Thermal Profiling Of Baking

What is Oven Heat Flux? Heat flux is one of five critical parameters in baking operations. It is defined as the amount of energy transferred per unit area per unit time from or to the product surface and is expressed in Btu/hr.ft2, J/S.m2 or W/m2. Three components constitute oven heat flux. Their ratios can influence the quality of baked products:

? Radiation

? Convection

? Conduction

How it works?

It is important to recognize that different products have their unique mix of heat flux components. Therefore, they’ll have their own heat flux profile. Knowing the total heat experienced by the product, and the contribution of individual heat fluxes is critical to understanding finished product characteristics. This information helps explain why one oven bakes differently from another. Analyzing and controlling these aspects are fundamental to product quality. Heat flux by radiation It occurs between two surfaces or materials at different temperatures by the emission of energy in the form of electromagnetic waves and subsequent absorption. Heating elements and oven walls emit energy in the form of electromagnetic waves. The energy flux (Q) depends mainly on the temperature of the object.

How is heat flux measured?

The heat flux sensor is designed to measure convective and radiant heat fluxes at the product level, and to identify which modes of heat transfer are predominant: Oven Air Velocity Also known as oven convective airflow

What is Oven Air Velocity?

Oven air velocity is defined as the flow rate of air in an oven chamber and is expressed in m/sec or ft/min. Measuring air velocity along with other oven parameters such as heat flux, humidity and temperature can have a significant impact on controlling:

? Amount of convective heat delivered to the product

? Moisture removal

? Crust formation

? Overall bake of the final product.

How it works ?

In a convection oven, for example, air velocity is a measure of how much air moves along and across the baking chamber. In terms of equipment, fans and blowers create hot air recirculation in the baking chamber. The faster the air moves, the higher the convective heat flux the product receives for a proper bake.

Increasing air velocity leads to an increase in the apparent heat transfer coefficient (h); hence making convective heat flux higher. This is certainly beneficial in processes which require high energy input in initial or middle oven zones. Products bake quicker in higher airflow ovens due to the greater temperature increase per minute at the product core level. Subsequently, relevant thermal events, such as yeast kill, starch gelatinization and arrival, occur sooner when tracked by an S-curve. Velocity of the circulating air also influences baking time. The higher the air velocity, the faster the product loses moisture and the sooner the crust forms. Weight loss, browning of crust, and firmness of baked product can be increased by increasing air velocity. For example, increasing air velocity results in drier products with a darker crust color. High-speed ovens have mechanisms which control the flow of air, these are often referred to as color-aiders.

Application

Ovens are designed to offer an even distribution of air velocity across the baking chamber or oven band / conveyor. Even distribution of airflow is vital to maintain the quality of product delivered by convection ovens. Side-to-side airflow variation is directly related to side-to-side variation in product bake. An uneven flow of air within the chamber often leads to differences in product volume (e.g. some loaves are larger than others), variations in crust color and other quality issues. Modern high-speed bread, cracker and cookie ovens rely heavily on air to transfer heat to the product. In most cases, the contribution of convective heat flux represents values as high as 80% of the total heat transfer; the other 20% are in the form of radiant and conductive heat.

How to measure oven air velocity ?

Oven air velocity can be recorded by an air velocity sensor. Arrays of sensors can provide a precise picture of airflow patterns inside an oven from side to side and end to end. These sensors collect data, at product level, as the array passes through the process. The number of sensors required vary with the width of the conveyor. Air velocity sensor arrays can help with spotting airflow differences between baking zones, concentrated air velocities on isolated parts of the conveyor, and unwanted air currents at the entrance or exit of the oven.

Air velocity profiles are useful in adjusting your process to maximize quality and reduce waste. The tighter the pattern of lines, the more even the air distribution is across the width of the process. By measuring air velocity design engineers are able to balance heat and airflow across the width and along the length of the process oven.

What is a Convection Oven?

A convection oven uses fans to circulate hot air around the product placed on racks in the baking chamber. This is the convective component of heat transfer. Convection ovens are perfect for baking small-sized goods such as pastry and other products that are baked free-standing on sheet pans or perforated racks. Convection heat transfer is different from natural convection:

? Instead of using fans, heat is transferred from air to the product.

? Temperature differences between air and product create a density gradient.

? As a result, there’s a vertical movement of air. The less dense hot air rises on top of colder air.

How does it work?

Heating products in a convection oven is done by a combination of the following heat transfer mechanisms:

? Radiation from a heat sources such as electric resistances, vapor or hot gas tubes, gas-fired burner flames. This readily delivers heat to air and products.

? Forced convection from hot air, with fans generating air currents. Convection is the predominant heat transfer mechanism.

? Conduction from hot surfaces in direct contact with the product (e.g. racks or pans).

