THE EFFECT OF MODIFIED ATMOSPHERE PACKAGING ON THE GROWTH OF MICROORGANISMS IN COTTAGE CHEESE

The effect of modified atmosphere packaging (MAP) on the growth of mic roorganisms in creamed cottage cheese was investigated. Dry cottage ch eese curd with elevated yeast and mold and standard plate counts was p repared by a regional dairy processing plant. Creamed cottage cheese ( 1% fat) was prepared by mixing the curd with cream dressing. In additi on, batches of the high count, low fat creamed cottage cheese were ino culated with Listeria innocua ATCC 33090. The cheese was packaged into high barrier pouches with different headspace gases (air, 100% CO2, 1 00% N-2 and a mixture of 50% CO2:50% N-2) and stored at 5 degrees C or up to 28 d. In all pouches of cheese packaged with 100% CO2, 100% N-2 and the mixture of 50% CO2:50% N-2, the headspace gas composition rem ained the same throughout the storage trial. Whereas, all samples pack aged in air showed declines in the O-2 concentration with concomitant increases in CO2 content. Increases in pH during refrigerated storage were seen only in cottage cheese samples packaged with air. Cottage ch eese inoculated with Listeria showed growth in packages containing air and packages containing 100% N-2 but not in packages containing eleva ted CO2 levels. The growth of other pseudomonads was observed in sampl es packaged in air. The growth of yeasts and molds was most strongly a ffected by modifying the storage atmosphere. In cottage cheese package d in air considerable growth of yeasts and molds was observed, while g rowth was suppressed in samples packaged with nitrogen. The yeast and mold counts of cottage cheese packaged in 100% CO2 and the mixture of 50% CO2:50% N-2 declined during storage presumably due to the combined effects of CO2 and the acidic environment of the cheese. These data i ndicate that MAP with CO2 can inhibit the growth of spoilage microorga nisms in creamed cottage cheese packaged into high barrier containers. The data also suggests that cottage cheese packaged with air or nitro gen and presumably in conventional containers could be a vehicle for l isteriosis and that packaging cottage cheese in gaseous environments c ontaining elevated CO2 levels could reduce the risk of Listeria in cot tage cheese.