When meat is thawing in a plastic freezer bag, such as a Ziploc bag, it is possible for blood or liquid to leach out of the bag due to a phenomenon known as osmosis. Osmosis is the movement of solvent molecules (in this case, water) across a semi-permeable membrane (the plastic bag) from an area of lower solute concentration to an area of higher solute concentration.
In meat, there are various solutes, including salts, proteins, and other molecules, dissolved in the water. When the meat is frozen, water inside the cells forms ice crystals. As the meat thaws, these ice crystals start to melt, releasing water containing these solutes. The concentration of solutes in the meat's cells is higher than the concentration in the surrounding water (the thawing environment).
Plastic freezer bags are semi-permeable membranes that allow the movement of water molecules but block larger molecules like proteins and other components of blood. As the water from the thawing meat tries to reach equilibrium with the lower concentration of solutes outside the bag, it moves through the membrane of the bag. However, the larger solutes, such as proteins and blood components, cannot pass through the membrane, so they remain inside the bag.
This process leads to the accumulation of water outside the bag and the loss of blood and liquid from the thawing meat inside the bag. The more permeable the plastic bag is to water, the more pronounced this effect will be.
To minimize the loss of blood and liquid during thawing, you can use thicker or higher-quality freezer bags that are less permeable. Additionally, thawing meat in a container or on a tray can help contain any leaking liquid and make cleanup easier. Always remember to follow proper food safety guidelines during thawing to avoid any potential health risks.