Convection ovens rely heavily on convection air currents and less on radiant heat sources. Therefore, the convective flow of air enhances the drying component of baking. This removes the fine water layer from the surface of dough/batter acting as a barrier (insulation) for heat penetration. Dough moisture typically evaporates faster in convection ovens. As a consequence, the product bakes more quickly and more uniformly than conventional ovens. This reduces the total baking time.

Conventional ovens vs. convection ovens

In conventional ovens, heat rises to the top. So, items placed on the top rack bake much faster than ones placed on the bottom. While in convection units, hot air swirls all around the product. Therefore, adjustments in oven settings are necessary when switching from conventional to convection ovens. For example, to maintain equivalent baking treatment at a constant baking time, temperatures should be reduced by 25–50°F (14–28°C). If no adjustments in oven temperature are made, then the bake time should be decreased accordingly to obtain equivalent baking treatment. Usually, it’s about 20% less.

Application

In convection ovens, hot air is circulated by a fan at an air flow/velocity of 2–22 mph (1–10 m/s). This rate must be carefully set depending on the product and baking conditions. The convective heat transfer coefficient (h) developed during baking depends on the air velocity and temperature inside baking chamber. It can range from 20–120 W/m2 K.4 Fast air currents can distort the shape of delicate products such as sponge cakes, batters and soft doughs such as pastry. They can also dry out the products, negatively impacting their texture, shelf-life and overall quality. Rapid surface drying of dough pieces may form a hard skin which can prevent dough piece expansion during oven spring. On the other hand, too slow air currents can reduce the rate of heat transfer and increase the baking time. This hinders convection.

Ideal bakery products for convection ovens

While convection ovens are very versatile and can handle both yeasted doughs and deposited batters, these are not as beneficial for products baked inside high-sided pans that do not allow for a full contact between air currents and product’s external surfaces. Ideally, they would be used for free-standing products baked on sheet pans or perforated racks.

What is Oven Design?

Ovens are an integral component of food and other manufacturing processes. Their design takes into consideration the overall plant layout as well as the type and estimated production volume. In baking operations, ovens are responsible for transferring heat to bring dough pieces to baking temperatures. Baking is a simultaneous heat and mass transfer unit operation. Heating primarily causes moisture evaporation (drying) and initiates a series of chemical reactions and physical changes in the baked product. To produce and transfer heat, ovens require one or a combination of energy sources, these are:

? Electricity

? Fuels (natural gas, liquefied petroleum gas)

? Steam (generated in a boiler) An oven is expected to achieve heating at an optimum rate and at the highest efficiency levels.

This is essential for minimizing energy waste and operating at maximum capacity.

How does it work?

Ovens are the most critical unit operation in baking plants and can be responsible for limiting their capacity. Therefore, the need for proper oven design to successfully satisfy present and future demands. No oven can be properly designed without completing the product development phase and understanding the requirements of the products to be baked. Composition, product quality characteristics, physical properties of input/output streams and baking specifications are few of the criteria that should be established.

Phases of oven design

? Mass and energy balances: flow rates (units or pieces /pounds per batch or min) and composition of input and output streams are determined.

? Baking process modeling: lab-scale experimentation and use of mathematical and empirical expressions (e.g. relevant process variables) to describe the required baking process .

? Process simulation: use of software and engineering data collected during modeling to measure and evaluate the real performance of the baking process.

Variables that can be effectively monitored during the process (baking temperature, conveyor speed, bake time, heat flux, air humidity and flow rate) are selected and values are assigned:-

? Equipment sizing: calculation of the size and characteristics of the equipment according to process specifications

? Equipment rating: calculation and selection of the operating conditions given process specifications and equipment size and characteristics

? Equipment costs.

Application

Relevant considerations for oven design:-

? Unit weight of dough pieces/finished product to be processed

? Product dimensions (width, thickness and length)

? Capacity/throughput required (according to current demand) plus long-term business growth (units/batch or units/min).

? Surface area required to bake products at a given production rate.

? Mode of operation: batch or continuous mode.

? Selection of the proper combination of heat transfer mechanisms (radiation, convection and conduction) required to obtain the correct baking profile and allow for maximum heat exchange efficiency.

? Total energy required to produce certain heat flux to bake a full batch of product (KWh), according to total bake time, heat surface, heat transfer mechanisms and heat losses within equipment ? Heat flux (BTU/hr·ft2 or W/m2) required to bake product

? Hygienic design.

