Ozone treatment keeps the quality of fresh-cut green leaf lettuce
by Hülya Ölmez and Meltem Y. Akbas
||The application of 2 ppm ozonated water treatment for 2 min was found to be the optimum processing conditions for ozone disinfection of green leaf lettuce, in terms of reducing the microbial load and maintaining the sensory quality during cold storage.
Need for disinfectant of fresh organic vegetables
Safety and quality during a defined period of shelf-life is an important requirement to fresh-cut organic vegetables. Therefore, disinfection is one of the most important critical points along the processing line.
Due to the environmental and health risks the use of chlorine is forbidden in organic production in Europe. Moreover, there is a trend in eliminating chlorine from the disinfection process. Thus, there is a need for alternative sanitizers to be used for fresh-cut vegetables not only for the organic food sector, but also for the conventional food processors.
Ozone as alternative disinfectant
A number of studies have shown the potential of ozone as an antimicrobial agent, but so far little has been demonstrated on the effect of ozone treatment on the nutritional constituents and sensory quality of the product. However, for technical as well as biological reasons (Ölmez et al., 2009) it seems preferable to use of the lowest required levels of ozone for the disinfection of fresh produce.
In the present study we seeked to optimize the ozone treatment of green leaf lettuce in terms of ozone concentration, exposure time and temperature and to assess the effect ozonated water washing on the microbiological, nutritional and sensory quality parameters of minimally processed green leaf lettuce.
Optimal washing conditions with 2 ppm ozone
The effect of ozonated washing water on the overall visual quality (OVQ) of lettuce was tested with three ozone concentrations (0.5, 2.5 and 4.5 ppm) applied for three exposure times (0.5, 2.0 and 3.5 min). The lettuce quality was tested after shelf-life for 5 days.
Likewise, the effect of the different ozone concentrations and exposure times was tested on the reduction of viable counts of Listeria monocytogenes, that were initially inoculated onto leaves of the lettuce.
As a result of the simultaneous consideration of reduction in L. monocytogenes counts and the change in overall visual quality of lettuce, 2.0 ppm ozonated water treatment for 2.0 min was determined as the optimum conditions for washing green leaf lettuce (Figure 1 and 2). Under these conditions higher than 100-fold reduction was observed in L. monocytogenes counts and the OVQ was maintained at a level almost equal to the freshly prepared samples.
Effects of ozone concentration (ppm) and exposure time (min) on Listeria monocytogenes counts on green leaf lettuce washed at 10 degrees celcius.
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Effects of ozone concentration (ppm) and exposure time (min) on the overall visual quality (OVQ) of lettuce washed at 10 degree celcius.
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Testing ozone and other sanitizing treatments
Using the optimized ozone concentration and exposure time, portions of shredded lettuce were prepared for shelf-life studies in comparison with lettuce treated with chlorinated water, organic acid (citric plus ascorbic acid) and cold water (control). Thus lettuce from all treatments were packed in portions under storage atmosphere of oxygen and carbon dioxide and stored for up to 12 days at 4 degrees celcius.
Effective reduction in bactarial numbers
Significant reductions in aerobic mesophilic bacteria, psychrotrophic bacteria and Enterobacteriaceae were achieved by each of the sanitizing treatments as compared with the control treatment. And, under the given experimental conditions, the effect of ozone was not significantly different from the effect of chlorine and organic acid.
Normal contents of vitamin C and beta-carotene
In relation to the nutritional quality of the lettuce it was found that the contents of vitamin C and beta-carotene (Vitamin A) was not significantly affected by the sanitizing treatments with chlorine and ozone. Thus, control samples and treated samples had a similar content of these vitamins during shelf-life.
Ozone protects sensory quality
In terms of sensory quality, such as cut edge tissue browning (CEBT), firmness and aroma, the ozone treatment contributed to an extended shelf-life. Thus, all treatments maintained good quality until day 7 (>> Box 1). However, in general a significant loss of OVQ in all the samples and a rapid increase in CETB levels in the chlorine and organic acid washed samples were observed after 7 days of storage. Ozone treatment was found to be more efficient in retarding the cut edge browning compared to the chlorine washing (>> Box 1). This may be attributed to the inhibitory effect of ozone on the browning-related enzymes, namely polyphenol oxidase (PPO) and peroxidase (POD).
Thus, at the studied conditions, 2 ppm ozone treatment was found to be better than the chlorine and organic acid treatments in maintaining the sensory quality of green leaf lettuce during 9 days of storage (>> Box 1).
Importance of optimizing ozone concentrations
From the present and previous studies it is obvious that the effect of ozone treatment on the quality of produce is concentration dependent; it may be advantageous up to a certain level to apply ozone, whereas after a critical level, it may speed up the browning reactions resulting in an inferior quality as compared with the chlorine treatment. Therefore, optimizing the treatment by simultaneously assessing its effects on the microbiological safety, nutritional and sensory quality of the product is essential.
Ölmez, H. and Akbas, M. Y. (2009). Optimization of ozone treatment of fresh-cut green leaf lettuce. Journal of Food Engineering 90, 487494.