Is Everything We Put in the Refrigerator Really Protected?
From My Food Engineering Notebook – Series #2
When we put food in the refrigerator, we rarely stop to think about what we are actually doing. We keep milk cold, freeze meat, place vegetables in the crisper drawer, and know that they will last longer this way.
However, behind what we call “cold storage,” there lies a vast system spanning from the structure of the food to the behavior of microorganisms.
While flipping through the lecture notes I took during my university years, I realized how often the concepts I once learned just for exams pop up in daily life. This piece is the second entry in a series where I expand upon those notes and turn them into articles.
Why Does Food Spoil?
Food spoilage does not merely mean the proliferation of microorganisms within a food product. Undesirable changes in taste, aroma, color, texture, or nutritional value are also part of spoilage.
Behind these changes, chemical and biochemical reactions can be just as responsible as microorganisms. For instance, reactions involving oxygen, enzymatic activity, or interactions between food components can alter the product’s quality over time.
The growth of microorganisms, on the other hand, is determined by the food’s intrinsic properties. Factors such as pH, moisture, water activity, nutrient composition, oxidation-reduction potential, and natural antimicrobial compounds serve as “intrinsic factors.” Alongside these, extrinsic factors – such as temperature, relative humidity, atmospheric gas composition, and processing methods – also influence microbial growth.
Therefore, extending the shelf life of a food product involves far more than controlling a single parameter.
The objective is to slow down or control the mechanisms causing spoilage as much as possible.
The Logic of Food Preservation
Various approaches can be utilized to combat microorganisms in food preservation. Physically removing microorganisms, creating environments unfavorable for their growth, or directly inactivating them are a few among these strategies.
Thermal processing, refrigeration, freezing, drying, filtration, irradiation, fermentation, and certain chemical treatments can be employed for these purposes.
Yet, a delicate balance must be maintained: while controlling microorganisms, the food itself must be preserved as much as possible.
When a treatment is too severe, it affects more than just the microorganisms. The food’s color, aroma, texture, and heat-sensitive nutrients can also be altered.
Hence, in food preservation, the goal is not simply to “kill microorganisms.”
It is about maximizing safety while preserving product quality as much as possible.
What Actually Happens When Heat Is Applied?
Thermal processing occupies a critical place in food preservation. Heat is utilized to destroy microorganisms and inactivate enzymes.
Mechanisms such as protein denaturation and the disruption of cell membrane lipids play a key role in the thermal inactivation of microorganisms. Moist heat, in particular, can damage cellular structures such as DNA, cell membranes, cell walls, and RNA.
However, heat resistance varies across different microorganisms. Factors including the species, growth stage, initial population, and the food’s pH, water activity, and composition modify the efficacy of thermal processing.
For this reason, focusing solely on temperature is insufficient in thermal operations.
Time and temperature must be evaluated together.
Pasteurization serves as a clear example. Pasteurization is defined as a relatively mild thermal process applied below 100 °C. The objective is to control vegetative cells, particularly of pathogenic microorganisms, thereby enhancing food safety and extending shelf life.
For instance, different time-temperature combinations can be utilized, such as LTLT (62.8 °C for 30 minutes) or HTST (72 °C for 15 seconds).
Sterilization, on the other hand, targets a much broader scope. It aims to control microorganisms and resilient structures (spores) through higher temperatures and appropriate pressure conditions.
Distinguishing between these two concepts is essential:
Pasteurization is not sterilization.
Following pasteurization, certain heat-resistant microorganisms or spores may survive. Consequently, pasteurization is often combined with refrigeration, proper packaging, or other preservation hurdles.
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Cold Slows Microorganisms Down Rather Than Eliminating Them
Unlike heat, low temperatures are primarily used to retard microbial growth.
As temperature drops, the growth rate of most microorganisms declines, while enzymatic and chemical reactions simultaneously slow down. However, this does not mean that microorganisms disappear completely in cold environments.
Certain psychrophilic and psychrotrophic microorganisms can continue to grow at low temperatures.
Thus, a refrigerator is not an environment that sterilizes food; it is a controlled environment that slows down spoilage processes.
Furthermore, not all foods are stored at the same temperature. While products like fresh meat and fish require lower temperatures, dairy products, ready-to-eat salads, fresh pasta, or certain juices can be kept within different cold storage ranges.
Low temperatures are not universally advantageous either. In certain fruits and vegetables, excessively low temperatures can trigger “chilling injury.”
Simply put, there is no basic rule stating “colder = better” in food preservation.
The right temperature is the correct temperature tailored to the specific product.
Freezing Is Not Pausing Time
Freezing is likewise not a process that completely eradicates microorganisms. By dropping the temperature of the food below its freezing point, a portion of its water transforms into ice crystals, thereby severely restricting microbial activity.
Here, the rate of freezing becomes paramount.
Rapid freezing forms smaller ice crystals. These smaller crystals cause less mechanical damage to the cellular structure of the food.
In contrast, slow freezing can form larger ice crystals, increasing structural cellular damage. Consequently, when the product thaws, greater drip loss and quality degradation can be observed.
This phenomenon is easily noticeable in the kitchen. When considering why a thawed frozen vegetable lacks its original crispness, part of the answer lies in how ice crystals interact with the food’s cellular matrix.
However, for a frozen product, the initial freezing step is not the only critical factor.
Maintaining the cold chain is equally vital to product quality.
Frozen foods must be maintained at -18 °C or lower throughout storage, transport, and retail display. Every stage a product undergoes from leaving the factory to reaching the supermarket shelf forms a link in this chain.
Minimizing temperature fluctuations, ensuring proper storage, maintaining correct stock rotation, and transferring products swiftly are therefore not merely operational details – they directly impact food quality.
Conclusion: Shelf Life Is More Than Just a Date
When we look at the shelf life date printed on a package, what we are actually seeing is the outcome of a comprehensive process.
A food’s composition, pH, water activity, applied treatments, packaging, storage temperature, transit conditions, and microbiological quality are all evaluated collectively.
Viewing food preservation as merely “storing food without letting it spoil” falls short.
In reality, what we are doing is managing the evolution of food over time.
We control microorganisms with heat, slow their growth with cold, reduce conditions suitable for microbial activity by freezing water into ice, and support this entire process with packaging and controlled storage environments.
Revisiting the notes I took in college today, what pleased me most was this realization: facts I once underlined for an exam now confront me on supermarket shelves, in the refrigerator, and in the kitchen.
This is the second entry in a series where I re-read, expand upon, and translate those university notes into written pieces.
Perhaps the true way to know you have learned something isn’t recalling it for an exam – it’s starting to notice it in daily life years later. :)
