Journal of Applied Microbiology 1999, 86, 429–438 Effect of elevated oxygen and carbon dioxide on the surface growth of vegetable-associated micro-organisms A. Amanatidou, E.J. Smid and L.G.M. Gorris* Agrotechnological Research Institute, Wageningen, The Netherlands 6790/07/98: received 1 July 1998, revised 12 October 1998 and accepted 14 October 1998 A. AMANATIDOU, E.J. SMID AND L.G.M. GORRIS. 1999. The impact of a novel type of Modified Atmosphere (MA), referred to as high O 2 -MA, on micro-organisms associated with the spoilage of minimally-processed vegetables was studied. Pure cultures of Pseudomonas fluorescens, Enterobacter agglomerans, Aureobacterium strain 27, Candida guilliermondii, C. sake, Salmonella typhimurium, Salm. enteritidis, Escherichia coli, Listeria monocytogenes, Leuconostoc mesenteroides var. mesenteroides, Lactobacillus plantarum and Lactococcus lactis were cultured on an agar-surface model system and incubated at 8 °C under an atmosphere composed of O 2 (80 or 90%, balanced with N 2 ), CO 2 (10 or 20%, balanced with N 2 ), or a combination of both gases. In general, exposure to high O 2 alone did not inhibit microbial growth strongly, while CO 2 alone reduced growth to some extent in most cases. Consistently strong inhibition was observed only when the two gases were used in combination. With minimally-processed vegetables, where CO 2 levels of around 20% or above cannot be used because of physiological damage to the produce, the combined treatment of high O 2 and 10–20% CO 2 may provide adequate suppression of microbial growth, allowing a safe, prolonged shelf-life. INTRODUCTION Modified atmosphere packaging (MAP) is used to extend shelf-life and maintain high quality of minimally-processed fruits and vegetables. The rapid increase in the market share of MAP of ‘ready-to-eat vegetables’ reflects the trends of today’s consumers for fresh, additive-free foods. Respiring products, like raw or processed vegetables and ready-made salads, generate equilibrium gas conditions inside the package that are typically very low in O 2 (2–3%) and moderately high in CO 2 (5–20%). These conditions reduce deterioration by limiting product respiration and maturation (Kader 1986; Day 1992; Gorris and Peppelenbos 1992) as well as by slowing down the proliferation of aerobic spoilage micro-organisms (Hotchkiss 1988; Kader et al. 1989; Moleyar and Narasimham 1994). The antimicrobial effect of CO 2 on micro-organisms has been intensively documented (Molin 1983; Eklund 1984; Daniels et al. 1985; Dixon and Kell 1989). Correspondence to: Dr Athina Amanatidou, Agrotechnological Research Institute, PO Box 17, 6700 AA Wageningen, The Netherlands (e-mail: A.AMANATIDOU@ATO.DLO.NL). *Present address: Unilever Research Laboratorium, Olivier van Noortlaan 120, 3133 AT Vlaardingen, The Netherlands. © 1999 The Society for Applied Microbiology Although MA packaging of respiring produce has come into substantial use in practice, some potential problems with regard to product quality and safety remain to be solved. The O 2 and CO 2 levels in an MA package are achieved mostly by the active respiration of the produce and are often difficult to predict and control (Ahvenainen 1996). All too frequently, O 2 is completely depleted, resulting in the production of off- odours and rapid deterioration of the product (Zagory and Kader 1988). In addition, excessive levels of CO 2 (generally over 20%) cause specific disorders such as the development of brown stains (Lougheed 1987; Kader et al. 1995). Bennik et al. (1995) showed that CO 2 levels from 20–50%, in com- bination with low O 2 (1·5%), affect the growth rate of spoilage bacteria relevant to minimally-processed vegetables but have no effect on the maximum population densities or the lag phase duration. Concerning product safety, psychrotrophic pathogens such as Listeria monocytogenes are not suppressed under MA con- ditions that are optimal for respiring produce (Berrang et al. 1989; Brackett 1994; Bennik et al. 1995; Carlin et al. 1996; Francis and O’Beirne 1997). On the contrary, growth may be enhanced in certain cases because of suppression of the natu-