Effect of oven modes on baking unit operations

There have been many articles, textbooks and literature in general that have dealt with the topic of oven baking technology, particularly for cookies, crackers and snacks. This particular article will focus on the effects and fundamental understanding of using different oven modes and how product attributes can be manipulated to meet consumer demand in the development of existing and unique new product platforms.

Baking is a critical unit operation in biscuit manufacturing and is defined as the high energy reaction phase, maximising chemical and physical transformations of raw dough into finished products. Traditional baking depends on the Direct Gas Fired oven (DGF) or convection systems. Recently, radiant and microwave energy have been added to hybrid combina tions of DGF and convection. These ‘multi-media’ ovens permit effective control of the reactions that determine finished product attributes through decoupling of conduction, convection, radiant and dielectric heat transfer modes. Microwave, DGF and radiant heat mainly impact structure, thickness and texture. Microwave and convection have the greatest effect on moisture and weight. Finally, colour can be readily adjusted by radiant and DGF modes of baking. This approach to baking processes through multi-media ovens develops understanding of the fundamental character – istics and interactions for baking reactions in terms of materials, process and product. It permits optimised process and oven designs through specific heat transfer data for scale-up from pilot plant to production ovens.

There are a lot of biscuit products currently in the marketplace. All these products have been manufactured by manipulating the product attributes by using the different modes and altering the heat transfer coefficients.

Product attributes are developed via a number of operations such as formulas, mixing, machining and baking. For the purposes of this article, the discussion will focus on oven baking. Ovens are divided into various zones but the principles of baking remain the same. Stage 1 is the structural development of the dough piece, stage 2 is moisture removal and stage 3 is for colour and flavour development. It should be noted that there is typical overlapping of the functions between stages 1 and 2 as well as stages 2 and 3.

Let’s first examine the fundamentals of baking unit operations in general for crackers and cookies before we proceed starting with crackers by investigating the functions occurring in stage 1 baking. Here, the front end heat is critical because the cracker structure starts to develop as the starches begin to cook. Simultaneously drying begins and ammonia, carbon dioxide gases and water vapour are formed and released, causing the cracker to lift. Bottom heat allows the cracker to heat up, without drying out the top surface too quickly. If the top surface dries out quickly, this may result in moisture getting trapped inside the cracker centre, the cracker will have low stack height and high moisture and there is a strong possibility of cracker checking problems which is defined as the formation of hairline cracks in the biscuit. This topic will be briefly discussed later in this article and how to minimise this phenomenon.

In stage 2, thermal energy or heat continues to remove ‘free water’ from the dough piece and the maximum gas/dough piece expansion is achieved. The product volume relaxes and the product structure is primarily fixed. The starches begin to cook and the gluten proteins de – naturise. Finally, crusting of the product surface is initiated. However, if crusting of the surface is initiated too early in the second stage, ‘blisters’ may result. This may not necessarily be a negative but may be desired depending on the final product appearance the consumer desires.

In stage 3 of cracker baking, the majority of the moisture has been removed in the previous stages and now colouring of the product is occurring. The structure is fully set and the product is firm. The colour development primarily is a result of sugar caramelisation and the sugar / protein reactions better known as the Mallard Browning reactions. Also, this is the step at which flavour is developed.

Cookie baking is significantly different from cracker baking because the formulations are different. For cookies, the levels of sugar and fat are much higher. Therefore when the dough pieces enter stage 1 and come in contact with the oven band, the sugar and fat melts causing spread (flow). The more sugar in the dough, the less the starch can be cooked. The dough becomes fluid and little or no structure develops. Drying is initiated and the release of water vapour in the early zones covers the product in a steam blanket. Evaporative cooling keeps the surface temperature down which allows the dough to be conditioned and keeps the product surfaces flexible so that the moisture and leavening gases can be released.

In stage 2 of cookie baking, heat continues to release ‘free water’ from the dough. Leavening gases and water are still lifting and spreading the dough until maximum volume is reached around the middle of stage 2. Good lift and spread improve the texture of the finished product and little or no structure is developing because of the high level of sugar and low level of water which reduces starch cooking. Colour development is once again initiated on the top surface as higher temperatures are achieved.

When the product enters stage 3, maximum lift and spread are achieved and due to the lack of cooked starch, the cookie surface collapses. The air pockets developed from the lift give the cookie its texture and lightness. If however the lift is poor, the cookie will have a harsh, unpleasant texture. The top and bottom colour are developed as the syrup / sugars react with the flour proteins at around 320°F, The syrup and sugars begin to caramelise at around 340°F.