To design an oven, several engineering aspects need to be integrated and brought together, including:

? Electrical components (e.g. motors)

? Mechanical components (e.g. conveyors, gears, transmission belts, chains, air blowers, air impingement or hot gas jetting systems, extraction systems)

? Thermal components (e.g. heat exchangers, steam generation, steam dampers, ribbon burners, insulation materials to minimize heat loses)

? Condensate collection systems

? Electronics and automation systems

? Instrumentation and control devices

? Fuel supply and ancillary systems

? Structure and construction components (e.g. equipment walls, supports, hoods) ? Explosion relief systems

? Heat recovery systems (use of waste heat and water vapor)

Thermal profiling is a very useful tool when designing ovens and testing their performance. Knowing the total radiant and convective heat provided by the equipment helps engineers improve energy transfer flaws and areas inside the equipment where very high/very low heat fluxes are generated.

How is a Thermal Profiling Used in Baking?

Thermal profiling is a baking process for optimization and control. Baking involves many stages such as proofing, baking and freezing operations. This tool allows bakers to read and analyze the change in temperature within a product while it goes through these stages.

A thermal profile helps ensure food safety and regulatory compliance for FSMA. It also helps improve product texture, quality and shelf life by monitoring things like:

? Yeast kill

? Bake out zone

? Product temperature arrival

? Color development

How does it work?

Thermal profiling measures key variables involved in thermal processing of foods and bakery products. It measures things such as:

? Internal product temperature over process time is done at one or more points of interest with sensors, and graphical plotting of temperature-time curve.

? Convective and radiant heat fluxes used to bake the products (Btu/hr·ft2 or W/m2).

? Total heat absorbed by the product during total bake time (Btu/ft2 or Joules/m2).

? Air velocity in convection ovens (m/s or fpm) and oven (air) temperature (°C/°F).

? Oven humidity as humidity mass ratio in lb water/lb dry air (kg water/kg dry air).

The fundamental principle of thermal profiling lies in the fact that physical, microbiological and chemical changes in the product during thermal processing occur as a function of internal temperature. Taking the baking step as example, dough enters the oven at normal proofing temperature of 35°C (96°F) and exits as bread at 93°C ( 200°F).

Changes taking place during baking encompass 4 major points:

? Yeast kill or deactivation: yeast cells in the dough are inactivated at 132°F/56°C. This happens after the product has passed the oven spring stage, the maximum expansion from yeast activity while dough matrix is still stretchable. Ensuring complete yeast kill should take 45–55% of total bake time.

? Critical change zone: the transition from a viscoelastic dough to a firm, drier and porous or sponge structure.

o Wheat starch gelatinization: starch granules gelatinization onset is at 65°C (150°F) and is concluded at 82°C (180°F). This process should be completed at 60% of total bake time.

o Gluten protein denaturation: gluten proteins unfold, re-associate and solidify at 160–185°F (71–85°C).

? Arrival to final product temperature: for a complete bake out so the crumb can set, the temperature of the loaf center should reach about 93°C (200°F) at standard atmospheric conditions. o This process should occur at 80–90% of total bake time.

? Crust color development: final 10–15% of bake time should provide good crust color through Maillard browning and caramelization reactions. Performing a thermal profile, and knowing what is happening in the oven, is critical to controlling the quality of the product. Excessive moisture loss results in a dry, crumbly product that’s prone to high staling rates. Inadequate bake out leaves too much moisture in the product, resulting in a gummy product that takes a longer time to cool. Too much moisture enhances microbial spoilage.

Application

Elements needed for thermal profiling:

? Product to profile (dough or batter)

? Oven (batch or continuous)

? Stopwatch for process time control

? Profiler (data logger)

? Product sensors (insertion thermocouples) and air sensors (humidity, air velocity, heat flux)

? Insulation box as thermal barrier for profiler

? Gloves (for safe handling of hot surfaces)

? Cooling fan

? Computer

? Profiling software for data analysis and record keeping.

Steps for determining baking thermal profile:

1. Connect sensors (for product and air) to data logger inputs (channels)

2. Insert sensors inside the test product (product core is the target). Air sensors are usually attached to the profiler’s body or come integrated with insertion thermocouples in a single probe to be exposed to the oven atmosphere.

3. Turn on data logger and press record button to start recording data

4. Place the thermal profiler inside the insulation box

5. Place the protected profiler and test products inside the oven (place them on the oven band, hearth or sheet pan)

6. Collect data during baking cycle

7. Run and complete baking cycle

8. Once bake time is completed, retrieve data logger from the oven (using gloves) and remove inserted thermocouples from test product

9. Disconnect profiler from sensors

10. Cool data logger with fan or allow it cool down naturally

11. Remove data logger from thermal barrier

12. Stop recording by pressing the record button

13. Connect data logger to computer (USB connection)

14. Open profiler software (it automatically graphs data collected during the test). Depending on the profiling equipment used, profiles for product temperature, air temperature, air velocity, air humidity and heat flux can be obtained.

Interpreting results from thermal profiling

Using this baking example, data collected and processed by the thermal profiling software is displayed as a temperature/time plot known as the S-curve. It explains how the product responds to oven settings. In this curve all the 4 major stages can be seen graphically.