Freezing is a unit operation in which temperature of a food is reduced below its freezing point (removal of sensible & latent heat of fusion), and a proportion of the water undergoes a change in state to form ice-crystals. Immobilization of water to ice and the resulting concentration of dissolved solutes in unfrozen water lowers the water activity of the food. Lowering of temperature, lowers the microbial activity as well as suppresses the chemical reactions in the food. Generally, we find two type of freezing method, quick freezing and slow freezing. In slow freezing, food products are introduced to low temperature for relatively longer time duration and hence the ice crystal formed are of larger in size, which are sometimes not at all desirable depending on characteristic of food product. In quick freezing, relatively even lower temperature is used for food product but exposure time is less. Hence the ice crystal formed are of smaller size. Even though small ice crystal formed during quick freezing helps in preserving the texture and other characteristic of product, sometimes it may cause to temperature shock to product. Choosing the method of freezing is hence of utmost importance.
Also known as freeze drying, lyophilisation is a preservation technique extensively used in food industry for preserving food material and for prolonging shelf life. Freeze drying technique is a non-thermal preservation technique used for majorly heat sensitive food product. Freeze drying employs treatment of food product in controlled temperature to preserve the main characteristic of product that can be lost in case exposed to uncontrolled condition. Freeze drying can be highly effective for preservation of wide range of product including fruits, vegetables, meats, flavours, herbs, coffee etc. Temperature used in lyophilisation is between -50 to -80 deg. C and product can be kept as such, without any characteristic changes for several years in case of unopened condition and can last for at least few days after opening if stored in refrigerated condition.
In lyophilisation, temperature of product is brought down to such a low temperature where the solid, liquid and vapour coexists, which is also known as triple point. In this process the initial liquid solution or suspension is frozen, the pressure above the frozen state is reduced and the water removed by sublimation. Thus a liquid-to-vapor transition takes place. If, however, solid ice is maintained at a pressure below the triple point then on heating the ice will sublime and pass directly to water vapor without passing through the liquid phase.
Lyophilisation involves three steps, involving freezing, primary drying and secondary drying.
- Freezing: Free water present in product is freezed to change its state from liquid to solid. Product is freezed below its triple point. Reason for bring down temperature to triple point is that it helps in the next step of the process. Freezing is the most important step of lyophilisation and it is preferred to have larger crystals to easily sublime it.
- Primary Drying: Second step involves lowering of pressure and providing enough heat to sublime the ice to vapour form. It gets easy due to the product existing in its triple point temperature. In this primary drying phase, about 95% of the water in the material is sublimated. In industrial level, this is most time taking process as slightly higher temperature can lead to deterioration of texture of product. Controlled pressure act as catalyst for sublimation.
- Secondary Drying: Then comes secondary drying, which primarily focus on the removing of unfrozen water. The temperature is raised higher than in the primary drying phase, and the pressure is also lowered in this stage to encourage desorption. Before sealing the product in packages, it is treated with inert gas to break the formed vacuum. For food product, nitrogen gas is used.
A lyophilisation plant consists of:
- Drying chamber having temperature controlling plates
- Capacitor to collect the water coming out of the product in the drying chamber
- Cooling system
- Vacuum system to reduce the pressure in the room
- Capacitor for the drying process.
The food product is firstly freezed to its freezing point, even liquid product is freezed below their freezing point. Then they are placed in the drying chamber where temperature of the chamber is set according to the product to be freezed and is always below 0 Deg. C. Applied vacuum facilitates ice evaporation which again lower the temperature of the product which are compensated by heating the product a bit by means of heating plate. The vapour evaporated is again freezed and during the process, freed water is drained.
Pros and cons with Freeze drying
Due to low temperatures, the chemical decomposition, particularly hydrolysis is minimized. There is no concentration of solution prior to drying. Hence, salts do not concentrate and denature proteins, as occurs with other drying methods. Again the basic importance or need for lyophilisation is due to decrease in weight and bulk of the product which helps in handling of bulk material during storage or transportation. Shelf life of product increases by reducing the microbial attack and self-metabolic activity.
Even Lyophilisation have their own limitation as it yields a very hygroscopic product and makes it difficult to maintain the required storage condition at every step. It is also very time taking process and the processing plant very complicated making it very expensive and can be used of higher quality product. Though it beneficial for heat sensitive product but again it can be used for certain types of products only. Chance of deterioration of fatty tissue by rancidity is the major drawback to the sensory values.
People having less time due to busy schedules are looking for alternatives to time taking cooking. For this purpose, product with longer shelf life are required which need to be coupled with less price. In larger food industries freeze drying are helping with convenient bulk handling of products and long shelf life which are the basic and most important requirement. Freeze drying segment is thriving on the basis of introduction of new technology, product type and innovation in packaging for holding freeze dried product. Increasing demand of canned and frozen products supports the growth of frozen food industry.