In physics and chemistry, flash freezing is the process whereby objects are rapidly frozen.^[1] This is done by subjecting them to cryogenic temperatures, or through direct contact with liquid nitrogen at −196 °C (−320.8 °F). It is commonly used in the food industry.

Flash freezing is of great importance in atmospheric science, as its study is necessary for a proper climate model for the formation of ice clouds in the upper troposphere, which effectively scatter incoming solar radiation and prevent Earth from becoming overheated by the sun.^[2]

The process is also closely related to classical nucleation theory, which helps in understanding many materials, phenomena and theories in related situations.

Overview

When water freezes slowly, crystals grow from fewer nucleation sites, resulting in fewer and larger crystals. This damages cell walls and causes cell dehydration. When water freezes quickly, as in flash freezing, there are more nucleation sites, and more, smaller crystals. This results in much less damage to cell walls, proportional to the rate of freezing. This is why flash freezing is good for food and tissue preservation.^[3]

Applications and techniques

Flash freezing is a process that rapidly freezes objects by subjecting them to extremely low temperatures. This technique is used in various fields, including the food industry and biology, due to its ability to minimise damage to cell structures.

Applications of Flash Freezing:

1. Food Industry: Flash freezing is used to quickly freeze perishable food items. The smaller ice crystals generated minimise damage to the cell membranes, preserving the food’s texture, flavour, and nutritional content.
2. Biology: Flash freezing techniques are also used in biology to prevent large ice crystals from damaging samples.
3. Home Use: Flash freezing can be done at home using dry ice or liquid nitrogen. This method is effective for preserving food’s texture and flavour.

Techniques of Flash Freezing: Flash freezing involves the quick application of extremely low temperatures to food, resulting in it effectively freezing in minutes. This rapid chilling prevents large ice crystals from forming, keeping the texture and flavour of the food and minimising moisture loss.

It’s important to note that proper safety precautions are imperative when using freezing agents like dry ice and liquid nitrogen, as direct contact can lead to burns.

In summary, flash freezing is a revolutionary technique in food preservation that maintains the quality of food by preventing large ice crystal formation and minimising moisture loss. It has transformed the frozen food industry and continues to find new applications in various fields.

How water freezes

There are phenomena like supercooling, in which the water is cooled below its freezing point, but the water remains liquid, if there are too few defects to seed crystallization. One can therefore observe a delay until the water adjusts to the new, below-freezing temperature.^[4] Supercooled liquid water must become ice at minus 48 C (minus 55 F), not just because of the extreme cold, but because the molecular structure of water changes physically to form tetrahedron shapes, with each water molecule loosely bonded to four others.^[5] This suggests the structural change from liquid to "intermediate ice".^[5] The crystallization of ice from supercooled water is generally initiated by a process called nucleation. The speed and size of nucleation occurs within nanoseconds and nanometers.^[2]

The surface environment does not play a decisive role in the formation of ice and snow.^[6] The density fluctuations inside drops result in the possible freezing regions covering the middle and the surface regions.^[7] The freezing from the surface or from within may be random.^[7] However, in the strange world of water, tiny amounts of liquid water are theoretically still present, even as temperatures go below −48 °C (−54 °F) and almost all the water has turned solid, either into crystalline ice or amorphous water. Below −48 °C (−54 °F), ice is crystallizing too fast for any property of the remaining liquid to be measured.^[5] The freezing speed directly influences the nucleation process and ice crystal size. A supercooled liquid will stay in a liquid state below the normal freezing point when it has little opportunity for nucleation; that is if it is pure enough and has a smooth enough container. Once agitated it will rapidly become a solid. During the final stage of freezing, an ice drop develops a pointy tip, which is not observed for most other liquids, and arises because water expands as it freezes.^[6] Once the liquid is completely frozen, the sharp tip of the drop attracts water vapor in the air, much like a sharp metal lightning rod attracts electrical charges.^[6] The water vapor collects on the tip and a tree of small ice crystals starts to grow.^[6] An opposite effect has been shown to preferentially extract water molecules from the sharp edge of potato wedges in the oven.^[6]

If a microscopic droplet of water is cooled very fast, it forms what is called a glass (low-density amorphous ice) in which all the tetrahedrons of water molecules are not lined up, but amorphous.^[5] The change in the structure of water controls the rate at which ice forms.^[5] Depending on its temperature and pressure, water ice has 16 different crystalline forms in which water molecules cling to each other with hydrogen bonds.^[5] When water is cooled, its structure becomes closer to the structure of ice, which is why the density goes down, and this should be reflected in an increased crystallization rate showing these crystalline forms.^[5]

Related quantities

For the understanding of flash freezing, various related quantities might be useful.

