Processing of Horticultural Crops (1+2)

Introduction

Introduction
Freezing is one of the oldest and most widely used methods of food preservation, which allows preservation of taste, texture, and nutritional quality in foods better than any other method. It is a process of bringing down the temperature of food below its freezing point and the frozen storage generally refers to storage at temperature below -18⁰C.

Theory of Freezing
The freezing process is a combination of the beneficial effects of low temperatures at which micro-organisms cannot grow, chemical reactions are reduced, and cellular metabolic reactions are delayed. Freezing is generally referred to as a unit operation in which the temperature of food is reduced below freezing point and a proportion of water undergoes a change in state to form ice-crystals. Immobilisation of water to ice and the resulting concentration of dissolved solutes in unfrozen water cause lowering of water activity of the food. Thus reduction in water activity and use of low temperature coupled with some pre- treatments like blanching is the basis for food preservation by freezing. Fruits such as strawberries, oranges, raspberries, black currants and vegetables like green pea, green beans, sweet corn, spinach, sprouts and potatoes are examples of commercially frozen products.

History of Freezing: The frozen food market is one of the largest and most dynamic sectors of the food industry. In spite of considerable competition between the frozen food industry and other sectors, extensive quantities of frozen foods are being consumed all over the world. In 1920, Clarence Birdseye introduced commercial freezing and storage equipments and soon the cold chain marketing system was evolved. The frozen vegetable industry mostly grew after the development of scientific methods for blanching and processing in the 1940s. The commercial freezing of small fruits and berries began in the eastern part of the US in about 1905. The main advantage of freezing preservation of fruits is the extended usage of frozen fruits during off-season. Additionally, frozen fruits can be transported to remote markets that could not be accessed with fresh fruit. Also, freezing preservation makes year-round further processing of fruit products possible, such as jams, juice, and syrups from frozen whole fruit, slices, or pulps. Thus, the preservation of fruits by freezing has become one the most important preservation methods.

Freezing process: The material to be frozen first cools down to the temperature at which nucleation starts. Common frozen storage temperature is -18⁰C or 0⁰F. Once the first crystal appears in the solution, a phase change occurs from liquid to solid with further crystal growth. Therefore, nucleation serves as the initial process of freezing and can be considered as the critical step that results in a complete phase change.
The freezing process involves lowering the product temperature generally to -18 °C or below. During freezing, sensible heat is first removed to lower temperature of a food to the freezing point. In case of fresh produce, heat produced by respiration (heat load) is also removed. Latent heat of crystallization is then removed to allow formation of ice crystals and hence to freeze the food. Besides, latent heat of other components of food like fat must also be removed before they can solidify. Usually in most of fruit and vegetables these components are present in smaller amounts and thus the removal of a relatively small amount of heat is needed for crystallisation to take place. However, since most fruit and vegetables containing a large proportion of water (78-95%), possess high specific heat (4200 J/kg/k) and high latent heat of crystallisation (335kJ/kg), as such considerable amount of energy is needed for freezing processes.

Freezing point of foods
Freezing point is defined as the temperature at which the first ice crystal appears and the liquid at that temperature is in equilibrium with the solid. The freezing point of pure water at normal temperature and pressure is 0°C (273°K). However, when food systems are frozen, the process becomes more complex due to the existence of both free and bound water. The presence of solutes in water alters both the boiling and freezing point of water. Bound water does not freeze even at very low temperature. Un-freezable water contains soluble solids, which cause a decrease in the freezing point of water lower than 0°C. During the freezing process, the concentration of soluble solids increases in the unfrozen water, resulting in a variation in freezing temperature. Therefore, the temperature at which the first ice crystal appears is commonly regarded as the initial freezing temperature.

The freezing point depression is given by the relation:

According to International Institute of Refrigeration (IIR), the freezing process is divided into three stages based on major temperature changes in a particular location in the product, as shown in Figures 13.2 and 13.3 for pure water and food respectively. In pre-freezing stage, the food is subjected to the freezing process until the appearance of the first crystal. For pure water, the freezing temperature will be 0°C and up to this temperature, there will be a sub-cooling until the ice formation begins.
In case of foods during first stage, the temperature decreases below the freezing temperature and with the formation of the first ice crystal, temperature again increases to freezing temperature. The second stage is the freezing period; a phase change occurs, transforming water into ice. For pure water, temperature at this stage is constant however; it decreases slightly in foods due to the increasing concentration of solutes in the unfrozen water portion. The last stage starts when the product temperature reaches the point where most freezable water has been converted to ice, and ends when the temperature is reduced to storage temperature.

