Effect of Gamma Irradiation on Microbial Quality of Minimally Processed Product in Tunisia: A Case of Ready to Eat Salad

Research Article

J Bacteriol Mycol. 2021; 8(2): 1167.

Effect of Gamma Irradiation on Microbial Quality of Minimally Processed Product in Tunisia: A Case of Ready to Eat Salad

Rahmani F1,2, Yahya M1, Jebri S1*, Amri I1, Mejri A3, Hamdi M4 and Hmaied F1

1Tunis El Manar University, National Center of Nuclear Sciences and Technologies (CNSTN), Tunisia

2Tunis El Manar University, Tunisia

3Ionizing Radiation Dosimetry Laboratory, National Center for Nuclear Sciences and Technologies (CNSTN), Tunisia

4University of Carthage, National Institute of Applied Sciences of Tunis (INSAT), Tunisia

*Corresponding author: Sihem Jebri Cnstn, Tunis El Manar University, National Center of Nuclear Sciences and Technologies (CNSTN), BP 2020, Sidi Thabet, Tunisia

Received: January 24, 2021; Accepted: March 15, 2021 Published: March 22, 2021


The use of gamma irradiation in food safety management as a tool to improve the microbiological quality of food products. Minimally processed product may contain a large number of spoilage microorganisms that constitute a potential health risk. In this study, raw carrot samples and fresh-cut products after each processing steps: water chlorination, peeling process and citric acid treatment were analyzed for the total aerobic plate count, Staphylococcus spp., yeasts and molds. Ready to eat products were also analyzed for these selective pathogens. The freshly packaged carrot salads were irradiated at various doses (0.5, 1.0, 2.0 kGy) and analyzed during 15 days storage period. The obtained results showed that raw carrots were highly contaminated by total aerobic plate count (7.23 Log10/25g), Staphylococcus spp. (3.77 Log10/25g), yeasts (5.62 Log10/25g) and molds (5.54 Log10/25g). Washing treatment and peeling process, were able to reduce the concentration of total aerobic plate count by 2.23 Log10 and to remove Staphylococcus spp. and molds. The mean concentrations of total aerobic plate count, Staphylococcus spp. yeasts and molds were 4.87, 2.08, 7.47 and 2 Log10/25g respectively for packaged salads. These results suggest that the contamination of carrot salads might occur through chain transformation. Regarding gamma irradiation effect, an optimal dose of 2kGy offered a pathogen-free, hygienic product in comparison with controls. Furthermore it increased shelf-life by 4 to 9 days at refrigeration temperature. The validity of the processing treatment at 2kGy was challenged by artificially inoculating Staphylococcus aureus in the product.

Keywords: Ready to eat salad; Gamma irradiation; Total aerobic plate count; Yeasts and molds, Staphylococcus spp.; Processing steps


In Tunisia, minimally processed product consumption is increasing, mainly fresh vegetables and fresh cut vegetables became one of the most important parts of the Tunisian food diet [1]. Vegetable salads usually used as a common supplement to urban fast food served in restaurants and canteens. The current pace of Tunisian community life pushes to the use of ready to eat vegetable products: they are offered in portions and can be consumed fast and easily [1]. Among Ready-to-Eat (RTE) vegetables, fresh salads do not undergo bactericidal heat treatment before consumption and may constitute potentially high-risk products. Consequently, spoilage microorganisms can proliferate in fresh vegetable salads and cause common foodborne diseases [2-4]. The first source of fruits and vegetables contamination is wastewater reuse in agriculture for irrigation and organic amendment of agricultural land [4-7]. In Tunisia, 43% of treated wastewater, are reused for an agricultural purpose such as irrigation of vegetable crops [8,9]. The microbiological contamination of fresh fruits and vegetables can occur throughout the food chain [10]. Hence, in the agri-food industry, the infected operator, who does not sufficiently respect the basic hygiene measures, can contaminate handled food products (processing, packaging, storage…) [11,12]. After the treatment of fruits and vegetables with citric acid and chlorination, the possibility of recontamination of these products is also possible [10,11]. It might occur through the formation of biofilms at the surface of the processing and packaging machinery or from the operators who have not followed the hygiene measures [13-15]. Total viable count, Staphylococcus spp. yeasts, and molds are known to dominate the microflora on fruits and vegetables [16]. Yeast and molds are spoilage microorganisms in carrot and could reach high concentrations in this product [17]. Staphylococcus spp. are considered as a biological hazard worldwide. They are potential pathogens causing several infections in human and animal. Staphylococcus spp. are also among common foodborne pathogens through the contamination of several foods such as fresh vegetables and dairy products [1,18]. Staphylococcus aureus is considered the third most important cause of disease in the world amongst the reported foodborne illnesses [19]. Staphylococcus aureus was responsible for 25% of all foodborne illnesses in the USA [20] and 5.1% of food poisoning outbreaks reported in Europe [21]. Several treatment methods are used to minimize health risks associated with collective food poisoning. Food irradiation is processing method used to improve the microbiological quality of several food types [3,22-24]. It is recommended to reduce the risk of food poisoning and extend food shelf life without detriment to health and with minimal effects on nutritional and sensory quality [25]. In this study, we aimed at evaluating the microbiological quality of carrot salads, evaluating the efficiency of treatment process throughout chain transformation and the effect of gamma irradiation on pathogenic microorganisms load.

