Fri. Apr 19th, 2019

Why do they sell bags of potatoes filled with air | Science

Why do they sell bags of potatoes filled with air | Science

The picaresque at the time of selling certain foods is the order of the day: misleading advertising, labeled ambiguous and other strategies that often lead us to distrust the honesty of much of the food industry. Among the products that cause most misgivings in this regard are chips. And when we open a bag and see that it is half empty, the first thing we think is that we have been cheated by selling air at the price of gold. But the explanation is not so simple.

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In general terms, we like that the French fries have a golden aspect, an appetizing flavor with a point of salt and a crunchy texture. This is not as easy as it might seem. For this, it is necessary to take into account numerous factors related to raw materials and the manufacturing process, such as the potato variety, its state of conservation, the type of cut made, the oil used for frying, the weather conditions and temperature applied in the operation or the amount of salt used. In addition, another important challenge is posed: to ensure that these attributes remain intact for as long as possible so that the chips reach our hands in optimal conditions.

This method of preservation allows to prolong the useful life of the product until 4-6 months, avoiding also the use of additives that have such a bad reputation.

In most foods the main concern when preserving its qualities is centered on the possible growth of microorganisms capable of causing alterations or causing diseases (especially bacteria and molds). However, this is not a problem in French fries because they have low water activity, they do not have enough for microorganisms to develop. This is so if the product does not pick up moisture from the environment, in which case not only would its safety be compromised, but it would also adversely affect its flavor and, above all, its texture: the potatoes would soften and stop being crispy. Therefore, humidity is one of the elements that must be taken into consideration when preserving the product.

To prevent the humidity of the environment from altering the characteristics of the potato chips, what can be done is to use hermetic containers formed by materials that act as a barrier to water vapor, such as a plastic bag. However, this is not enough to keep the product intact because there are other elements that can also damage it. Among the most important are light and oxygen, which are responsible for causing the oxidation of fats, a phenomenon that causes the development of rancid odors and flavors and alterations in color, as well as the deterioration of certain nutrients ( polyunsaturated fatty acids and fat-soluble vitamins) and the formation of potentially toxic compounds. In other words, eating rancid potatoes is not only unpleasant because of its bad taste, it can also negatively affect our health.

Without going into many details, what happens during this biochemical process is that light induces the decomposition of certain fatty acids, giving rise to free radicals that, in the presence of oxygen, are oxidized. Subsequently, the latter remove hydrogen atoms from the unsaturated fatty acids, so that hydroperoxides and new free radicals are obtained, which continue to react. Thus begins a chain reaction that only stops when there is no oxygen or fatty acids to continue the process. That is, there is no way to stop this series of reactions, so to avoid its undesirable consequences it is necessary to prevent it from starting. For this, two of the most important elements that participate in it must be controlled and, as we have already mentioned, are light and oxygen.

In order to make the protective atmosphere effective, the relationship between the volume of gas and the volume of the food must be equal to or greater than two.

Therefore, to protect fried potatoes it is necessary to use a container made of a material that acts as a barrier against light, oxygen and humidity. But not only that. In addition, you must meet a series of mechanical skills (resist tearing and tensile forces, be able to close with the application of heat, etc.), commercial (must be suitable for printing, easy to open, attractive, etc.) and of another type (it must be economic, light, inert, etc.). As it is difficult for a single material to fulfill all these characteristics simultaneously, what is usually done is to use a structure formed by multiple layers with sheets of different materials, such as polypropylene and metallized polypropylene, which serve as protection against light and prevent diffusion of gases (water vapor, oxygen, etc.).

At this point, there is still a loose end. As we have seen, the container would act as a barrier to stop the action of agents that can alter the product from the outside, but in the interior there would still be air and its oxygen content could cause oxidation of the lipids. To avoid this, what is known as packaging in a modified atmosphere or protective atmosphere is used. As can be deduced from its name, this consists of replacing the air inside the product with a gas or a mixture of gases that protect it from deterioration, allowing to extend its useful life. These are usually mixtures of oxygen, carbon dioxide and nitrogen in different proportions, depending on the type of product (for example, meat, fish, bag salads, etc.) and their characteristics. For example, fried potatoes with sunflower oil are more susceptible to rancidity than those that have been fried in high oleic sunflower oil, due to the lower proportion of polyunsaturated fatty acids of the latter. In the case in question, what is usually used is nitrogen, which is a colorless, odorless, tasteless and inert gas, that is, it does not react with other compounds. Although it is not the only option. For example, a mixture of nitrogen is sometimes used together with some noble gas, such as argon.

In order to make the protective atmosphere effective, the relationship between the volume of gas and the volume of the food must be equal to or greater than two. What is sought is that the oxygen proportion is below 0.2%, since this way it is achieved that the alterations of smell or taste due to the oxidation of fats are negligible. So, mystery solved. This explains why the bags of potatoes are sold "half empty".

We must observe the labeling to see if it is packed in protective atmosphere and to know the net weight, the amount of chips

This method of preservation allows to prolong the useful life of the product up to 4-6 months, avoiding also the use of additives that have such a bad reputation, despite being safe in the doses of employment allowed. By the way, the large volume of gas present in the container protects the product against breakage by impact or crushing. But not everything are advantages. It is an expensive method and that makes the product more expensive than another without protective atmosphere. In addition, it generates distrust because it tends to make us think that the package contains more quantity of product than it might at first glance. So what we should do in these cases is to observe the labeling to see if it is a product packed in a protective atmosphere and, above all, to know the net weight, which clearly indicates the amount of chips contained in the package.

We must not forget that the moment we open a bag of chips packed in a protective atmosphere, the leakage and protection offered by the gases that were contained inside are lost, so the product is exposed to moisture, oxygen and, if we're not careful, in light, which are the riders of the apocalypse of the potato chips.

Miguel A. Lurueña (@gominolasdpetro) is a doctor, a graduate in food science and technology, a technical agri-food engineer and a scientific disseminator (

NUTRIATE WITH SCIENCE It is a section on food based on scientific evidence and knowledge contrasted by specialists. Eating is much more than a pleasure and a necessity: diet and eating habits are now the public health factor that can most help us prevent many diseases, from many types of cancer to diabetes. A team of dieticians-nutritionists will help us to better understand the importance of food and to tear down, thanks to science, the myths that lead us to eat badly.


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