Crystal growth or nucleation is the formation of a new thermodynamic phase or a new structure via self-assembly. Nucleation is often found to be very sensitive to impurities in the system. For nucleation of a new thermodynamic phase, such as the formation of ice in water below 0 °C (32 °F), if the system is not evolving with time and nucleation occurs in one step, then the probability that nucleation has not occurred should undergo exponential decay. This can also be observed in the nucleation of ice in supercooled small water droplets.^[8] The decay rate of the exponential gives the nucleation rate and is given by

${\displaystyle R\ =\ N_{S}Zj\exp \left({\frac {-\Delta G^{*)){k_{B}T))\right)}$

Where

• ${\displaystyle {\displaystyle \Delta G^{*))}$ is the free energy cost of the nucleus at the top of the nucleation barrier, kBT is the thermal energy with T the absolute temperature and kB is the Boltzmann constant.
• ${\displaystyle {\displaystyle N_{S))}$ is the number of nucleation sites.
• ${\displaystyle {\displaystyle j))$ is the rate at which molecules attach to the nucleus causing it to grow.
• ${\displaystyle {\displaystyle Z))$ is what is called the Zeldovich factor Z. Essentially the Zeldovich factor is the probability that a nucleus at the top of the barrier will go on to form the new phase, not dissolve.

Classical nucleation theory is a widely used approximate theory for estimating these rates, and how they vary with variables such as temperature. It correctly predicts that the time needed for nucleation decreases extremely rapidly when supersaturated.^[9][10]

Nucleation can be divided into homogeneous nucleation and heterogeneous nucleation. First comes homogeneous nucleation, because this is much simpler. Classical nucleation theory assumes that for a microscopic nucleus of a new phase, the free energy of a droplet can be written as the sum of a bulk term, proportional to a volume and surface term.

${\displaystyle {\displaystyle \Delta G={\frac {4}{3))\pi r^{3}\Delta g+4\pi r^{2}\sigma ))$

The first term is the volume term, and, assuming that the nucleus is spherical, this is the volume of a sphere of radius${\displaystyle {\displaystyle r))$. ${\displaystyle {\displaystyle \Delta g))$ is the difference in free energy per unit volume between the thermodynamic phase nucleation is occurring in, and the phase that is nucleating.

critical nucleus radius, at some intermediate value of${\displaystyle {\displaystyle r))$, the free energy goes through a maximum, and so the probability of formation of a nucleus goes through a minimum. There is a least-probable nucleus occurs, i.e., the one with the highest value of ${\displaystyle {\displaystyle \Delta G))$ where

${\displaystyle {\displaystyle {\frac {dG}{dr))=0))$

This is called the critical nucleus and occurs at a critical nucleus radius

${\displaystyle {\displaystyle r^{*}=-{\frac {2\sigma }{\Delta g))))$

The addition of new molecules to nuclei larger than this critical radius decreases the free energy, so these nuclei are more probable.

Heterogeneous nucleation, nucleation with the nucleus at a surface, is much more common than homogeneous nucleation. Heterogeneous nucleation is typically much faster than homogeneous nucleation because the nucleation barrier ${\displaystyle {\displaystyle \Delta G^{*))}$ is much lower at a surface. This is because the nucleation barrier comes from the positive term in the free energy${\displaystyle {\displaystyle \Delta G))$, which is the surface term. Thus, in conclusion, the nucleation probability is highest at a surface instead of the center of a liquid.

The Laplace pressure is the pressure difference between the inside and the outside of a curved surface between a gas region and a liquid region. The Laplace pressure is determined from the Young–Laplace equation given as

${\displaystyle \Delta P\equiv P_{\text{inside))-P_{\text{outside))=\gamma \left({\frac {1}{R_{1))}+{\frac {1}{R_{2))}\right)}$.

where ${\displaystyle {\displaystyle R_{1))}$ and ${\displaystyle {\displaystyle R_{2))}$ are the principal radii of curvature and ${\displaystyle {\displaystyle \gamma ))$ (also denoted as ${\displaystyle {\displaystyle \sigma ))$) is the surface tension.

The surface tension can be defined in terms of force or energy. The surface tension of a liquid is the ratio of the change in the energy of the liquid, and the change in the surface area of the liquid (that led to the change in energy). It can be defined as${\displaystyle \gamma ={\frac {W}{\Delta A))}$. This work W is interpreted as the potential energy.