Patrick Varoquaux

The influence of raw material characteristics on the storage life of fresh-cut butterhead lettuce

Abstract The physiological characteristics of 5 butterhead lettuce cultivars ( Lactuca sativa L.) were investigated using etiolated leaves. Their storage life under modified and controlled atmospheres was assessed. When prepacked butterhead lettuce was maintained under a low oxygen atmosphere to prevent enzymatic browning, high CO 2 content was the main factor increasing the rate of decay. Shelf life was negatively correlated with respiration rate and susceptibility to CO 2 . Potassium leakage was a good indicator of physiological disorders. High oxygen and low CO 2 enhanced enzymatic browning, while low oxygen and, more significantly, high carbon dioxide enhanced CO 2 injury (brown stain). Maintaining CO 2 concentration within the packs below 5% resulted in an improved preservation of the lettuce leaves. Practical means for obtaining modified atmospheres which were in equilibrium yet were low in both O 2 and CO 2 are discussed.

Respiratory parameters and sugar catabolism of mushroom (Agaricus bisporus Lange)

The respiration rate (RR) of mushroom (Agaricus bisporus Lange, strain X25) under air follows Arrhenius’ law with an apparent activation energy of 43 400 J mol−1 corresponding to a Q10 of 2.9 between 10 and 20°C. Oxygen from 20 to 1 kPa did not affect RR at temperatures ranging from 5 to 20°C, confirming previous studies. The initial RR of mushroom (same cultivar, flush, growing conditions) measured 24 h after harvesting can vary from 1 to 2 mmol O2 kg−1 h−1 at 10°C. The apparent Km value was below 0.1 kPa at 10°C, and could not be estimated more accurately due to instrumental limitations. Decrease in dry matter of modified atmosphere-packed mushrooms matched the theoretical carbon consumption calculated from the constant respiration rate. Glucose and glycogen were rapidly catabolized, whereas mannitol consumption began 3 and 8 days after harvest at 20 and 10°C, respectively. It was concluded that no extension of mushroom shelf life was attainable through modified atmosphere packaging. Controlling RH within the package is likely to be more effective.

Modified Atmosphere Packaging of Fresh Beansprouts

Freshly harvested beansprouts displayed a respiration rate of about 1 mmol O 2 kg -1 h -1 at 10°C which was strongly dependent on temperature, a 10-fold increase being observed every 16.5°C (z =16.5°C, ie Q 10 = 4.4). This commodity is also characterised by a high initial microbial load (about 10 7 cells g -1 ). During storage at various temperatures from 1 to 20°C, oxygen uptake rates dramatically increased with time and this phenomenon was well correlated with the development of aerobic microorganisms which reached 10 9 cells g -1 after 2 days at 20°C or 9 days at 1°C. Beansprouts were packaged in films, with permeabilities ranging from 950 to 200 000 ml O 2 m -2 day -1 atm -1 , and stored at 8°C. Due to plant and microbial metabolism, oxygen concentrations decreased steadily within all packs until the onset of plant tissue decay. The latter occurred after 5-6 days with the least permeable films but did not occur within when the film permeability was over 100000 ml O 2 m -2 day -1 atm -1 . However, such films favoured brown discolouration, exudation texture and breakdown. The orientated polypropylene film (OPP) induced anoxic condition within 2 days and favoured anaerobic metabolism and necrosis of the sprouts. In all packages there was a rapid development of aerobic microorganisms and lactic acid bacteria that resulted in the accumulation of acetate and lactate and a decrease in pH. Thus, it clearly appeared that tissue decay was enhanced by microbial activity. At 8°C, 0.24 m 2 of film per kg of sprouts provided the optimal atmosphere composition (ie 5% oxygen and 15% carbon dioxide) when a film permeability of 50000 ml O 2 m -2 day -1 atm -1 was used. These conditions allowed a shelf-life of 4-5 days.

Respiratory parameters of onion bulbs (Allium cepa) during storage. Effects of ionising radiation and temperature

The O 2 and CO 2 respiration rates of untreated and irradiated onion bulbs (Allium cepa )a t 0.15 and 0.30 kGy were measured at 4, 10 and 20 °C. The O 2 respiration rate increased for 24h after treatment from 0.19 mmole kg ˇ1 h ˇ1 at 20 °C for control samples up to 0.26 and 0.39 mmole kg ˇ1 h ˇ1 for 0.15 and 0.3 kGy irradiated onions respectively. Respiratory quotient (RQ) increased with tempera- ture. The Q 10 of the respiration of the control samples (1.61) was lower than that of any other plant tissue, but it increased with storage duration and irradiation dose. The respiration rate of control onions increased steadily over 25 weeks of storage at 4 °C, while that of the irradiated samples decreased during the same period after a peak observed after irradiation treatment. The apparent K m for the Menten-Michaelis equation was determined on a new respirometer and averaged 1.6 kPa at 10 °C and 6.3kPa at 20 °C. However, at this higher temperature (20 °C) apparent K m varied with O 2 partial pressure, proving that the respiration of onion bulbs does not follow a Menten-Michaelis-like process. The Fermentative Index (FI) of onions was measured under anoxic conditions as CO 2 production rates in mmole kg ˇ1 h ˇ1 at 4, 10 and 20 °C. # 2000 Society of Chemical Industry