Now that we have discussed the detailed baking unit operations for cookies and crackers, let’s refresh ourselves with how heat passes from the oven to the product. From our basic heat transfer courses conventional heating methods pass heat from the oven to the product by three mechanisms; conduction, radiant, and convection. The top of the product is primarily heated by convection and radiant. The bottom of the product is heated by conduction through the heated oven band and the bottom of the oven band is mainly heated by convection and radiant. Let’s examine each one more closely and what its effects are on the final product attributes.

Conduction in a Direct Gas Fired oven occurs as the heat transfers directly to the hot oven band into the bottom of the dough pieces. Conduction heats the piece from below, causing a phase change and changing the water into steam. This helps lift the stack height and pushes steam up through the surface for moisture removal.

Radiant heat in an oven comes from the hotter metal surfaces, burners’ flames and combustion gases. The heat is then directly transferred to the product. Radiant heat how – ever has a particular impact on uneven surfaces. The shallow areas on the product surface will receive less radiant heat than the exposed areas which will result in brown highlights at exposed points. Radiant burners operate at extremely high temperatures in order to radiate heat to the product. They can be installed in any location in the oven but the location is key in being able to manipulate the product attributes. If they are installed in the first zones they can be used for immediate dough colouring and forming surface bubbles. If they are installed in the end zones they are used for final product colouring. The burner’s surface temperature operates at around 1000°F. One may consider them like a giant toaster.

Natural convection in an oven is defined as air moving in the oven chamber which is continually circulating and incoming cooler air warms, lightens and rises. Convection heating is important in lifting moisture in the form of steam away from the surface of the product (boundary layer). This aids in achieving finished product moisture specification. The speed of moisture removal affects the product quality.

Forced convection comes from the oven fans that heat the air and blow it into the oven chamber. Forced convection helps to even out heat difference in an oven. If zone temperatures need to be altered to adjust the oven profile they can be controlled by zone baffles. It is also important to note that since in a convection oven you typically have only one main burner per zone, that burner must be functioning at peak efficiency. Unlike a DGF oven which has multiple burners per zone, if some of the burners fail there are typically enough burners in that particular zone to compensate. However in a convection oven if the burner fails, the other burners in the other zones are not designed to be able to compensate.

In recent years, dielectric oven modes have been introduced into the food industry as alternative oven modes. Two modes of dielectric are microwave and radio frequency, each having distinct functions. Let’s examine microwave technology. There has been a significant amount of literature published but I will attempt to summarise in terms of baking unit operations.

Microwaves are a form of electromagnetic energy just like radio waves, television signals and light waves. In cooking and baking applications, microwaves operate by vibrating water molecules at 2450 MHz. This vibration generates heat which helps speed up the baking process by ‘baking from the inside out’ by heating the centre of the product equally. Changes with microwave energy are immediate as compared to conventional baking which bake the product from the ‘outside in’ thus requiring relatively long baking times. Microwaves help control the moisture and stack height of the product. In crackers, the bake time can be reduced by 10 – 15 per cent to provide a 10 – 15 per cent throughput increase. Microwave processing enhances coupling to the product, increases reaction rates, shortens bake times and manages the attributes of moisture and stack height. It decouples the attributes of moisture and stack height and increases product throughput without requiring add – itional oven zones. Further processing information is listed in a granted US Patent 5,945,022 on 31 August 1999.

A Radio Frequency (RF) dryer is primarily used for post conditioning. They are located either after, or in the oven itself. The product is usually baked to a slightly higher moisture level in the oven. The process forces the water molecules in the product to vibrate and heat up which releases the moisture. Higher moisture levels attract more dielectric energy therefore it is called a ‘self-regulating’ moisture control process. This is typically used as a last resort to control cracker checking as we briefly mentioned earlier in this article. The dryer’s primary function is to equilibrate the moisture gradient in the finished biscuit. This occurs for several reasons but the primary reason is that cracker baking is done at relatively fast bake times of two to six minutes. Because of these bake times, the product emerging from the discharge end of the ovens results in a product that has uneven moisture levels in the baked product piece. As the moisture levels try to equilibrate within the piece, this results in stresses and strains that over the course of time results in hair line fractures which results in cracker breakage. The radio frequency process levels out the moisture gradient and minimises the level of checking. There have been many articles discussing checking and its formation and solutions to which the author will direct one to research separately as this article will not cover this topic in detail. The unit operates when the positive and negative sides of two magnets face each other and the magnets attract. When the positive sides of both magnets face each other, they repel. The electrodes oscillate positive and negative energy, causing polar molecules to rotate. Molecules with a high polarity (water) rotate at a faster rate than the molecules with a low polarity (flour proteins). The spinning of the water molecules causes an increase in temperature and phase change into steam.

