Developing food structure through thermal processing

Developing Food Structure through Thermal Processing

Summary

The increasing demand of ready to eat food such as breads, cakes, biscuits, ice cream, chocolates, etc. has necessitated the development of the food structure thermally to improve quality, shelf-life and availability. In this report the methods of developing the structure of the food thermally through freezing and baking have been described, which are employed to structure and preserve perishable and inedible food product into stable and edible forms. The mathematical model of the simultaneous transient heat and mass transport within the food structure during the freezing and baking processes are also presented and described. The individual heat transfer mechanisms conduction, convection and radiation to the food structure during baking are also presented mathematically, while the population balance model for ice crystals nucleation and growth during freezing is also described. Finally the similarities and differences between freezing and baking processes are presented.

1.1 Introduction/Literature Review

Most food materials are perishable by nature due to water activity and some are not edible in their natural state such as meat, fish, yam, rice, and so on, therefore they require processing for them to be edible, palatable, preserved, packaged, stored and extend their shelf life. In addition, the increasing demand for ready to eat food by working class in the society has influenced the growth and development of food processing techniques (baking, canning, freezing, etc.) to improve food quality that will deliver sensory characteristics, taste and flavour to the consumers (Demirkol et al., 2006). Food products are structurally complex due to the multiphase materials and multi-material interaction in their structure such as in ice cream, breads, cakes, pizza etc. therefore, developing food structure through thermal processing is complex, as it involves the creation of microstructure, coupled with heat and mass transfer, and the rheological behaviour of food materials is highly non-Newtonian of which our engineering understanding is still limited (Zhang et al., 2005; Norton et al., 2006). Food structure can be developed thermally (i.e. heating or cooling) through drying, frying, baking, freezing, crystallization, freeze-drying, etc. In addition, to developing food products structure to be metastable to deliver taste, flavour, texture and eating pleasure on consumption requires an integrated understanding of chemistry and material science in conjunction with knowledge of how the processing conditions affects its structure, chemistry and appearance (Norton et al., 2006). The objective of developing food structure through thermal processing is to inactivate pathogenic microorganisms, improve shelf life and make the food product safe and palatable on consumption.

Most food processing operations depends on heating such as baking, canning, drying, frying and pasteurisation (Fryer and Robbins, 2005; Miri et al., 2008). The main aim of heating is:

  • For preservation, that is extend shelf life by killing and decreasing microbial growth rate and inactivates their enzymes such as in sterilisation of canned foods and pasteurisation of milk, so that the food product is safe for consumption (Bellara et al., 1999).
  • To develop taste, flavour, colour and lower microbial load, such as in cooking of fish, meat, vegetables; baking of breads, biscuits, cakes etc.
  • To develop the structure of the food, such as in drying and baking of bread, cakes, biscuits etc. where heating acts both to change the starch structure and also develop bubble structure within the material (Fryer and Robbins, 2005).

Many food product structures rely on solid state to determine their taste, texture and flavour such as in cakes, breads, pizzas, biscuits, etc. this can be achieved through baking process. Baking involves the simultaneous heat and moisture transport in the dough, and brings about water vaporisation at the dough interface, volume increase, starch gelatinisation, protein coagulation and crust formation and browning (Vanin et al., 2009). Baking process has been studied experimentally, design and optimization of baking ovens and mathematical modelling conducted to understand the basic mechanism of heat transfer and water transport in the dough and also to determine baking time (Fahloul et al., 1995; Sablani et al., 1998; Broyart and Trystram, 2002; Demirkol et al., 2006). Mondal and Datta reported the mathematical models of the coupled heat, moisture content, volume expansion, weight loss, crust formation and browning changes during the baking of bread (Mondal and Datta, 2008). This concept of thermal processing based on heating the food materials for certain length of time at a given temperature may directly impact on the quality and sensory properties of the food due to the degradation of heat sensitive ingredients and reactions. Consequently, food is significantly over processed to ensure safety (Fryer and Robbins, 2005). Therefore, there is need to control over processing to maintain quality, nutritional value of the food product such as vitamins and proteins which may be denatured by heating, and sensory characteristics such as colour, flavour, taste and texture to ensure consumers acceptability (Lewis and Heppell, 2000; Miri et al., 2008).

