Background
Frozen desserts have existed for centuries, but modern manufacturing processes use a high degree of control to ensure efficient operation to produce products with the desired characteristics. The latest Scraped Surface Freezer (SSF), built on the basic freezer of the past, efficiently creates the structure needed for frozen desserts. However, the internal operation of the freezer (that is, the scraped surface heat exchanger) remains a bit confusing. This is also due to the fact that the ice changes very quickly as the temperature changes, although analytical measurements may not show the internal workings. In particular, controlling ice phase production is important for product quality and shelf life. Many of the less than about 50 μm ice crystals produce a desirable smooth and creamy texture. The freezer must make these small ice crystals and other phases are controlled (e.g. air and fat). However, small ice crystals produced in the refrigerator are unstable. Its high surface energy causes recrystallization.
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Cryogenic Materials
Cryogenic materials are liquids with extremely low boiling points, usually around -73 degrees Celsius or lower. Examples of such materials include liquid Nitrogen, liquid Oxygen, and Carbon Dioxide. To produce such materials, an effect known as the Joule-Thomson effect was applied. This effect states that if a liquid is made to expand below a certain pressure and temperature, its temperature will decrease. Cryogenic liquids, however, might prove to be a problem. If they are not stored properly (containers with high pressures), they might turn back into gas, just like the case for liquid Nitrogen. On the other hand, they can cause several health problems and other hazards if not handled correctly.
Joule-Thomson Effect
The Joule-Thomson Effect is the observable change of temperature in an isenthalpic (a process that proceeds without any change in enthalpy)
Ice Cream
In the United States, ice cream has a Standard of Identity as defined by the U.S. Food and Drug Administration (FDA) in the Code of Federal Regulations. It must contain no other fat than milk fat, and the minimum fat content is 10%. There is also a maximum density requirement that effectively caps the amount of air that can be incorporated at 100% overrun (air incorporation on a volume basis). If the requirements of the Standard of Identity are not met, the product cannot be called ice cream on the label, but instead must be called a frozen dessert.
In Indonesia, Badan Standardisasi Nasional defines ice cream as a semi-solid food made by freezing ice cream flour or from a mixture of milk, animal or vegetable fat, sugar, with or without other food ingredients and permitted food ingredients. It’s minimum fat content is 5%, and the sugar classified as sucrose must contain a minimum saccharose of 8%, as per SNI 01-3713-1995
Scraped Surface
Scraped surface is a surface of an apparatus in which the cooling fluid freezes to form a cylindrical surface as a thin film, which is removed by wipers.
The advantage of SSHE for ice cream is that it creates many small crystals before the product hardens in the cold room, which improves the smoothness. Recently, many studies have been reported on ice slurry used as a two-phase secondary refrigerant, SSHE is used to generate them continuously. In both applications the goal is to produce the maximum amount of ice without exceeding the viscosity threshold to ensure pumpability. Therefore, the heat transfer efficiency should be predicted considering the crystallization of ice.
Making Ice Cream Traditionally
The process of making ice cream begins with creating a mixture from the ingredients. Cream, or another dairy ingredient, and sugar are the main ingredients that make up most of the cream mix. A variety of components to improve the product or reduce costs can also be added. Emulsifiers are often needed to control the emulsion interface. Stabilizers (e.g. hydrocolloids such as gelatin, gum or cellulose) are often added to increase viscosity and extend shelf life. Other ingredients that may be added include corn syrup solids, skim milk solids and of course, colors and flavors. After freezing, types (eg caramel, marshmallow, fudge) and nuts (eg chocolate crumbs, cookie dough, nuts, fruit) can be added to create special originality.
Freezing Process
During the freezing process, the water contained in the food thermodynamically converts into ice, in which the entire system is under equilibrium conditions. The rate of equilibrium is obtained through kinetics/rate. The first step to freezing is the formation of ice crystals, followed by the growth of ice crystals. In this stage, latent and sensible heat is removed.
According to the graph, as the product reaches freezing point, the system exists in equilibrium, or a state in which the liquid and solid phase can exist simultaneously. During this time, water is converted into ice and sensible heat is lost at a very fast rate. The next step is known as a period of thermal arrest, where the temperature increases and all the water is frozen and there is no more latent heat present in the food. The next stage is the third and final stage, where the phase change temperature is further reduced to a final freezing temperature, and if there is any remaining water at this stage, it will surely freeze as well. Hence, the important factors that are considered in cryogenic freezing would be time and freezing rate.
Is Liquid Nitrogen Safe?
There are actually many health risks towards consumers and workers when operating with cryogenic materials, such as liquid nitrogen in particular. Liquid nitrogen, if inhaled, can cause asphyxiation, especially if individuals are working with it in a small room, which can lead to instant death. If skin was to come into contact with liquid nitrogen, then it could lead to frostbite, blisters, ulcers, edema, and tissues necrosis. This can be more dangerous if it happens inside the mouth.
Liquid nitrogen is sometimes used in food and consumption, with the assumption that by the time it is served to customers, the liquid nitrogen would have evaporated already. However, that is not always the case. The most severe case that could happen to a person who accidentally ingested liquid nitrogen would be gastrointestinal barotrauma, which develops due to increased pressure in the stomach. On the other hand, liquid nitrogen can be a potential storage place for biohazards such as bacteria, fungi, yeasts and viruses, which can be given through unhygienic practices in the workplace or from other sources of contamination. Even though this rarely happens, it is still a possibility.
References
Ehs.gatech.edu. n.d. Cryogenic Materials | Environmental Health & Safety. [online] Available at: <https://www.ehs.gatech.edu/chemical/lsm/16-3> [Accessed 17 March 2022].
Ali, D., Farber, J., Kim, J., Parto, N. and Copes, R., 2021. A Qualitative Risk Assessment of Liquid Nitrogen in Foods and Beverages. Food Protection Trends, 41(3), p.293.
Hartel, R., 2019. Engineering Frozen Desserts. [online] Aiche-cep.com. Available at: <https://www.aiche-cep.com/cepmagazine/may_2019/MobilePagedArticle.action?articleId=1487932#articleId1487932> [Accessed 17 March 2022].
Lakhdar, M., Cerecero, R., Alvarez, G., Guilpart, J., Flick, D. and Lallemand, A., 2005. Heat transfer with freezing in a scraped surface heat exchanger. Applied Thermal Engineering, 25(1), pp.45-60.
Dash, K. and Chakraborty, S., 2021. Food Processing. Milton: Taylor & Francis Group, pp.185-186.
De la Cruz Martínez, et al., 2020. Estimation of Ice Cream Mixture Viscosity during Batch Crystallization in a Scraped Surface Heat Exchanger. Processes, 8(2), p.19.
