When I was younger, I always wondered why my Grandmother would put bread in the freezer. Bread is not eaten frozen, so was she trying to make a bread popsicle? Years later, having gained knowledge of food science, I now know that grandma’s generation was using a clever technique to preserve the quality of the bread.

What Causes Bread Quality to Deteriorate?

Two major processes contribute to the deterioration of bread i.e. spoilage and staling. Spoilage refers to the deterioration of food to the point where it is no longer palatable or edible (this is seen when bread goes mouldy). It is caused by small organisms that cannot be seen with the naked eye, known as microorganisms. Staling is a process that happens in starchy foods, such as bread. It results in a change from the original desirable texture of the food to a hard, dry, and unpalatable texture (Wikipedia, 2021).

Why Freezing?

Grandma would have realized that freezing prevents both spoilage and staling. But how? To explain this, we must understand the science behind spoilage and staling.

The Science Behind Spoilage

As mentioned before, spoilage is caused by microorganisms. There are several types of microorganisms including bacteria, mould, yeast and viruses. Special microorganisms known as spoilage microorganisms specifically target the types of foods that humans consume. When this happens, the food may change in texture, odour and flavour, making it unpalatable and, in some cases, completely inedible (Gram et al., 2002). Therefore, if we want to be able to store our food over time, we have to find ways to control spoilage microorganisms.

How Do We Control Spoilage Microorganisms?

To control them we must understand the factors that make them grow in the first place. In the food processing industry, we utilize the acronym FATTOM (Food, Acidity, Time, Temperature, Oxygen and Moisture) to remember this:

Food: Organisms need food to survive. Food contains nutrients that are needed for organisms to grow and thrive.

Acid: Most microorganisms do not like acidic environments, therefore the higher the acidity of food the lower the rate of spoilage.

Time: Spoilage naturally occurs over time as microorganisms get the opportunity to grow.

Temperature: Spoilage microorganisms thrive at room temperature (20-30oC) but grow much more slowly at refrigeration temperatures (0-8oC) (Fellows, 2000). At freezing temperatures (≤18oC) growth is immobilized and at high temperatures (e.g. boiling at 100oC) spoilage microorganisms are destroyed.

Oxygen: Most spoilage organisms are aerobic, which means they need oxygen in their environment to survive. If you were to completely remove all oxygen from the environment, these aerobic microorganisms would not survive.

Moisture: Water is needed for all living organisms to thrive. If water is unavailable, spoilage microorganisms will not grow.

The parameters of food, acid, and oxygen are near impossible to control at home. Bread is rich in nutrients and is a low-acid food, so will always be susceptible to spoilage. Oxygen is present throughout the home environment. It can penetrate storage containers and bread bags as they do not provide a vacuum seal. Special technology known as modified atmosphere packaging (MAP) would have to be used to eliminate oxygen from your bread storage container. As we are looking for ways to store bread, consuming it in a short time (before microorganisms like mould can take it over) is not an option either. Therefore, moisture and temperature are the parameters of focus. Moisture can largely be avoided by storing in a dry environment. High temperatures such as boiling would degrade the texture and palatability of the bread, so low temperatures would be more suitable.

Therefore, you may think to store your bread in the refrigerator. While refrigerating bread does effectively delay spoilage, you may notice that your bread becomes hard and dry very quickly. That is because as storage temperature decreases, staling occurs more rapidly (Gray & Bemiller, 2003). However, once you reach freezing temperatures, staling stops. Why is this? To understand this, we must delve into the science of staling.

The Science Behind Staling

Staling is a process that is unique to starch-rich foods such as bread (bread is about 70% starch). Therefore, to understand staling we have to look at the chemistry of starch. Starch is a macromolecule made up of several small chains. Starch contains two types of chains – amylose (a straight chain) and amylopectin (a branched chain).

Figure 1. Amylose and amylopectin

These chains form bonds together known as hydrogen bonds.

Figure 2. Hydrogen bonding between amylose

Because amylose molecules form straight chains, they are able to form many hydrogen bonds with each other and “stack” upon each other closely in an orderly fashion. This is known as crystallization.

Figure 3. Crystallized amylose

When heat is applied to starch in the presence of water (such as when flour and water are mixed to make dough and baked into bread), the hydrogen bonds that hold up the “stacked” structure of amylose begin to break.

Figure 4. Breaking of hydrogen bonds in the presence of heat

As these bonds break, water molecules insert themselves in the spaces between the amylose chains where the hydrogen bonds were. The space between the amylose chains expands and the starch becomes swollen and hydrated via a process known as gelatinization.

Figure 5. Hydrated amylose

This gelatinization is what gives freshly baked bread its soft, tender texture as opposed to the coarse texture of raw flour. As the bread begins to cool down, however, the straight-chained molecules begin to draw closer together again. As they increase in proximity, they force out the water molecules that were absorbed between the chains and slowly return to their crystalline state. This process is known as retrogradation. As water leaves the starch molecules they begin to evaporate from the surface of the bread, leaving it tough and dry or “stale” as we know it.

Figure 6. Retrogradation

Cooling happens more rapidly at refrigeration temperatures, causing the amylose molecules to align themselves and push out water at a faster rate. So, while the rate of spoilage may be reduced, the rate of staling rapidly increases.

Why Grandma Was a Genius

This brings me back to my grandmothers’ clever solution. If you wish to store bread for an extended period, freezing is the better option. Freezing immobilizes the water molecules in the bread. When water molecules reach freezing temperatures, they begin to align themselves in a fixed and organized (crystalline) structure (Berk, 2009). The combination of low temperatures and immobilized water suspend the growth of spoilage microorganisms while also maintaining the water absorbed by the starch molecules in a fixed position, inhibiting retrogradation. When the bread is removed from the freezer and thawed, the water molecules rehydrate the bread, reinstating its soft texture. It looks like Grandma, in her own way, was a food scientist after all!

References

  1. Berk, Z. (2009). Food Process Engineering and Technology (1st ed., pp. 391-400). Burlington, MA, USA: Elsevier.
  2. Fellows, P. (2000). Food Processing Technology – Principles and Practice (2nd ed., pp. 387-389). Abington Hall, Abington Cambridge CB1 6AH, England: Woodhead Publishing Limited.
  3. Gram, L., Ravn, L., Rasch, M., Bruhn, J., Christensen, A., & Givskov, M. (2002). Food spoilage—interactions between food spoilage bacteria. International Journal Of Food Microbiology78(1-2), 79-97.
  4. Gray, J., & Bemiller, J. (2003). Bread Staling: Molecular Basis and Control. Comprehensive Reviews In Food Science And Food Safety2(1), 1-2
  5. Wikipedia. 2021. “Staling.” Last modified January 2, 2021. https://en.wikipedia.org/wiki/Staling

Jacquel Johnson
Jacquél Johnson is a Food & Agro Processing Technology Student at the University of the West Indies, Mona Campus, Jamaica.