Furthermore, some food product structures can also be developed through the removal of heat (i.e. cooling) such as freezing as in ice cream and sorbet which rely on their frozen state to provide their structure, freeze drying as in coffee, and crystallisation as in chocolate. Freezing is also used to generate food texture from an amorphous protein as in myco-protein and kori-tofu which rely on the compression of the protein paste between ice crystals to form fibres (Mousavi et al., 2005). The heat transfer and phase change during freezing process of food material has been model theoretically and empirically to determine the freezing time (Delgado and Sun, 2001; Mittal and Zhang, 2000; Becker and Brian, 1999). But the freezing time is complex to predict due to the variation of the thermophysical properties of food during freezing and phase change (Mittal and Zhang, 2000). However, a large number of models and equation for estimating the freezing and thawing times for food products have been proposed starting from the Planks equation which ignores the removal of sensible heat and gradual phase change as freezing proceeds (Plank, 1913; Nagaoka et al., 1955; Pharm, 1986). Lian et al. reported a combined computational fluid dynamics (CFD) and discrete population balance model to describe ice crystallisation and growth kinetic in a scraped surface freezer during the production of ice cream (Lian et al., 2006). Many food products are structurally complex such as ice cream composing of five phases; fat, protein, air bubbles, matrix, and ice crystals phases, and chocolate with different polymorphic forms, which makes their processing exceedingly complex.

Therefore, in order produce safe and quality food product through thermal processing requires sound understanding of the influences of process variables on their microstructure and sensory properties (Miri et al., 2008; Aguilera, 2005) as illustrated in Fig.1. This can be done by using suitable microscopy and imaging technique available such as X-ray micro-computed tomography, scanning electron microscope (SEM), transmission electron microscope (TEM), cryo-scanning electron microscopy, confocal laser scanning microscope (CLSM), differential scanning calorimetry (DSC), etc. to probe into the food microstructure and to obtain valuable information that will enhance design processes that will improve quality of the food product and structure (Aguilera, 2005). Aguilera investigated the relationship between the quantitative data obtained from the food microstructure by image analysis and the nutrition values, chemical and microbiological stability, texture and physical properties, transport properties and product engineering (Aguilera, 2005). However, food product manufacturers are under pressure to change their processes to produce more healthy foods, with lower salt and fat content due to consumers awareness and tighter regulations on food additives (Norton et al., 2006).

2.0 Developing the structure of the food through cooling

2.1 Freezing

The development of food structure through freezing is a complex process involving the removal of heat (i.e. cooling) with simultaneous phase change from liquid to solid (ice). Freezing is a unit operation in which heat within the food material is removed below its freezing point and the water content is converted into ice crystals. Prior to freezing, the sensible heat within the food material is removed (i.e. cooling) to decrease its temperature to about -18oC (or below) the initial freezing point of the foodstuff which is lower than the freezing point of pure water due to the ingredients in the food product (Goswami, 2010; Delgado and Sun, 2001). As cooling continues, ice crystals begins to form and the concentration of the solute content increases depressing the freezing point of the food material. In view of this, the freezing process involves four stages pre-cooling, nucleation and crystallization, the growth of the ice crystals and the cooling of the foodstuff to reach the temperature of the thermal centre. The size and nucleation of the ice crystals during freezing influences the quality of the food product after thawing causing drip loss, textural and colour changes (Chevalier, et al., 2000). Some food products (in particular ice cream and sorbet) rely on freezing to create the microstructure that will provide the desirable traits, texture and sensory properties on consumption. Food products rely on freezing:

  • For preservation lowering the temperature of food products reduces biochemical, microbial and enzyme activities, biochemical reactions that lead to food spoilage and crystallizes liquid water that supports microbial growth.
  • Food structuring some food product rely on their frozen state to develop their structure such as in ice cream and myco-protein.
  • Freeze drying to remove moisture from frozen food material without damaging the structure through partial vacuum and heat supplied to sublimate the ice such as in coffee (Mousavi et al., 2005).

