Controlling Water Activity

The principles and practice of controlling water activity via heat transfer.

Darren Olive – 06067578

Year 4 module towards the award of BSc (Hons) degree in food manufacturing. FDT 3001M – Technical Management.

Submitted: 5th May 2014

Tutor: Mrs Lindi Tizi

Extension code: MJS2013101

Contents Page
1. Introduction 3
2. Process operation that controls the water activity 3
3. Heat transfer. 5
4. Impact on nutritional and organoleptic qualities. 8
5. Reference 9

1. Introduction
The control of water activity in food products can be critical to provide a safe consumable product. In many cases water activity is used in co-junction with other preservation methods to give the ‘hurdle effect’ as discussed by Garbutt, J. (1997, Page 107):-
Hurdle technology exploits scientifically something that has been used for centuries, i.e. the use of more than one factor to preserve foods. Traditionally fruit conserves, for example, employ naturally occurring organic acids and sugar as preservative. Cheese manufacture uses lactic acid and salt.
In some cases applying all of the hurdles are not possible, such as the customer requiring a pH neutral conserve, in these cases the control of water activity takes a larger role in the risk limitation for the particular food.
In this paper we will be reviewing the control of water activity by the means of heat transfer, in particular evaporation used for neutral pH conserves.
2. Process operation that controls the water activity.
The water activity in the conserve is partly controlled by the evaporation of water in an open pan evaporator.
In many cases the available water is also controlled by the order of added ingredients, for example, the starch used in these recipes will need enough water to ensure complete hydration if other ingredients are added at the incorrect time this could result in incomplete starch hydration and more water being free to increase the water activity.
The diagram below details an example of the steam heated jacketed cooking vessel used to process conserves.

Walburton,G.E & Hallam, E.
The conserve is heated in a jacketed cooking vessel; the heat is generated from steam. The vessel will also contain some mixing heads to avoid any burnt areas to form.
Earle, R.L (1983) highlighted some important factors regarding steam jacketed cooking vessels:-
1. There is the minimum of air with the steam in the jacket.
2. The steam is not superheated as part of the surface must then be used as a de-superheater over which low gas heat-transfer coefficients apply rather than high condensing coefficients.
3. Steam trapping to remove condensate and air is adequate.
The action of the agitator and its ability to keep the fluid moved across the heat transfer surface are important. Save for boiling water, which agitates itself, mechanical agitation is assumed. Where there is no agitation, coefficients may be halved.
While the mixing heads avoid burnt areas they are also very important in the speed of heat transfer throughout the vessel.

Once all the ingredients have been added to the vessel, the product is then heated to a final temperature and held for an allocated time.
As mentioned previously, while the heating does reduce the amount of available water by evaporation, it is not the only control to reduce the available water, some of the water will be bound to other molecules within the conserve, the amount of free water (unbound) is the water activity that can be measured on a scale of 0 – 1.0 with pure water having a value of 1.0.
Food poisoning micro-organisms can occur in water activity values from as low as 0.83.
Conserves would generally be considered as high moisture foods with water activity between 0.90 to 0.95, and therefore unless other controls are utilised in the preservation of the finished product the potential for microbiological growth exists.
Thus emphasising the need to carefully consider the recipe constituents and define detailed shelf life trials.
Preservatives can be added to recipes but consumers are becoming more aware of additives and as such customers are particularly looking for clean label products.
These recipes would normally contain large quantities of sugar which also aid in the control of available water as sugar will try to achieve equilibrium with the food product with which it is in contact, and therefore drawing any available water to form a sugar solution.
The main concern for these products would the spoilage organisms such as yeasts and moulds, which usually require even lower available water to prevent growth.
3. Heat transfer.
Heat transfer is the movement of heat from one medium into another, by the heating of the conserve an open pan cooking vessel with steam used to heat the outer vessel jacket.
The water is evaporated off as it changes states from liquid to gas as detailed in the following diagram:-

