Freezing is a unit operation in which the temperature of food is reduced below its freezing points. In this process step, a proportion of the water undergoes a change in state to form ice crystals. The immobilisation of water to ice and also the resulting concentration of dissolved solutes in unfrozen water decrease the water activity of the food (Fellows, 2000). Fellows (2000) reported that preservation can be achieved by a combination of low temperatures, reduced water activity, and in vegetables, pre-treatment by blanching. By applying correct freezing and strong procedures, there are only small changes to nutritional or sensory qualities of foods. In freezing because of the low temperatures, the microorganisms cannot grow, the chemical reactions are reduced, and the cellular metabolic reactions are delayed (Delgado and Sun, 2000).
Fennema (1977) reported that freezing as a method of long-term preservation for fruits and vegetables is considered as superior to canning and dehydration, with respect to retention in sensory and nutritive properties. The freezing process consists of lowering the product temperature (the controlled removal of heat from the product), until the heat remaining in the product (thermal center) reaches -18°C, resulting in crystallization of most of the water, the main component of plant tissues and some solutes (Canet and Alvarez, 2012; Silva et al., 2008). The product to be frozen first should be cooled down to the temperature until the appearance of the first seed of crystal. This temperature at which the first ice crystal appears is regarded as the initial freezing temperature.
The process producing this seed or nucleus, upon which the crystal can grow, is called nucleation (Barbosa-Cánovas et al., 2005). For an efficient freezing process, industries generally recognize the target temperature of -18°C at the thermal center of the product (Martínez-Romero et al., 2004). The freezing process comprises several stages as shown in Figure 3.2. The first stage is a pre-freezing stage; the product is subjected to the freezing process and the temperature of the product is reduced to the freezing point by releasing sensible heat from the product. The initial freezing temperature depends on the moisture content and can be varied with the product. The second stage is the super-cooling stage; where the temperature falls below the freezing point.
This stage is not usually observed, and the length of it depends on the food variety and the freezing rate. The third stage is freezing stage. This stage is the period of transformation of water into ice which is an example of crystallization. In this stage, the latent heat is extracted and ice crystals are formed.
The last stage is the sub-freezing stage; where the product temperature is lowered to the end temperature (storage temperature) and mostly sensible heat is removed (Silva et al., 2008; Alexandre et al., 2013). The freezing time and freezing rate are the key factors to design freezing systems. Freezing time can be defined as the time elapsing from the start of the pre-freezing stage until the final temperature has been attained. The freezing time depends on several factors, including the product size (especially thickness) and shape, composition of the vegetable, as well as by the parameters of the heat transfer process and the temperature of the cooling medium (Silva et al., 2008; Canet and Alvarez, 2012; Barbosa-Cánovas et al., 2005). The duration of the freezing process depends on the freezing rate (°C/h). Freezing rate for a product defines as the ratio of the difference between the initial and final temperature of the product divided by freezing time (the outside to the inside of the product).
It depends on the freezing system applied (mechanical or cryogenic), the initial temperature of the product, the size and form of the package, and also the type of product. Frozen food quality is highly affected by freezing rate (Barbosa-Cánovas et al., 2005; De Ancos et al., 2006). During slow freezing, large sharp ice crystals are formed particularly in extracellular space and may cause cell shrinkage. It means that the cells will lose water by osmosis, and this leads to an extensive dehydration of the cells, resulting in the release of enzymatic systems and their substrates, leading to different effects such as off-flavours, colour and textural changes, etc.
On the contrary, during rapid freezing, heat is removed so quickly and there is little time for dehydration of cells, resulting in producing many small ice crystals that are uniformly distributed in intracellular and extracellular spaces (Silva et al., 2008; Barbosa-Cánovas et al., 2005; De Ancos et al., 2006). The individually quick frozen (IQF) technology was developed with the purpose of quick freezing. Most vegetables are frozen using individual quick freezing methods (Barbosa-Cánovas et al., 2005). 3.2.2.4.
Packaging Packaging of frozen fruits and vegetables plays an important role in protecting the product from external contamination, from damage that may occur along the distribution chains from the manufacturer to the consumer and from air and oxygen that produce oxidative degradation (Silva et al., 2008; Barbosa-Cánovas et al., 2005; De Ancos et al., 2006). Therefore, the main packaging function is product preservation by protecting the frozen food from ingress of oxygen, water vapour and light. Each of them may cause deterioration of colours, denaturation of proteins, oxidation of lipids and unsaturated fats, degradation of ascorbic acids, and a general loss of sensory and nutritional qualities (De Ancos et al., 2006). De Ancos et al. (2006) noted that barrier properties of the package should protect against the loss of moisture from the frozen product to the external environment to avoid external dehydration or “freezer burn” and weight loss. At the same time, the packaging materials should not affect or contaminate the food in any way and should have a high heat transfer rate to facilitate rapid freezing (Silva et al., 2008; De Ancos et al., 2006).
The choice of the packaging material must be in line with the existing legislation (Silva et al., 2008). The packaging material should be impermeable to liquids, resistant to moisture, weak acids and low temperatures, reflective and as astonished as possible and permeable and resistant to microwave energy (Villalvilla, 1988). Frozen vegetables may undergo dehydration during storage, as a result of fluctuations in storage temperature, and water permeability of packaging. Such dehydration is irreversible, giving rise to ice formation inside the package and exerting detrimental effects on quality such as alterations in colour and flavour, freezer burn, increased risk of oxidation, and structural deterioration.
Therefore, packages should ideally be airtight, totally impermeable to water vapour, and also effective as thermal insulators to limit possible temperature fluctuations within the product (Ahvenainen and Malkki, 1984; Canet and Alvarez, 2012).