Developing food structure through freezing can be achieved using conventional freezers (examples; plate, contact and air blast freezers) and Cryogenic freezer (examples; immersion and spray freezers) in which the food to be frozen is placed in direct contact with a liquid refrigerant such as liquid nitrogen which is odourless and colourless with a boiling temperature of -196oC and cooling capacity of about 400 kJ/kg (Goswami, 2010). The freezing rate is rapid and freezing time is short for cryogenic freezing, and the food materials may suffer a high degree of thermal shock which may result to internal deformation and loss of texture compared to conventional freezers (Smith, 2007). Freezing does not destroy the nutrients, vitamins, and flavour of the food product. However, the growth of ice crystals within the food structure during freezing may result to substantial textural damage or generate a texture as in myco-protein (Mousavi et al., 2005). A sound understanding of the relationship between the freezing conditions and the size and growth rate of ice crystals is necessary to controlling food product quality and texture (Mousavi et al., 2005). The rate of freezing/cooling plays a key role in the size, number of crystals and quality of the frozen food product. Slow freezing rate causes undesirable large ice crystals while high freezing rate produces smaller and more numbers of ice crystals (Resende et al., 2007; Goswami, 2010).

The freezing of food products is more complex compared to the freezing of pure water, because food structures contain multi-materials with many ingredients which make the freezing to occur at a lower range of temperatures (Kennedy, 2000). The freezing temperature time curve of pure water and food material is shown in Fig.3 below. The supercooling point S in Fig.3 represents the onset of crystallization during the freezing process. The freezing rate (i.e. the velocity of the phase change) depends on the size, surface area and thermal properties of the food product and the characteristics of the freezing system such as the temperature and heat transfer coefficient of the freezing medium. In addition, to the removal of heat to cause phase change the rate of nucleation of ice crystals and ice growth are also involved to bring the food product to frozen state. As the foodstuff is continuously cooled below its freezing range the moisture content within it is converted into ice crystals and complete when the liquid water at the thermal centre of the food product has been converted to ice, hence reduces water activity.

2.1.1 Ice formation, nucleation and crystallization

Freezing involves the removal of heat accompanied by the formation of ice crystals. Therefore, freezing process involves nucleation (i.e. the formation of ice crystals) and the growth of the crystal size. Nucleation is the generation of ice nuclei to serve as an active site for crystal growth term crystallization. The energy barrier must be overcome before nucleation can be initiated, and nucleation is necessary for freezing to commence (Kennedy, 2000). Homogenous nucleation occurs in pure systems, while heterogeneous nucleation takes place in foodstuffs (Kennedy, 2000). The rate of nucleation is depends on the temperature of the freezing system, volume of the material and the cooling rate. Once nuclei are formed, crystals growth sets in, the curve in Fig.2 present a typical nucleation and ice crystal growth rate during freezing. The crystals growth is influenced by the rate of removal of the heat of crystallization, phase change and the rate of mass transfer from the solution to the surface of the ice crystals (Lian et al, 2006). But as the freezing progresses, the freezing temperature decreases because of the increasing solute concentration as free water is converted into ice. The formation of ice crystals may cause some physico-chemical changes to the food structure such as cracking, lost of quality due to drip losses during thawing, re-crystallization of ice, colour changes, disruption of tissues and the glassy transition state. Understanding the relationship between the size, shape and distribution of the ice crystals and the freezing conditions is essential in controlling the food structure, quality and texture (Mousavi, et al., 2005; Lian et al., 2006). Freeze-cracking will occur if the internal stress is higher than the frozen foodstuff strength; the food structure will crack during freezing depending on the freezing rate (Kennedy, 2000). Ice crystallization during freezing is complex and involves the interactions of heat transfer, nucleation and growth of ice crystals to give an ice crystal size distribution (CSD) within the food structure.

Population balance (PB)

The population density that is the number of ice crystals per unit size per unit volume of the foodstuff and the ice crystal size distribution are derived from the population balance. The ice crystal nucleation and growth kinetics as freezing progresses is described by the population balance equation (1):

Where; n is the population density of ice crystals, U is the velocity of the mixed phase (fluid), B and G are the ice crystal nucleation rate and growth rate, l is ice crystal size, kb (m-1) and kg (m/s) are ice crystal nucleation and growth rate constant, nb and ng are power indices,?i and ?e are initial ice content and the equilibrium ice content at which the solute in the unfrozen phase of the foodstuff is in equilibrium with the freezing temperature.

