Lesson 16. DESIGN OF FOOD FREEZING EQUIPMENT

Module 2. Food freezing

Lesson 16

DESIGN OF FOOD FREEZING EQUIPMENT

16.1 Introduction

Freezing is one of the common processes for preservation of foods. Preservation of food by freezing occurs by several mechanisms. The reduction of temperature to levels below 0oC causes a significant reduction in growth rates for microorganisms and the corresponding deterioration of product due to microbial activity. In addition to this enzymatic and oxidation reactions will be slow downed or arrested. In addition, formation of ice crystals with in the product changes the availability of water to participate in reactions. The engineering aspects of food freezing include:

  1. Design of food freezing system
  2. Design of refrigeration system
  3. Prediction of the rate at which freezing progresses.

16.2 Freezing Process

Actual freezing process in food products is somewhat more complex than freezing of pure water. In water the temperature decreases as heat is removed from the system until freezing point is reached. After small amount of sub cooling / super cooling, the temperature remains constant as latent heat is removed from the water system. Following this latent heat removal, the temperature deceases again.
fig

Fig. 16.1 Freezing point depression

In a food product removal of heat energy results in a temperature decrease until initial freezing point is reached. Initial freezing results in crystallization of a portion of the water, resulting in a concentration of the remaining solution and further reduction of the freezing point of that unfrozen portion. This results in additional decrease in temperature. This process continues till eutectic point of the solute present in the food product. In single solute system, the removal of heat energy beyond this point does result in temperature decrease, but with crystallization of solute as well as ice crystal formation.

In actual food product system, it is very probable that more than one solute will be present. If the temperature is monitored at thermal center of a food (the point that cools most slowly) as heat is removed, the freezing curve obtained will be like

fig

AS – The food is cooled to below its freezing point Tf, with the exception of pure water, is always below 0oC . At point S the water remains as liquid, although the temperature is known as super cooling and may be as much as 10oC below the freezing point.

SB – The temperature rises rapidly to the freezing point as ice crystals begins to form and latent heat of crystallization is released.

BC - Heat removed from the food at same rate as before. Latent heat is removed and ice forms, but the temperature remains almost constant. The freezing point is depressed as solute concentration increases and the temperature therefore falls slightly. It is during this phase that the major part of ice is formed.

CD – One of the solute becomes super saturated and crystallizes out. The latent heat of crystallization is released and temperature rises to eutectic temperature * for that solute.

DE – Crystallization of water and solute continuous. The total time, Tf taken is determined by the rate at which heat is removed.

EF - The temperature of the ice water mixture falls to the temperature of the freezer. A portion of water remains unfrozen at the temperature used in commercial freezers, the amount depends on type and composition of the food and temperature of storage.

table

16.3 Ice Crystal Formation

The freezing point of a food is the temperature at which a minute crystal of ice exists in equilibrium with surrounding water. However before an ice crystal can form, a nucleus of water molecules must be present. There are two types of nucleation: homogeneous nucleation (the chance orientation and combination of water molecules) and heterogeneous nucleation (the formation of a nucleus around suspended particles or at cell wall). Heterogeneous nucleation is more likely to occur in foods and takes place during super cooling.

The length of super cooling period depends on the type of food and the rate at which heat is removed. High rates of heat transfer therefore produce a large number of small ice crystals. The rate of ice crystals growth is controlled by rate of heat transfer. The rate of mass transfer does not control the rate of crystal growth except towards the end of freezing period. The time taken for the temperature of a food to pass through the critical zone determines both the number and the size of ice crystals.

16.4 Enthalpy Change During Freezing

One of the basic considerations in the design of a system for the freezing process is the refrigeration requirement for reducing the food product temperature to the desired level. The enthalpy changes required will reduce the product from some temperature above the freezing point to some temperature below the freezing point and can be represented by

Δ H = Δ Hs + Δ Hu + Δ HL + Δ HF

where the terms on right hand side represent the sensible heat required to reduce the product solids temperature from initial to storage temperature ( Δ Hs), the sensible heat removed to reduce the unfrozen portion of the product to the storage temperature( Δ Hu), the latent heat removed ( Δ HL), and the sensible heat removed to reduce the frozen water portion of the product to the storage temperature( Δ HF).

Sensible heat Δ Hs is given by,

Δ Hs = M Cp (Ti-TF) where TF = freezing point temperature

Evaluation of other components is somewhat complex because of changing state of product below initial freezing point. Mass of unfrozen product and frozen product are changing and are temperature dependant. Enthalpy change required to reduce the unfrozen portion of the product to various temperatures below initial freezing point, TF is given by:

Δ Hu = Mu (T) Cp,u (T) (TF-T)

Similarly,

Δ HF = MF (T) Cp, F (TF-T)

these equations can be writen in differential form as:

dHu = Mu (T) Cp,u(T) dT and

dHF = MF (T) Cp, F dT

Latent heat portion is given by:

Δ HL = MF (T) L

Unfrozen and frozen portions of product at any temperature below the initial freezing point can be calculated by

ln XA = l ’/Rg (1/TAo – 1/TA)

Where, TAo - freezing point of pure liquid, K

XA - is mole fraction of water in solution

Rg - gas constant , 8.314 kJ/ kg mol K.

TA - absolute temperature of aqueous solution, K

l ’ - Latent heat of fusion, J/ mol (6003 J/ mol for dilute liquids)

After obtaining information on the frozen and unfrozen fractions as function of temperature and specific heat of unfrozen fraction the above equations can be evaluated by integration.

Volume changes:

The volume of ice is 9 % greater than that of pure water hence expansion of food after freezing is expected. The degree of expansion depends on:

1. Moisture content (higher moisture content produce greater changes)

2. Cell arrangement: Plant material with intra cellular air spaces will absorb internal increases in volume without large changes in their overall size.

