The remaining challenge is to further upscale the technology and apply it to storage of apple fruit in commercial storage rooms, typically loaded with 100 - 500 tons of fruit. Hereto, the leakage model developed in this work should be validated for commercial rooms. More relevant criteria for airtightness should be developed as well as more precise and standardized measurement methods of the airtightness of storage rooms. In this work, gas composition of the storage atmosphere was measured every minute, while in a commercial storage environment gas composition can only be analyzed less frequently. This will have an effect on the accuracy of obtained RQ estimates which should be quantified and a minimum frequency of gas composition measurement should be determined.

Jaar van publicatie:2018

Toegankelijkheid:Closed