2.1.2 Heat load

The heat load is the amount of heat energy to be removed from the food material to keep it in a frozen state. The physical model of the freezing process is non-linear unsteady state heat transfer due to phase change, changing thermal properties and heterogeneity of food structures (Resende et al., 2007). This involves the following stages shown in Fig.3 of a typical freezing curve:

  • A-S from the freezing curve represents the removal of sensible heat from the foodstuff to the freezing temperature. The point S is the supercooling region, followed by a rise in temperature to the freezing point B/B due to the release of heat of crystallization as water within the food structure crystallizes out forming ice crystals (Goswami, 2010; Kennedy, 2000).
  • B- C on the curve represent the removal of the latent heat of fusion, this brings about phase change (liquid to ice) and the crystallization of the water as ice crystals.
  • C-D represents the removal of sensible heat from the freezing temperature to the final frozen/storage temperature. However, for food products there are freezing range rather than freezing point due to soluble components in the food structure (Robbins, 2010; Smith, 2007).

The total heat load, ?h (J/kg), to be removed during freezing is the sum of the heats removed for the three stages shown Fig.3 above and is given by equation (4):

The specific heat capacity of food materials is non constant due to the different ingredients that are contained in the food structure and the phase change during freezing causes changes in thermal properties; therefore the heat capacity is temperature dependent which implies that heat load in equation (4) can be expressed as equation (5):

Where; Ti is the initial temperature of the food (K), Tf the freezing temperature (K), Tend is the final frozen temperature of the food, Cpl is the specific heat capacity of the non-frozen food (J/kg K), Cps the specific heat capacity in the frozen state (J/kg K) and ? is the latent heat of fusion (J/kg).

2.1.3 Freezing time

The standard freezing time/holding time as defined by Eek is the total time required to lower the food product temperature from its initial value to a given final one at the thermal centre (Eek, 1991). However, for most foodstuffs a final temperature of -18oC is more reasonable considering storage at temperature below -18oC (Delgado and Sun, 2001). Food engineers developing food structure through freezing are interested in estimating freezing time and the corresponding refrigeration load to able them optimise the design of the processing equipment and estimate the refrigeration requirements for the freezer (Resende et al., 2007; Becker and Brian 1999; Delgado and Sun, 2001). The freezing time can be divided into three stages:

  • Pre-cooling time that is, the time it takes to cool the foodstuff from its initial temperature to the freezing point temperature.
  • Freezing time this is the time involve for phase change, that is the time takes for the freezable water in the foodstuff to freeze (turn to ice).
  • Sub-cooling time that is the time it takes for the food product, after freezing, to reach the final temperature of the thermal centre of the food (-10oC or -18oC) (Lopez-Leiva and Hallstrom, 2003).

However, the temperature profile within the food structure decreases with respect to time and position during freezing process which can then be described by an unsteady-state heat transfer in equation (7), in which the temperature within the food product changes with time according to Fouriers second law of heat conduction.

Where; a is the thermal diffusivity (m2s-1), x, y and z are coordinate axis (m), and k thermal conductivity (Wm-1K-1). The above equation does not consider heat transfer by convection, the removal of latent heat of fusion and phase change (Smith, 2007).

Plank (1913) presented a simpler model for estimating the freezing time for a block of ice as shown in equation (8) with the assumptions:

  • Thermal conductivity of the foodstuff is constant and the frozen and unfrozen layer have equal density.
  • Stead-state process due to slow heat transfer process.
  • The heat capacity of the frozen food layer is negligible.

Where; h is the heat transfer coefficient (Wm-2K-1), T1 is the temperature of the medium (K), a is the geometric dimension (m), t is the freezing time (s) and P and R is depends on geometry of the food material as presented in Table 1 below. Planks equation does not consider the time required to remove sensible heat, the variation of thermal conductivity with temperature and the gradual phase change during freezing.

In addition, Nagaoka et al., (1955) developed an empirical model for freezing time by modifying the Planks equation as presented in equation (9), to account for the time required to remove sensible heat (i.e. pre-cooling time) and as well as phase change (freezing time).

