Probiotics are live microorganisms, such as strains of Lactobacillus and Bifidobacterium, that confer a health benefit when administered in adequate amounts. These beneficial bacteria are highly sensitive to environmental factors like heat, moisture, and oxygen. The question of whether they can survive the extreme cold of a freezer is a common concern for individuals looking to extend the shelf life of supplements and fermented foods. Viability depends heavily on the preparation, the specific bacterial strain, and how the freezing and thawing processes are managed.
Probiotic Survival and Viability After Freezing
Many probiotic strains can survive freezing temperatures, but the process typically results in a loss of viability. Viability is measured in Colony Forming Units (CFUs), which represents the number of live, active microbial cells present. When a probiotic product is frozen, the total number of live CFUs generally decreases compared to the unfrozen state.
The degree of this loss varies widely depending on the product’s formulation and the bacterial species involved. Studies on cryopreservation (the scientific term for freezing living cells) show that survival rates can drop significantly without protective agents. For some strains, the reduction in viable cells can be as high as 50% or more, while others show much greater resilience.
The remaining viable probiotics are preserved in a state of suspended animation, which can prolong their shelf life for months or years. The rapid drop in temperature halts metabolic activity, essentially pressing the pause button on the natural decay and death of the cells. This preservation is the principle behind the industrial freeze-drying process used to create shelf-stable probiotic supplements.
Biological Effects of Freezing on Bacterial Cells
The loss of viability in bacterial cells during freezing is due to several damaging physical and chemical stresses. One primary culprit is the formation of ice crystals, which can physically rupture the bacterial cell walls and membranes. The size and location of these ice crystals are directly related to the rate of freezing, with slower rates often leading to the formation of larger, more damaging crystals.
As water turns to ice, the concentration of dissolved solutes outside the cell rapidly increases, creating a high-osmolarity environment. This osmotic shock draws water out of the bacterial cell, causing it to shrink and potentially damaging cellular proteins and DNA. Furthermore, the extreme cold itself, known as cold shock, can lead to the denaturation of macromolecules.
These combined stresses compromise the cell’s integrity, leading to a loss of function and ultimately cell death upon thawing. Researchers work to counteract these effects by introducing protective substances that mitigate the physical and chemical damage of the freezing process.
Strain Differences and Protective Agents
The ability to withstand freezing varies significantly among different probiotic species and even between strains of the same species. For example, strains of Bifidobacterium and Lactobacillus show different levels of sensitivity to cryopreservation conditions. The inherent biological differences in cell wall structure and metabolic pathways determine a strain’s natural cold tolerance.
To protect sensitive strains, manufacturers incorporate cryoprotectants into the formulation before freezing or freeze-drying. These protective agents work by limiting ice crystal formation and reducing osmotic stress on the cells. Common cryoprotectants include sugars like trehalose and sucrose, which help stabilize the cell membrane, and proteins such as skim milk, which form a protective matrix around the bacteria.
Skim milk, used alone or in combination with other sugars, has demonstrated a strong performance in helping various strains of Lactobacillus maintain viability during long-term storage. The presence of these agents is why commercially prepared probiotic foods and supplements often retain a high percentage of live cultures even after being subjected to industrial processing.
Consumer Freezing and Thawing Guidelines
For consumers who wish to freeze probiotic-containing foods like yogurt, kefir, or homemade ferments, proper technique can help limit the loss of viable cells. Before freezing, it is helpful to divide the product into smaller, single-serving portions to facilitate a faster freezing rate. Using an airtight, freezer-safe container minimizes exposure to oxygen and prevents freezer burn, which can further degrade the product.
When ready to consume, the thawing process is as important as the freezing process for cell survival. The most effective method is a slow thaw, which involves moving the frozen product directly to the refrigerator for several hours. Thawing slowly minimizes thermal shock, which can cause additional damage to the already stressed bacterial cells.
Rapid thawing in a microwave or at room temperature should be avoided, as the quick temperature change and potential for heat exposure can rapidly kill the remaining live bacteria. Once a product has been fully thawed, it should not be refrozen, as this double exposure to freezing and thawing stresses will significantly reduce the number of viable probiotic cells.