Dr Cruzan’s Math (2012)
The heating of the conserve causes the molecules in the product to vibrate, as the vibrating molecules come in contact with other slower moving molecules there is a transfer of kinetic energy whereby the faster vibrating molecule will slow down but the slower one will speed up and gain kinetic energy, this chain reaction will carry on while the heating is applied.
There are three mechanisms of heat transfer being:-
Conduction
Follows ‘Fourier’s Law’ whereby, the more energetic molecules interact with the less energetic molecules and passing this energy on.
Conduction is normally associated with heat transfer of solids.
The calculation is described in the Engineering toolbox (2014) as:-
Fourier 's Law expresses conductive heat transfer as q = k A dT / s where q = heat transfer (W, J/s, Btu/s)
A = heat transfer area (m2, ft2) k = thermal conductivity of the material (W/m.K or W/m oC, Btu/(hr oF ft2/ft)) dT = temperature difference across the material (K or oC, oF) s = material thickness (m, ft)
Convection
Follows ‘Newton’s law of cooling’ that states the rate of change of the temperature of an object is proportional to the difference in temperature between the items own temperature and the surrounding ambient temperature.
Convection is normally associated with the heat transfer of liquids.
The calculation as detailed by Endomeme (2014):-
Newton 's Law of Cooling equation is: T2 = T0 + (T1 - T0) * e(-k * Δt) where: T2: Final Temperature T1: Initial Temperature T0: Constant Temperature of the surroundings Δt: Time difference of T2 and T1 k: Constant to be found
Radiation
Is governed by Stefan-Boltzmann Law that states, Thermal energy radiated by a blackbody radiator per second per unit area is proportional to the fourth power of the absolute temperature.
The heat transfer is carried out by electromagnet waves.
Earle R.L. (1983) confirms this information:-
In general, heat is transferred in solids by conduction, in fluids by conduction and convection. Heat transfer by radiation occurs through open space, can often be neglected, and is most significant when temperature differences are substantial. In practice, the three types of heat transfer may occur together
Heat transfer can occur in two ‘states’:-
Unsteady state heat transfer, during processing the temperatures may change and therefore the rate of transfer would also alter, this is the definition for unsteady state heat transfer, which would be applicable for the jacketed heating method used during the conserve processing.
Steady state heat transfer is the exact opposite with controlled temperature that does not alter.
All products have a heat transfer coefficient, this basically is a value assigned to any material for its overall ability of heat transfer.
The higher the heat transfer coefficient the quicker the heat will be transferred.
4. Impact on nutritional and organoleptic qualities.
The heating of products will cause chemical changes and can affect the colour, texture, taste, aroma and nutritional factors.
Not all heating gives a negative affect though, for instance if you are producing a caramelised conserve the heating will generate the maillard effect with the sugars resulting in the caramelised flavour.
Many vitamins are adversely affected by heat processing and can therefore be destroyed during cooking such as vitamin C.
Colour changes will occur due to chemical reactions within the food, for instance direct heat applied to bread will eventually turn the bread brown, toasting it due to the breakdown of sugars forming carbon, this is a chemical change as it cannot be reversed, in very much the same way the conserve that will contain sugars will also have colour changes.
Texture, a classic example of texture change due to heating would be an example of an egg cooking, not only does it have a massive texture change it also has a very visual change. This occurs due to the protein in the egg being denatured as described by Encyclopaedia Britannica (2014):-
The denatured protein has the same primary structure as the original, or native, protein. The weak forces between charged groups and the weaker forces of mutual attraction of non polar groups are disrupted at elevated temperatures, however; as a result, the tertiary structure of the protein is lost.
Carbohydrates are also affected by heat processing whereby the cell walls will breakdown resulting in a softer texture. Starch that is used in conserve recipes achieves it thickening properties by the addition of water and heat, at the correct temperature the starch will absorb the water causing the granules to swell, the interaction between all the granules causes a gel to form, and therefore thickening the conserve, but over heating can cause the swollen starch granules to rupture and lose all thickening properties.
While heating food products do have effect chemical changes within the products, you have to also consider the effect of freezing, this heat transfer method can also cause loss of texture due to the formation of ice crystals during the freezing stage. Loss of colour and vitamins in fresh fruit and vegetables as the enzymes will still be active but at a very reduced rate.
5. References
Dr Cruzan’s Math (2012) The Most Important Solvent. [Online]. Available from http://www.drcruzan.com/Water.html [Accessed 3rd May 2014]
Encyclopaedia Britannica (2014) Protein denaturation. [Online]. Available from http://www.britannica.com/EBchecked/topic/479680/protein/72545/Protein-denaturation [Accessed 4th May 2014]
Endomeme (2014) Newton’s Law of cooling. [Online]. Available from http://www.endmemo.com/physics/coollaw.php [Accessed 4th May 2014]
Engineering ToolBox (2014), Conductive Heat Transfer. [Online]. Available from http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html [Accessed 4th May 2014]
Earle, R.L (1983) HEAT TRANSFER THEORY. [Online] Available from http://www.nzifst.org.nz/unitoperations/httrapps1.htm#jacketed [Accessed 5th May 2014]
Earle, R.L (1983) HEAT TRANSFER APPLICATIONS. [Online] Available from http://www.nzifst.org.nz/unitoperations/httrapps1.htm#jacketed [Accessed 5th May 2014]

Garbutt, J. (1997) Essentials of Food Microbiology. Arnold, a member of the Hodder Headline Group: London

References: Dr Cruzan’s Math (2012) The Most Important Solvent. [Online]. Available from http://www.drcruzan.com/Water.html [Accessed 3rd May 2014] Encyclopaedia Britannica (2014) Protein denaturation. [Online]. Available from http://www.britannica.com/EBchecked/topic/479680/protein/72545/Protein-denaturation [Accessed 4th May 2014] Endomeme (2014) Newton’s Law of cooling. [Online]. Available from http://www.endmemo.com/physics/coollaw.php [Accessed 4th May 2014] Engineering ToolBox (2014), Conductive Heat Transfer. [Online]. Available from http://www.engineeringtoolbox.com/conductive-heat-transfer-d_428.html [Accessed 4th May 2014] Earle, R.L (1983) HEAT TRANSFER THEORY. [Online] Available from http://www.nzifst.org.nz/unitoperations/httrapps1.htm#jacketed [Accessed 5th May 2014] Earle, R.L (1983) HEAT TRANSFER APPLICATIONS. [Online] Available from http://www.nzifst.org.nz/unitoperations/httrapps1.htm#jacketed [Accessed 5th May 2014] Garbutt, J. (1997) Essentials of Food Microbiology. Arnold, a member of the Hodder Headline Group: London