Effects of UV-C irradiation upon the reference strains of some food-associated microorganisms

SUMMARY

Within this paper are presented experimental researches on the possibility of using non-ionizing ultraviolet radiation UV-C to decrease the microbiological load existing on the external surfaces of horticultural products. Four microbial reference strains often associated with food (bacterial strains: Escherichia coli ATCC 11229, Bacillus subtilis subsp. Spizizenii ATCC 6633 and fungal strains: Aspergillus nigerATCC 15 475, Fusarium oxysporumMUCL 791) were irradiated to test the efficiency of UV-C decontamination performed by an installation for the decontamination of external surfaces of horticultural products. Due to the lethal mutations caused by UV-C radiations, in all cases the CFU/mL values were lower than the control. Also, it has been found that with a proper adjustment of the speed and distance from the UV-C source, it can be obtained an optimum configuration that will supply a maximum efficiency of the installation, in the given conditions.

Keywords: microbial populations, post harvest treatment, UV-C radiation.

INTRODUCTION

Consumed fresh, fruits and vegetables, can be carriers of some optional pathogenic microorganisms such as bacteria, yeasts, molds, coming from soil, water, air, and other environmental sources. These microorganisms can cause either loss of horticultural products in the storage process, due to the post harvest decay process, or food-borne diseases with direct effects on consumer human health.

The presence of air, high humidity, and higher temperature during storage, increases the chances of spoilage. The most common spoilage is caused by different types of molds, some of those from genera Aspergillus, Fusarium, Penicillium Phytopthora, Altemaria, Botrytis. A large number of molds produce toxic substances designated mycotoxins, as secondary metabolites. Some are mutagenic and carcinogenic, some display specific organ toxicity, and some are toxic by other mechanisms (Ray and Bunia, 2008).

Aspergillus is widely distributed and contains many species responsible for postharvest decay of fresh fruits including apples, pears, peaches, citrus, grapes.

Fusarium molds are associated with rot in citrus, apples, potatoes, and grains. Among the bacterial genera, species from Pseudomonas, Erwinia, Bacillus and Clostridium are most important.

Escherichia coli is a Gram negative, facultative anaerobic bacterium, belonging of Enterobacteriaceae. Many strains are nonpathogenic, but some of them are pathogenic to humans and animals and are involved in foodborne diseases.

Bacillus are cells aerobes or facultative anaerobes. All species form endospores with high resistance to radiation and heat. The genus Bacillus includes many species, some of which are important in foods, because they can cause foodborne disease and food spoilage. Bacillus is present in soil, dust and plant products, especially spices. Many species and strains can cause food spoilage.

Yeasts from genera Saccharomyces, Candida, Torulopsis. and Hansenula are associated with fermentation of some fruits, such as apples, strawberries, citrus fruits and dates (Jay et al., 2005).

In order to limit these losses, there have been used treatments with synthetic fungicide substances, whose residues remain on the surface of horticultural products, after the tratment. These residues are considered a potential threat to consumer health and especially children (Kasim and Kasim, 2007). Many outbreaks of gastroenteritis have been associated with the consumption of contaminated horticultural products (Franz and Van Bruggen, 2008). In order to reduce microbial contamination, there have been used widely, chlorine-based cleaning systems, being a significant interest in developing methods for safe and efficient decontamination of horticultural products (Hinojosa et al., 2013). Several alternative disinfectants (including hydrogen peroxide, organic acids and ozone) have been tested to reduce bacterial populations (Allende et al., 2006; Silveira et al., 2008; López-Gîlvez et al., 2009).

Conventional thermal methods of food sterilizarion are unsuitable for fruits and vegetable destined for fresh consumption because of the heat which cause inevitable changes of color, smell, flavor and a loss of nutritional value (Perni et al., 2008).

Conventional antimicrobial treatments for fresh produce rely on chemical compounds and physical contact to inactivate and remove bacterial contamination. Recent research has identified a number of energy-based alternative technologies to improve the safety of fresh and fresh-cut fruits and vegetables: ultraviolet radiation, electron-beam irradiation, technology with pulsed visible light and technology with cold plasma. In some cases, such as UV light, these technologies have a substantial database of information regarding the use in other domains, and can be adapted to use with fresh produce. In other cases, such as with electron-beam irradiation, advances in technology need new researches. Other technologies, such as pulsed visible light and cold plasma, are newer areas of research that hold promise as antimicrobial processes which can reduce the viability of bacterial pathogens on fresh produce. Within the methods earlier mentioned, a special potential has the use of non-ionizing ultraviolet radiation UV-C. The wavelength range that varies between 200 and 280 nm, which is considered lethal to most types of microorganisms, affects the DNA replication of these microorganisms (Bintsis et al., 2000; Char et al., 2010). Non-Ionizing UV radiation can cause breaks of molecular chemical bonds and can induce photochemical reactions. The biological effects of UV radiation depends on the wavelength and the exposure time. UV-C ultraviolet radiation is already successfully used in various fields such as medicine (decontamination of air and medical instruments), environment (wastewater treatment), packaging industry (decontamination of packaging for various food products) etc. Worldwide, there are initiatives in using this method for decontaminating the outer surfaces of food products. As a postharvest treatment on fresh produce, UV-C irradiation has been proven beneficial to reduce respiration rates, control rot development and delay senescence and ripening in different whole or fresh-cut fruits and vegetables, such as apples, citrus, peaches, watermelon, grape berries, tomatoes, lettuce, baby spinach and mushrooms (De Capdeville et al., 2002; Lamikanra et al., 2005; Allende et al., 2008; Artés-Hernîndez et al., 2010; Escalona et al., 2010; Jiang et al., 2010; Fava et al., 2011; Manzocco et al., 2011).

According to existing studies in the field, among the most common microorganisms that can contaminate horticultural products, with adversely affect on storage or human health, are shown in table 1. For the destruction of these potentially pathogenic microorganisms, it is recommended to apply certain doses of UV-C radiation.