Food package comprised of polymer with thermally responsive permeability

A method of storing a biological material which is actively respiring and which is selected from the group consisting of foods and flowers, which method comprises

• (A) placing the biological material inside a container which
• (1) retains its shape at temperatures between 0.degree. C. and 40.degree. C.,
• (2) comprises one or more control sections which provide the only way in which oxygen, carbon dioxide and water vapor can enter or leave the container, at least one of said control sections being a first control section
• (a) which is composed of a polymeric material which comprises a crystalline polymer having a melting point T.sub.m from 0.degree. C. to 40.degree. C. and a heat of fusion of at least 5 J/g, and
• (b) said polymeric material being sufficiently permeable to oxygen, carbon dioxide and water vapor and the polymeric material exhibiting an increase in permeability to at least one of oxygen and carbon dioxide by a factor of at least 2.5 over a temperature range of 10.degree. C. between 0.degree. C. and 40.degree. C. such that said control sections cause said package to provide an environment around the biological material which adjusts to the respiration of the biological material to ex tend its shelf-life when the temperature of the container and biological material changes within the temperature range of 0.degree. C. and 40.degree. C.
• (B) storing the container in air, with the biological material inside it, at a temperature T.sub.1 which is at least 0.degree. C. and less than 40.degree. C.;
• and
• (C) storing the container in air, with the biological material inside it, at a second temperature T.sub.2 which is above T.sub.1 and at most 40.degree. C. and at which the permeability of the first control section of at least one of oxygen and carbon dioxide is at least 2.5 times the permeability at T.sub.1.