Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Steaks from gluteus medius m. (GM) and longissimus dorsi m. (LD) were packaged in atmospheres containing two low levels of CO (0.2 and 0.4%) in combination with 21% O-2 or without oxygen. The other components of the modified atmosphere packaging systems were carbon dioxide (30%) and nitrogen (balance). Steaks were evaluated on 0, 7, 14 and 21 days of storage at about 2 degrees C to investigate the effects of the gas mixtures on the microflora, instrumental (CIE L*a*b*) and visual colour and odour. The preservation of colour and odour of GM and LD steaks packaged under low carbon monoxide was best achieved using anoxic mixtures containing 0.2% CO and 30% CO2. Steaks packaged without oxygen retained a desirable colour and practically did not present a visible discoloration or perceptive off-odour along the entire storage time. Colour stabilization with 0.2% CO and 30% CO2, without O-2, following storage up to 21 days at 2 degrees C did not increase microbial risk of products. The increase in CO from 0.2 to 0.4% had a negative effect on the colour of LD steaks which was regarded as 'artificial' by some panellists. The higher stability achieved in this study using 0.2% CO may result from increased stability of the colour from Bos indicus x Bos Taurus cross. Discoloration and off-odour were evident in the GM and LD steaks packaged aerobically after 14 days of storage when the counts of deteriorant bacteria achieved approximately 6.5 to 9.7 log CFU/g. Highest bacteriostatic effect could be obtained if the volume of injected CO2 was sufficient to saturate the meat and keep in balance around the product during storage. Copyright (C) 2010 John Wiley & Sons, Ltd.236327338Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)FAPESP [04/14644-8]