EFSA has assessed the risks to public health from Listeria contamination of vegetables that are blanched – scalded in hot water or steam for a short time – before they are frozen. They conclude that the risks associated with the consumption of these products is lower than for ready-to-eat foods such as smoked fish, cooked meat, sausages, pâté, soft cheese – which are usually associated with listeria contamination.
http://www.efsa.europa.eu/en/news/listeria-frozen-vegetables-how-reduce-risks
http://www.efsa.europa.eu/en/news/listeria-frozen-vegetables-how-reduce-risks