2.1.4 The impact of freezing on food quality

Although developing food structure through freezing is less destructive, preserve taste and nutritional value, but as freezing progresses liquid water within the cells of the food structure is converted into ice which may cause disruption of cellular walls and changes in osmotic pressure due to increase in concentration of solute as crystallisation proceeds (Chevalier et al., 2000). This formation of ice crystals may induce some physico-chemical changes that decrease the quality of the food. This modification in quality of the food depends on the nature of the food structure, packaging, temperature and the duration of storage. The common physico-chemical changes are lipid oxidation, flavour deterioration, protein denaturation, degradation of pigments, freezing cracking, recrystallisation of ice due to temperature fluctuations and drip losses during thawing (Kennedy, 2000). High freezing/cooling rate leads to the formation of small ice crystals which are homogeneous in their structure and give a better quality of the food product, but may cause freeze-cracking in some food structure as a result of thermal shock. The formation of ice crystals during freezing also causes dehydration of the food cells which induced protein denaturation due to the disruption of the protein-solvent interaction to protein-protein interaction such as in meat, fish, dough, fruits, vegetable, etc and also decreases the water holding capacity of the tissues on thawing (Kennedy, 2000). The freezing process slows the microbial and enzymatic reactions that occur in the foodstuffs, but the hydrolytic enzymatic reaction causes colour changes, produce off-odours, off-flavours such as in fruits, vegetables, meats, etc. The growth of ice crystals as freezing continues may cause substantial textural and cell membranes damages within the delicate structure of the food product, there by reducing quality (Mousavi et al., 2005).

2.1.5 Freeze- drying

Freeze-drying (lyophilization) is a coupled heat and mass transfer process which involve the freezing of the food structure and applying heat to sublimate the ice from the frozen state under reduced pressure conditions (vacuum). Freeze-drying is used to develop food structures which have to conserve its flavour, retain aroma, texture, structural integrity, reduce weight and preserve quality at ambient temperature for a long period of time (Boss et al., 2004; Chakraborty et al., 2006). The process involves three stages: freezing, primary drying and secondary drying as shown in Fig.4.

In the primary drying stage, ice is sublimated as the ice-air interface recedes in the food structure by convective mass transport while during the secondary drying the temperature of the product is increased so that residual moisture is removed. Freeze-dried food product structure remains stable without shrinkage compared to convectional drying. In addition, the products are of high quality due to the low temperature of the process creates less impact on the flavour, nutrients, vitamins and colour on the foodstuff. However, it is highly expensive due to the energy and long drying time involved.

3.0 Developing the structure of the food through heating

3.1 Baking

Baking process involves the simultaneous heat and mass transport that bring about physical, chemical and structural modification of the food product inside the oven. In the oven during baking, the mechanisms of the heat transfer to the food material are natural convection or forced convection by a fan from the hot air flow over the food product, the radiative heat transfer from the heated oven walls to the baking material and by conduction from the contact area between the food product and metal tray surface as shown in Fig.5 below (Sablani et al., 1998; Sakin et al., 2009). Concurrently, the moisture in the food structure diffuses outward to the food surface. The physical, chemical/biochemical and structural changes brought about by baking process in food products such as in breads, cakes, biscuits, pies, pizza, etc. includes increase in product temperature, volume expansion, gelatinization of starch and protein coagulation transforms the viscous dough into an elastic crumb, evaporation of moisture, formation of porous structure, denaturation of protein due to heat reactions, crust formation and browning (Sablani et al., 1998). The heat transfer rate to the food material depends on the air temperature and velocity. The chemical, physical, rheological and structural states of the food determine the quality of the food product and they are dependent on the moisture transport, temperature and baking time (Demirkol et al., 2006). These structural modifications improves texture, makes the food product edible, extend shelf-life, preserve the food, add value, extend availability and provides the mouth feel pleasure on consumption. However, the following process parameters will impact on the quality of the baked product temperature, air humidity and velocity, process time and the physico-chemical changes in the food product during baking.