Material and Methods

Sample collection

A total of 42 of minimally processed carrots were investigated in this study. In fact, 26 freshly carrot salad packaged in expanded polystyrene container (22.5 X 13,5 cm) wrapped with the stretch film, were collected from an agri-food industry located in the north of Tunisia. The shelf life indicated on this product was 4 days. The treatment process used through chain transformation includes chlorination treatment, peeling process and citric acid treatment. 16 carrot samples were collected through processing treatment. Raw carrot samples (n=4) as well as after each treatment: chlorination treatment (n=4); peeling process (n=4) and citric acid treatment (n=4). Samples were carried to the laboratory and processed within 24 h.

Irradiation of packaged carrot salad

The freshly packaged carrot salad was irradiated at various doses (0.5, 1.0, 2.0 kGy) using Cobalt 60 source at a dose rate of 5.305 Gy/min and homogenization index of 1.05. Nonirradiated samples served as controls. The effect of gamma irradiation was evaluated using artificially contamination of sterilized packaged carrot salad (exposed to 2kGy dose) by 106 CFU/ml of Staphylococcus aureus (ATCC 25823). The packaged carrot salad were dipped into the selective strain of Staphylococcus aureus for 10 min and repacked in a polystyrene container. The irradiated samples as well as controls, were stored during 15 days at refrigeration temperature 4°C.

Microbiological analysis

Firstly, 25g of each sample was diluted with 225 ml of Peptone Water (Biokar diagnostics, France) and homogenized by stomacher (AES, 400ml) for 2 min. Then, serial dilution was performed and 100 μl from each dilution was dispensed onto Petri dishes with appropriate media in triplicate.

Enumeration of total aerobic plate count

The detection of total aerobic plate count was performed using Plate Count Agar (Biokar diagnostics, France) and incubated at 37°C for 24 h.

Isolation of Staphylococcus spp

Staphylococcus spp. were isolated using Baird Parker medium supplemented with egg-yolk tellurite emulsion (Biokar diagnostics, France) and incubated at 37°C for 24 h to 48 h.

Yeast and molds isolation

Yeast and molds counts were determined by surface spreading of 0.1 mL sample on sabouraud chloramphenicol agar (Biokar, diagnostics, France). Incubation of the plates was performed at 25°C for 3-5 days.

Data analysis

Statistical analysis was performed using STATGRAPHICS Centurion XVI software version 16.2.04. Statistical data comparisons of pathogens concentrations after irradiation processing were conducted using Analysis of Variance (ANOVA) tests.


After water chlorination process of raw carrots, the mean concentration of total aerobic plate count, Staphylococcus spp. yeasts and molds was decreased by 0.29; 1.25; 0.85 and 1.04 Log10/25g respectively. The peeling process was able to decrease the concentration of total aerobic plate count and yeasts from previous process by 1.55 and 0.17 Log10. Also peeling process, was able to remove Staphylococcus spp. from carrot samples. After citric acid washing treatment Staphylococcus spp. and molds were not detected. Furthermore, this treatment reduced the mean concentration of total aerobic plate count and yeasts from peeling process by 0.39 and 0.21 Log10/25g respectively. After all processing steps, the microbiological profile of carrots showed, a decrease of total aerobic plate count and yeasts by 2.23 and 1.23 Log10/25g respectively, a removal of Staphylococcus spp. and molds (Table 1). D10 values were determined, as dose of irradiation needed to elicita1-log 10 reduction of bacteria for irradiated samples. They are shown in Table 2 for each pathogens.