Development of a small and flexible sensor-based respirometer for real-time determination of respiration rate, respiratory quotient and low O2 limit of fresh produce

• We developed a small and flexible respirometer for non-invasive determination of real-time respiration rate.
• The gas sampling implied little manual handling.
• The respiratory quotient and the low O2 limit could be acquired.
• The respirometer was tested with fresh produce kept at varying temperature and O2 contents.

Information on the respiration rate and the low O2 limit (LOL) is important for optimization of packaging and storage systems for fresh fruit and vegetables. In this study, a small and flexible sensor-based respirometer was developed for real-time determination of the respiration rate, respiratory quotient (RQ), and LOL of fresh produce. The respirometer consisted of a wide mouth 1-L glass jar with a screw-type metal lid and an electrochemical and an infra-red sensor mounted directly on the lid of the glass jar to take continuous and non-invasive measurements of the O2 and CO2 contents. Data from the respirometer was compared with data obtained from two fluorescence-based spot sensors (OpTech and PreSens) and a headspace gas analyzer (CheckMate). A test with strawberry showed that similar respiration rates (14.1–16.2 mL O2 kg−1 h−1 and 13.4–16.4 mL CO2 kg−1 h−1 at 10 °C) were obtained with all instruments. Further on, a Savitzky–Golay smoothing filter was implemented on the data from the respirometer to estimate the real-time respiration rate. The result showed that the respiration rate could be acquired in 2–3 h after filling of the respirometer or even after 1 h if the produce was equilibrated to the target storage temperature before the measurements. Detailed information on the respiration rate of wild rocket, strawberry, and carrot showed that the respiration rate decreased with time as the O2 content decreased; however, the RQ remained almost constant throughout storage until the LOL was reached. Information on the RQ and the LOL value is rare in the literature; however, the RQ and the LOL could easily be determined by the use of the respirometer. The RQ was 1.0, 1.0–1.5, and 0.5 for wild rocket, strawberry, and carrot, respectively, during storage under an O2 content above >2.0 kPa. As the O2 content dropped to 0.5, 1.0 and 2.0 kPa O2, for wild rocket, strawberry, and carrot, respectively, the RQ values increased sharply. The described respirometer made it easy to analyze the impact of a dynamic temperature and O2 content on the respiration rate, the RQ, and the LOL as handling was limited and real-time data could be obtained. With such detailed information, a knowledge-intensive design of packaging and storage systems for fresh horticultural produce is enabled.