The baking process is divided into two periods which are heating up and crust crumb formation periods (Sakin et al., 2007). The baking time for a typical baking process ranges from 5-60 minutes depending on the temperature, the nature and size of the food product, and the operating conditions of the oven. The moisture transport is heat transfer stimulated, and leaves the surface of the food structure by convective mass transfer. This implies that baking is the combination of drying and structure creation to improve flavour, texture and taste foodstuff.

The objective of baking are: extend product stability, texture and flavour creation, minimization of moisture content, edibility and palatability of the food, preservation, product discoloration, reduce weight, improve packaging and transportation, stabilise storage and structural development and stability of the food structure. Some common ovens employed for baking includes tunnel, natural gas, electric, microwave, and hybrid ovens. Most baked products contain the following ingredients:

  • Wheat flour provides starch and gluten protein to help create the food structure
  • Water - to help mixing, binding and hydrolysis reaction with the starch.
  • Yeast convert the starch via fermentation to sugar releasing alcohol and CO2 to give the dough airy internal texture and cause expansion due to rising temperature.
  • Salt/sugar salt add flavour, strengthen the gluten and controls the fermentation process for optimum volume expansion, while sugar is for sweetness.
  • Fat help to slow staling process and increase shelf-life.
  • Eggs to provide proteins that coagulate on baking to give the food product its texture.
  • Raising agent example bicarbonate of soda to provide CO2 to help enhance air bubbles.

Emulsifiers and anti-staling agents are added to improve crumb structure, extend shelf-life and improve quality, improve sensory properties and the mouth-feel characteristics (Mondal and Datta, 2008). The whole ingredients are mixed to form dough. The rheological properties of the dough are vital for structural, textural, volume expansion and product quality attributes. Therefore, baking is aimed at engineering the food structures under a control heating to trap some leaving gas bubbles, create crust, aroma and browning due to Maillard reaction that will deliver the desired flavour and taste on consumption. During baking the dough absorbs heat, which increases its internal temperature towards the centre and causes evaporation of water, loss of weight as result of mass transfer, and the water vapour leaves with its latent of vaporisation which will lead to some steam condensing. The baking time, moisture content and the temperature of the oven will determine the quality of the food product.

Baking process requires high energy, therefore mathematical modelling and optimization of the process and oven will aid the design to reduced energy consumption while quality of product is maintained as well as process economics. The energy is used to dry the dough, generate structure, create flavour and texture, browning of the crust colour, and cook the food product to make it edible and palatable for consumption. The heat and mass transport between the oven environment and the food product during baking can be described by Fouriers and Ficks second laws of diffusion. Sablani et al. reported that there is no constant drying rate period during baking, and that the formation of crust causes resistance to heat and mass transfer between the oven environment and the surface of the food product (Sablani, et al., 1998; Sakin et al., 2007).

3.2 Heat fluxes

The three mechanisms by which the dough (i.e. the food product) receives heat fluxes during baking are conduction, convection and radiation heat transfer modes as shown in Fig.5 above. Standing experimental and mathematical modelling of the heat transfer in the baking of biscuit in a band oven revealed that amount of heat contributed by each of these heat transfer mechanisms are 20% by conduction, 37% by convection and 43% by radiation (Standing, 1974). This implies that the convective and radiative heat fluxes are dominant in the oven during baking.

3.2.1 Conductive heat transfer

The conductive heat transfer through adjacent molecules from the tray walls to the food structure is due to temperature gradient. Also the total heat absorbed by the food product progressively increases the internal temperature as it diffuses into the interior parts by conduction. The heat transfer by conduction is described by Fouriers law as shown in equation (10):

3.2.2 Convective heat transfer

The convective heat transfer from the hot air within the oven to the dough is due to the motion of the fluid in the oven. It is described by the heat transfer by force convection equation (11):

3.2.3 Radiative heat transfer

The radiative heat transferred via electromagnetic waves from the hot refractory surfaces of the oven to the dough due to temperature difference, can be described by Stefan-Boltzmann law as shown in equation (12):

Where; qc, qf, and qR are heat transferred by conduction, force convection and radiation (J/s)

AS surface area of the dough (m2), s is the Stefan-Boltzmann constant (Wm-2K4), Ta, TB, Tb, Tr and Ts are temperature of hot air, tray support, contact temperature of dough with the tray, temperature of refractory oven surface and dough surface temperature (K), h is the heat transfer coefficient (Wm-2K-1) and ? is the emissivity of the dough surface (dimensionless).

