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How do probiotics stay alive until they are consumed?

How do probiotics stay alive until they are consumed
Kefir grains from a freeze-dried Kefir starter

The idea of consuming live microbes to promote health is not new. Back in 1907, Élie Metchnikoff, a disciple of Louis Pasteur, the Father of Microbiology, was attracted by a new discovery made by Bulgarian scientist Stamen Grigoroff, who found the reason for the fermentation of Bulgarian yogurt. Later, Metchnikoff associated the intake of fermented milk containing live lactobacilli with a prolonged and healthy life in Bulgarian peasants at the time.

So how do probiotics stay alive until they are consumed?

Life is the condition that distinguishes animals and plants from inorganic matter. Life includes the capacity for growth, reproduction, and metabolic activity.

In order to sustain life, certain environmental conditions must be met, which differ for different organisms. The availability of water and nutrients, adequate temperature, pH (acidity), and the absence of growth inhibitors are essential for microbes.

It is also possible to manipulate certain conditions to bring about a state where growth may be put in “standby mode,” yet the microbe remains alive. We cannot imagine ourselves in a condition where life is preserved even without any metabolic activity, but for microbes, it is possible.

Probiotics can be in foods (yogurts, fermented milk, fruit juices, cereal bars) or in supplements (capsules, compressed pills) in a “hibernation” state, characterized by no growth, no reproduction, and no metabolic activity, waiting for the proper conditions to come back to full metabolic life. This occurs when the microbes reach the gut having the proper temperature, nutrient availability, lack of inhibitors, adequate acidity, and water. Thus, in the case of microbes, there is an uncoupling of life and metabolic activity. Even without having any metabolic activity, they can still be alive but in a dormant state.

Open a food supplement containing probiotics, and you will probably find a dry powder. This is what the microbes may look like in their dormant state due to a technological process called “lyophilization,” also known as freeze-drying.

Freeze-drying is a two-stage process where cells are first quickly frozen at very low temperatures (-40 to -70°C or less, using liquid nitrogen, for example). Then, frozen water is removed by a gentle process of evaporation at low pressure and temperature, called sublimation. This process removes most of the water from around and inside the cells, leaving the microbes in a dormant state. Water activity is a scientist’s way of measuring water availability for the microbes. This technological measure ranges from 0 (no water) to 1 (pure water). A water activity close to 0 impairs growth. In food supplements and also starters for fermented foods like yogurt and kefir, freeze-drying leaves water activities less than 0.2, ensuring that no metabolic activity will take place during the shelf life of the product.

So yes, probiotics in supplements, pills, freeze-dried yogurt, and kefir starters are alive in their own way. This is also the case for probiotics in certain foods, such as cereal bars. In the case of food products with water activities closer to 1, such as yogurts, fermented milk, cheeses, or fruit juices containing probiotics, the factor that limits metabolic activity is the low temperature at which these products are stored, combined in certain cases (yogurts, fermented milk, fruit juices) with the low pH (or high acidity) of these products. The combination of low temperature and acidity is effective in maintaining probiotic cells in a dormant state, impairing any metabolic activity that may lead to cell stress and cell death. Yet, even while tightly controlled factors impair metabolic activity, some cell death may occur during the shelf life of the probiotics in the products that deliver them. In this case, responsible manufacturers are sure to add extra probiotic cells so that the necessary number of viable cells needed to deliver a health effect is present through the end of the product’s shelf life.

In both probiotic foods and food supplements, the number of viable cells is commonly expressed as a certain number of colony-forming units. As probiotics are present in high concentrations, the number of viable cells often reaches into the billions within a capsule or in a serving of yogurt.

To be able to count such enormous numbers of cells, microbiologists must make serial dilutions of the probiotic product. Then, they will put a small drop of a solution onto the surface of a Petri dish containing the culture medium, on which probiotics will grow. Each probiotic cell (or clump of cells) will grow in place and form a visible colony that can be observed by the naked eye and counted.

Live probiotics are present in food and supplements, but in a life stage different from that of higher organisms, where metabolic activity is taking place at all times. During shelf life, the metabolic activity of probiotics is stopped by freeze-drying them (food supplements or starters for yogurt and kefir) or by a combination of low temperature and acidity (ready yogurts and kefir and fruit juices, for example).

Active growth returns when these microbes enter our gut and find the proper conditions of nutrients, temperature, acidity, and water to be active and deliver their health effects.


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