To summarise, the traditional baking process consists of convection, conduction and radiant heat transfer mechanisms. By under – standing the fundamentals of the different oven modes and how they behave, which have been described above, one can change the heat generation systems by altering the different forms of convection, DGF, radiant and surr – ounding them with dielectric modes results in changing the heat transfer rates. This altering of the baking constants allows the product attributes or characteristics to be manipulated to achieve the desired products the consumer is seeking in the marketplace.

One additional item worth discussing is the use of hybrid ovens. Hybrid ovens are defined as the combination of Direct Gas Fired (DGF) zones and forced convection baking. Typically, the front end of the oven has anywhere from one to four DGF zones and the rest forced convection, the reason being is that the DGF heat as discussed earlier achieves the product structure at the beginning of the baking process and the convection zones are primarily used for moisture removal and colour development. The convection heat also enhances the uniform side to side baking resulting in improved balancing of the product attributes. So one gets the benefits of both oven modes in a single oven which provides unique flexibility in baking different products.

Product humidity also plays a key role in oven baking. Generally, the concept behind using humidity for controlling baking is that higher humidity air has an increased heat capacity resulting in more efficient heat transfer to the product thereby reducing the bake time. Also, the higher humidity in the oven zone delays the moisture migration from the dough piece. Baking becomes ‘even’ over a longer portion of the oven (potential reduction on checking / breakage; reduces case hardening and burned edges). This is a topic that would require a separate discussion for the future.

So to summarise the high points of this article, multi-media oven baking consists of convection, conduction, radiant and dielectric in various aforementioned combinations. These alternative baking modes deliver the unique products or attributes by altering the heat transfer or baking constants. They provide productivity opportunities via the baking process by enhancing the oven with alternative dielectric sources. Uniform baking and moisture removal is enhanced as well as improving control of the product thickness (stack height) and reducing finished product losses. It also provides the means and capability of scaling up processes from pilot plant to production in conjunction with data loggers and most notably heat flux sensors, which measures the energy per square area at the product level, currently available in the industry.

Biography

Mihaelos (Michael) Mihalos is a Senior Associate Principal Engineer of Snacks Growth in Product Development /RD&Q for Mondelēz International. He is responsible for managing leading edge, very complex technology development projects.

Mihalos joined Nabisco in 1988 as a Process Engineer in Biscuit Engineering and holds various US patents for his development work. He has held a variety of technical positions in his 25 years at Nabisco/Kraft/Mondelēz International in areas such as Biscuit Process Engineering, Process R&D, Technical Services, Manufacturing Development, Pilot Plant, Growth Engineering, Global Biscuit Product Development (PD) and currently in North America Snacks Growth & Innovation/PD as part of Research and Development. He is a recipient of the Kraft Foods 2011 Technical Leadership Award for Research, Development and Quality.

He spent time in biscuit manufacturing facilities understanding bakery biscuit operations and processing and currently continues to provide pilot plant and technical support in the area of process development for Snacks Growth and Snack Platforms for cookies, crackers and snacks, Responsibilities include management of all schedule, budget, scoping components and leading overall efforts in process development of new products. He identifies and develops new processes and process technologies in support of the product development business plan and implements ‘Field Ready’ technologies used in product / platforms. He has developed multiple process capabilities in innovative technologies implemented this expertise to improve productivity, product quality, and commercialisation of new products resulting in over 50 US and International Patent and pending patent applications. He provides process leadership by hosting a Process Development Community of Practice conference within Mondelēz International organisation to share new technologies as well as brainstorming opportunities from the various business categories such as beverages, coffee, cheese, gum and candy, etc.

Prior to joining Nabisco, Mihalos worked in both research and development and engineering as a chemical engineer for Colgate Palmolive in Jersey City, NJ.

Mihalos is a member of the American Institute of Chemical Engineers, Institute of Food Technologists and both a member as well as certified by the American Chemical Society. He is also an alumnus of both Columbia University and Fordham University. Mihalos received a BS in Chemistry from Fordham University, a BS in Chemical Engineering from Columbia University and an MS in Chemical Engineering from Columbia University. He is an alternate for the Biscuit & Cracker Manufacturer Education Committee and presents at the annual technical conference. He also edited the processing section in the 4th edition of ‘Baking Science and Technology’ Volume II.