Therefore, the total heat transferred to the food product during baking is the sum of the conduction, convection and radiation heat transport mechanism, given as:

The total heat balance on the food product during baking is given by eqn. (14)

3.3 Temperature profile

Fig.5 above shows the simultaneous heat and mass transfer within the food product during baking. Therefore, the temperature profile inside the food product as the absorbed heat energy diffuses into the interior parts of the structure can be described by the Fouriers equation for unsteady state heat conduction. This mathematical model of the temperature is given by equation (15); it will enable us to predict the temperature profile within the food structue as a function of position and time during the baking process. Fig.6 illustrates how a typical internal temperature profile of foodstuff with time during baking increases as heat is diffusing through the interior part by conduction.

Where; W is the moisture content (kg water/kg dry product), ? is the latent heat of vaporization of moisture (J/kg) and ? is the density (kg/m3)

3.4 Moisture content profile

Due to the temperature gradient inside the food structure during baking, a partial water vapour gradient is induced in the food material to drive moisture counter to the heat gradient accompanied by condensation, thus creating evaporative-condensation mechanism. The amount of moisture diffusing out or evaporated depends on how much heat is absorbed by the food material, which causes the drying of the product. The moisture removal occurs by external convective mass transfer and by internal diffusion which are temperature and relative humidity dependent. The internal moisture diffusing out of the food structure due to temperature gradient during baking process can be described by Ficks second law of unsteady state mass transfer by diffusion as shown in equation (16). This equation will predict the moisture content profile within the food structure as a function of position and time, this reduction of the moisture contents brings about drying of the foodstuff and creates the structure of the food. Typical moisture content reduction profile within the foodstuff as baking proceeds is shown in Fig.7 below with the stage at which crust formation and browning starts.

Where; Deff is the effective moisture diffusivity (m2s-1), which is sensitive to temperature changes as baking proceeds.

Equations (15) and (16) shows the transient temperature and moisture content distribution in the food structure during baking process, this nonlinear partial differential equations can be solved given initial and boundary conditions.

3.5 Weight loss

The loss of moisture through evaporation during baking causes reduction in weight of the food product. The percentage loss in weight is given by equation (17):

Where; mo is the initial weight of the foodstuff (kg) and mt is the weight (kg) in time t (s).

3.6 Crust formation and browning

The formation of crust and browning is a common phenomenon associated with baking process and also a yardstick of quality index. The crust crumb structural layer at the surface of the dough and its browning contribute to the flavour, pleasant aroma, slicing characteristics and texture of the baked food structure. This crust layer formed at the surface of the dough during baking constitutes resistances to heat and mass transport, resist further volume expansion and weight loss (Mondal and Datta, 2008). The Maillard reactions are responsible for the formation of colour and the release of flavours during baking (Vanin et al, 2009). The crust browning is due to Maillard browning reaction, caramelisation as a result of heating of the carbohydrate in the foodstuff or chemical reactions induce by heat. The formation of crust and browning depends on the moisture content of the foodstuff during baking process and oven temperature as illustrated in Fig.7 above.

4.0 Comparing and contrasting freezing and baking processes

4.1 Similarities between freezing and baking processes

  • Physically both processes involve heat and mass transfer process. In baking heat is added to remove moisture and create structure of the food while in freezing heat is removed to convert water into ice.
  • The mechanism of heat transfer through the food structure in both processes is thermal conduction, which can be described by Fouriers equation for heat transfer by conduction.
  • Both processes are dynamic (i.e. they are time dependent) and can be described by unsteady state heat transfer equations.
  • Both processes inhibit the growth of pathogenic microorganisms, retard biochemical and enzymatic reactions and preserve foodstuffs.
  • The driving force in freezing and baking processes is temperature gradient within the food structure.
  • Both processes involve phase change at some point during the process. In freezing, water is converted into ice (liquid to solid) during supercooling while in baking water is converted into vapour (liquid to gas) as the food product undergoes drying due to adsorption of heat.
  • The freezing time and baking time depend on the size of the food product and the operating conditions of the system.
  • Both processes induce physico-chemical changes to the food product such as moisture migration, expansion, protein denaturation, etc.
  • The rate of heat transfer during freezing and baking processes depend on size, surface area and thermal properties of the food product and the operating system conditions such temperature, etc.
  • Frozen and baked products are less dense than their original starting material. This is due to the weight losses during freezing and baking processes. Unless the freezing foodstuff is packaged in an impervious film during freezing (Kennedy, 2000).
  • Freezing and baking processes result in the volume expansion of the food product.
  • Internal thermal stresses are generated within the food structure during both processes.
  • Freezing and baking process involves the migration or transport of moisture from the inner parts of the foodstuffs to the surface. At the surface, moisture water/moisture is loss via evaporation and in frozen state by sublimation.