3. The concentrations of solute: High concentrations reduce freezing point and do not freeze or expand.

4. Freezer temperature: This determines the amount of unfrozen water and hence the degree of expansion.

Prediction of Freezing Rates/ Time

Most important consideration in food freezing problems is the prediction of time required to accomplish a given freezing process. The concept of “thermal arrest time”, which may be defined as the time required to reduce the temperature of the product to some stated temperature below the initial freezing point.

During freezing, heat is conducted from the interiors of a food to the surface and is removed by the freezing medium. The factors which influence rate of heat transfer are:

1. The thermal conductivity of food

2. The area of food available for heat transfer

3. The distance that the heat must travel through the food

4. The temperature difference between the food and freezing medium

5. The insulating effect of boundary film of air surrounding the food.

Prediction of Freezing Time is Complicated because of The Following Reasons:

1. Differences in initial temperature of the food

2. Differences in initial size and shape of pieces of food

3. Differences in the freezing point and the rate of ice crystal formation with in different regions of pieces of food.

4. Changes in density, thermal conductivity, specific heat, and thermal diffusivity with reduction in temperature of food.

Factors Influencing Freezing Time

There are several parameters that influence freezing time and that will influence the design of equipment used for food freezing.

  1. Freezing medium temperature, where lower magnitude will decrease freezing time.
  2. Product size will influence freezing time.
  3. Convective heat transfer coefficient, hc will influence freezing time significantly.
  4. The initial freezing point.
  5. Product properties (TF,ρ,k,CP) will influence freezing time predictions.

Design Criteria For Selection Food Freezing Equipment

  1. It should be of sanitary design.
  2. All product contact surfaces should be made of Stainless Steel.
  3. The equipment should be adaptable for different food products.
  4. It should have high efficiency.
  5. Quick freezing & continuous operation.
  6. Material of construction should be non-toxic, non-corrosive and odour less & taintless.
  7. The equipment should be of simple design and easy to operate.
  8. Type of product processed.
  9. Reasonable freezing rates.
  10. Airflow rates should be uniform and air should make intimate contact with the product.
     

Changes in Food during Freezing

In any food freezing operation the amount of water getting frozen depends on

1. Initial water content

2. The way in which it is bound.

During freezing the water inside food is converted into the ice and freezing progresses i.e. the constituents like salts, acids, sugar etc., which are dissolved in the water, become more and more concentrated. This concentrated solution may adversely affect the properties of the product. Hence fast freezing is recommended. During rapid freezing the size of ice crystals will be small and the constituents are trapped inside them and almost in unchanged state. It is important to maintain lower temperature as frequent changes in temperature may lead to recrystallization of water in which the water molecules will migrate from the smaller ice crystals which have higher osmotic pressure, to larger ones which have lower osmotic pressure. Apart from rapid freezing, rapid thawing is also important to preserve biological value of foods. Rapid thawing by use of microwaves will prevent recrystallization.

Important Changes during Freezing

1. Freezer burns:

Due to irreversible drying of the surfaces leading to brown spots on the surface. This occurs due to changes in water vapour pressure in the product. This occurs mainly when the freezer is switched off –and- on frequently. Suppose during a particular interval of time the product has higher temperature which is the case when the freezer is switched on, the water from the product may come out and condense on colder surfaces. However when the temperature changes are reversed, the water does not return to original sites and this gives rise to freezer burns. This can be prevented by wrapping the food products in polyethylene films which are impervious to water vapor before freezing.

2. Destruction of cells:

This takes place during freezing. The cause is not clear yet but reasons may be manifold.

1. Purely mechanical action of growing ice crystals may destroy cells.
2. Osmotic drying of cells due to increasing concentration of solution.
3. Concentrated solution of acid/sugar/salt may damage the cells.

    To prevent this, addition of sugars like sucrose at 10%, gelatin etc has been recommended especially in freeze drying of eggs.

    1. Change in proteins: Precipitation of calcium caseinate commonly called as denaturation by cold may take place.

    2. Enzymes: Enzymes are not destroyed completely during freezing. Peroxides are found to be active even at – 20oC . Hence vegetables are given blanching treatment prior to freezing (either by steaming or boiling water). To prevent browning of fruits 0.1 % ascorbic acid or 0.1% citric acid solution is used. SO2 gas is also used to prevent browning.

    3. Vitamins: No loss of A, B, D, E during freezing and some loss of C especially at below –20oC may occur.

    4. Microorganisms: They are inhibited – no growth. But psychotropic organisms may grow. Destruction of microorganisms by denaturation of protein is possible.

    5. Metabolism: It is slowed down.

    Example 16.1:

    A milk chilling unit can remove heat from the milk at the rate of 41.87 MJ/hr. Heat leaks into the milk from the surroundings at an average rate of 4.187 MJ/hr. Find the time required for cooling a batch of 500 kg of milk from 45oC to 5oC . Take Cp of milk as 4.187 kJ/kgoC .

    Solution:

    Heat to be removed from milk Q1 = 500 (45-5)(4.187) kJ

    = 8.374 x 104 or 83.74 x 103 kJ = 83.74 MJ

    Heat leakage to milk Q2 = 4.187 MJ/hr

    41.87 MJ of heat is removed in 60 min (1 hr)

    Therefore, 83.74 MJ of heat wil be removed in (83.74 x 60)/41.87 = 120 min

    Heat added in two hours = 4.187 x 2 = 8.374 MJ

    Now additional time required is determined as follows –

    83.74 MJ of heat is removed in 120 min,

    Then 8.374 MJ of heat is removed in 12 min

    Therefore, total time required is 120 min + 12 min = 132 minutes
Last modified: Friday, 12 October 2012, 6:42 AM