FOOD ENGINEERING

Lesson 12. PREDICTION OF FREEZING RATES

Module 2. Food freezing

Lesson 12

PREDICTION OF FREEZING RATES

12.1 Introduction

The most important consideration associated with food freezing rate is the rate of the process. This rate not only establishes the structure of the frozen product but the time required for freezing is the basic design consideration for the process. An analysis of current literature indicates significant variation in the definition of freezing rate. Fennema, et.al(1973) have identified four methods to describe rate of freezing including: (a) Time-Temperature methods (b) velocity of ice front (c) Appearance of specimen and (d) Thermal methods. The most frequently encountered methods are time – temperature including (a) Temperature change per unit time or (b) time to transverse a given range of temperatures. The temperature change per unit time is the most appropriate indicator when the primary concern is structure of the frozen product and resulting influence on quality. It must be emphasized that temperature change per unit time will vary significantly during the freezing process and an average value has limited meaning.

The most appropriate indicator of freezing rate for purpose of process design is the time to transverse a given range of temperature. The international institute of refrigeration (1971) has proposed the following definition: The freezing rate of a food mass is the ratio between the minimum distance from the surface to the thermal center and the time elapsed between the surface reaching 0 ° C and the thermal center reaching 5 ° C colder than the temperature of initial ice formation at the thermal center. Where depth is measured cm & time in hour, the freezing rate will be expressed as cm/hr. A variation of the IIR definition is referred to as “thermal arrest time” and represents the time required for the slowest cooling point in the product to decrease 0 ° C to – 5 ° C. Long class used thermal wrest time to describe the rate of freezing in fish. The result of this research indicates two significant factors about the use of thermal arrest time. The first factor was the location of the temperature sensor. Small deviations in location of the temperature sensor from the slowest cooling or freezing point in the product resulted in considerable error in determining the thermal arrest time for a given product. The second factor was the influence of initial product temperature. Results reported by long (1955) indicated that an increase in initial product temperature decreased the thermal arrest time. In other words, the total freezing time was longer when the initial temperature was higher, but the time required to reduce the product temperature from 0 ° C to – 5 ° C was less, for purposes of the discussion which follows, the time required to reduce the product temperature at the slowest cooling location from the initial freezing point will be utilized as the time to describe freezing rate. Although this definition is not without limitations, it seems to provide the best compromise when considering advantages & disadvantages of other methods.

Fennema and Powrie (1964) have listed four factors which influence freezing rates: (a) The temperature difference between the product & cooling medium (b) The modes of heat transfer to from & within the product (c) The size, type & shape of the package containing the product & (d) The size, shape & thermal properties of the product. Although considerable information is available in heat transfer literature to assist in describing the rates of heat transfer in various shaped packages & products, the major limitation appears to be in the description of transient heat transfer with thermal properties being a function of temperature. The latter must be the case during the freezing of food products, since the apparent specific heat & thermal conductivity are both significant function of temperature in the freezing zone or below the initial freezing point of the product. Many of the methods utilized to obtain expressions for freezing time have involved simplifying assumptions which do not account for the thermal diffusivity being a function of temperature, in an effort to obtain a solution to a complete heat conduction problem.