4.2 Differences between freezing and baking process

1. Freezing involves the removal of heat from the food material (i.e. cooling).

Baking involves the addition of heat to the food material (i.e. heating).

2. The heat removed to cause freezing isless compared to that added to baked a the foodstuff of the same size.

The heat required for baking is more compared to that removed to freeze foodstuff of the same size.

3. The freezing time is more compared to baking time.

Baking takes less time compared to freezing due to high energy involved.

4. Heat diffuses out of the foodstuff from the thermal centre to the surface during freezing.

Heat diffuses into the foodstuff from the surface to the thermal centre as baking proceeds.

5. Heat transfer is by convection and conduction mechanisms.

Heat transfer during baking is by conduction, convection and radiation.

6. Liquid water is been converted into ice (solid) as freezing proceeds.

Liquid water is converted into vapour (gas) during baking process to induce drying.

7. Less operating cost.

High operating cost due to more energy requirement.

8. Less labour intensive compared to baking.

Much labour intensive compared to compared to freezing.

9. Less destructive to proteins and vitamins in foodstuff compared to baking.

High degradation of proteins, vitamins and nutrients due to over heating of the foodstuff during baking.

10. Freezing can structure food products in liquid, paste and solid states.

Baking can only structure food products in solid state.

11. The temperature of the food product decreases with time towards the centre.

The temperature of the food structure increases with time towards the centre.

5.0 Conclusion

Developing the structure of the food through thermal processing either through cooling or heating is aimed at creating structure, extend availability, reduce bacterial growth and load, make the food edible and palatable, extend shelf life, provide convenience for packaging and handling, add value and to provide variety and choice for consumers. To create the structure of the food thermally with the desired properties requires an understanding of the interaction involved during processing line such as the process, rheology and the microstructure of the food. The mathematical model of the simultaneous heat and mass transport during thermal processing of food product will provide clues for improving quality and structure. Many foods are structured in solid state, whose microstructures determine their taste, texture and the release of the sensory characteristics. Baking therefore is used to manipulate the food structure in solid state to enhance flavour, extend shelf life, and make it edible and palatable to deliver the desired sensory properties on consumption, while freezing is mostly used for food preservation, structuring as in ice cream and freeze drying.

Nomenclature

A Surface area of food product (m2)

B Ice crystal nucleation rate (kg/s)

Cp Heat capacity (J/kg K)

Deff Effective moisture diffusivity (m2s-1)

G Ice crystal growth rate

h Heat transfer coefficient (Wm-2K-1)

?h Heat load (J/kg)

k Thermal conductivity (Wm-1K-1)

kb Ice crystal nucleation rate constant (m-1)

kg Ice crystals growth rate constant (ms-1)

l Ice crystal size (m)

m Weight of the foodstuff (kg)

n Population density of ice crystals

nb and ng Power indices

Q Total heat transfer (J/s)

qc, qf, and qR Heat transferred by conduction, force convection and radiation (J/s)

T Temperature (K)

t Time (s)

WL Percentage weight loss

x, y and z Dimensions in the 3-cartessian coordinate (m)

Greek letters

a Thermal diffusivity (m2s-1)

? Emissivity of the dough surface

? Density (kg/m3)

s Stefan-Boltzmann constant (Wm-2K4)

?i and ?e Initial and equilibrium ice content

? Latent heat of vaporization (J/kg)

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