Probiotics are defined as live microorganisms that, when administered in adequate amounts, confer a health benefit on the host. Because these are living cells, maintaining the colony-forming unit (CFU) count requires careful temperature and moisture management throughout the product’s lifespan. The viability of these beneficial bacteria depends entirely on protecting them from environmental stresses, which include both extreme cold and heat.
How Freezing Affects Bacterial Life
Freezing does not necessarily kill probiotic bacteria, but it introduces significant stress that can dramatically reduce their numbers. The primary threat during freezing is the formation of ice crystals, which physically puncture the bacterial cell walls and membranes. This mechanical damage leads to the rapid loss of viability, a process known as freeze-thaw injury.
Commercial probiotic products, particularly those that are shelf-stable, undergo lyophilization, or freeze-drying, which is carefully controlled to prevent this damage. Manufacturers add specialized compounds known as cryoprotectants, such as trehalose, glycerol, or skim milk. These substances lower the freezing point of the water inside and outside the cell, preventing large, destructive ice crystals from forming. The cryoprotectants also form a protective glassy matrix that stabilizes the bacterial cell structures during drying and storage.
When probiotic capsules are frozen at home, they lack these specialized cryoprotectants, making the bacteria highly susceptible to ice crystal formation. The cold itself merely slows the metabolic rate of the bacteria, inducing deep dormancy. However, without the protective matrix, the physical damage from freezing water can compromise the integrity of a substantial portion of the culture.
Other Environmental Factors That Compromise Viability
While freezing poses a risk, other environmental factors like heat and moisture are often greater threats to probiotic potency. Probiotic bacteria are highly sensitive to elevated temperatures, which rapidly accelerate their death rate. Exposure to temperatures exceeding \(115^\circ\text{F}\) (\(46^\circ\text{C}\)) causes irreversible damage by denaturing the cells’ proteins. Leaving a supplement bottle in a hot car can destroy the majority of the live cultures in a matter of hours.
Moisture and high humidity are equally damaging, as they prematurely reactivate the dormant, freeze-dried bacteria. Once reactivated inside the capsule, the bacteria consume their limited resources, leading to a quick loss of viability before they can reach the digestive tract. Moisture can also trigger unwanted chemical reactions and degradation within the capsule matrix. Light and oxygen exposure are minor contributing factors that stress the cultures, accelerating the decline in the overall CFU count.
Guidelines for Optimal Probiotic Storage
The most important step for maintaining probiotic viability is to closely follow the storage instructions printed on the product label. Probiotics are categorized into those requiring refrigeration and those considered shelf-stable. Refrigerated formulas should be stored consistently in the main body of the refrigerator, not the door, where temperature fluctuations occur frequently.
For both refrigerated and shelf-stable products, moisture control is paramount to preserving potency. Always keep the product in its original, tightly sealed container, as the packaging materials are designed to block humidity. Storing probiotics in high-humidity environments, such as a bathroom cabinet or above a kitchen stove, should be avoided. If the bottle contains a desiccant packet, it should be left inside to absorb ambient moisture.
Home freezing of standard probiotic capsules is generally not recommended because the lack of commercial cryoprotectants increases the risk of ice crystal damage. While the cold slows the bacterial metabolism, the physical stress of the freezing and subsequent thawing process can compromise the capsules and the live cultures. Choosing a high-quality product that guarantees the CFU count until the expiration date provides the best assurance of potency.