The initial freezing point of some of the foods is given along with water contents in Table-13.1. Most of the foods have a freezing point in the range of 28-30⁰F or -1 to -2⁰C. The freezing time and freezing rate are the most important parameters in designing freezing systems.

Table 13.1: Average freezing point and water contents in some of commodities and products.

Commodities and products	Water contents (%)	Initial freezing point (0C)
Apple juice	87.2	- 1.44
Apple juice concentrate	49.9	- 11.3
Asparagus	92.6	- 0.67
Carrots	87.5	- 1.11
Grape juice	84.7	- 1.78
Orange juice	89.0	- 1.17
Peaches	85.1	- 1.56
Pears	83.8	- 1.61
Raspberries	82.7	- 1.22
Strawberries	89.3	- 0.89
Sweet cherries	77.0	- 2.69
Tomato pulp	92.9	- 0.72

Freezing time: Freezing time is defined as time required to lowers the product temperature from its initial temperature to a pre-determined temperature of -10 or -18⁰C at its thermal centre. Since the temperature distribution within the product varies during freezing process, the thermal centre is generally taken as reference. Freezing time depends on various factors including the initial and final temperatures of the product, the quantity of heat removed, dimensions (especially thickness) and shape of product, difference in freezing point and rate of ice crystal formation, heat transfer process and temperature. The freezing time for food cubes is calculated by:

Freezing rate: The freezing rate (°C/h) for a product or package is defined as the ratio of difference between initial and final temperature of product to freezing time. The quality of frozen products is largely dependent on the rate of freezing. Generally, rapid freezing results in better quality frozen products when compared with slow freezing.

• If freezing is instantaneous, there will be more locations within the food where crystallization begins.
• Large ice crystals are known to cause mechanical damage to cell walls in addition to cell dehydration.
• Rapid freezing is advantageous for freezing of many foods; however some products are susceptible to cracking when exposed to extremely low temperature for long periods.
• Several mechanisms, including volume expansion, contraction and expansion, and building of internal pressure, are responsible for product damage during freezing.

Several methods for estimation of freezing time and rate exist as:

• Use of analytical solution to heat transfer equation involving phase change problems under appropriate boundary conditions.
• Use of empirical equation based on assumptions and approximations that can give estimates of freezing time.

Energy requirements: For fruits and vegetables, the amount of energy required for freezing is calculated based on the enthalpy change and the amount of product to be frozen. Refrigeration requirements for fruits and vegetables can be calculated from the following equation:




Refrigeration
Refrigeration is defined as the elimination of heat from a material at a temperature higher than the temperature of its surroundings. The main elements in a closed mechanical refrigeration system are the condenser, compressor, evaporator and the expansion valve. Hydro-chlorofluorocarbon (HCFC) and ammonia are used as refrigerants in mechanical refrigeration systems. A simple scheme for the closed mechanical refrigeration system is shown in Fig 13.4.

Fig 13.4: Closed mechanical refrigeration system (one-stage).

Starting at the suction point of the compressor, fluid in a vapour state is compressed into the compressor where an increase in temperature and pressure takes place. The fluid then flows through the condenser where it decreases in energy by giving off heat and converting to a liquid state. After the phase, a change occurs inside the condenser, the fluid flows through the expansion valve where the pressure decreases to convert liquid into a form of liquid-gas mixture. Finally, the mixture flows through the evaporator and is converted into a saturated vapour state and removes heat from the environment in the process of cooling.

Refrigerants: A refrigerant can be selected on the basis of physical, thermodynamic and chemical properties of the fluid. Environmental considerations are also important in refrigerant selection, since leaks within the system produce deleterious effects on the atmospheric ozone layer. Ammonia is commonly used for industrial applications whereas; chloro-fluoro-methane and tetra-fluoro-ethane are also recommended as refrigerants.

Freezing capacity: Freezing capacity (tonnes per hour) is defined as the ratio of the product quantity that can be loaded into the freezer to the holding time of the product in that particular freezer. The amount of food product loaded into the freezer is affected by both the dimensions of the product and the mechanical constraints of the freezer. The holding time of the product has an important role in freezing systems and is based on the calculation of the amount of heat removed from the product per hour, which varies on the type of product.