Packaging makes food more attractive and convenient in use. It also secures greater safety of the food against biological and chemical changes. Packaged foodstuff can enjoy longer shelf life. On the other hand, the food may become contaminated with components or degradation products of plastic packaging materials. Some chemical and physical properties of polymeric materials may change due to temperature changes, low pH, mechanical stress, and the presence of oxygen. Chemical changes of polymers are the source of food contamination. In certain situations, decomposition products from additives and monomers may migrate into the foodstuffs. Physical changes may occur as decreased structural, mechanical and barrier properties of the materials commonly used in food packaging. As a result, external microbiological and chemical contaminants may migrate through the packaging into the food. Till now, little information has been collected on the influence of biological factors on packaging properties. The paper discusses the effects of biological factors (lactic acid bacteria) on mechanical and morphological properties of plastics commonly used for fermented food such as cottage cheese. The preliminary investigations of bacterial biofilm forming on the polymer surface are also reported. (c) 2005 Elsevier Ltd. All rights reserved.