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Ready-to-eat cereals are defined as processed grains for human consumption made in such a way that no further cooking is required. (Lee, 1995) Cereals are excellent sources of carbohydrates and dietary fibres. They also contain various artificial minerals and vitamins. RTE cereals are a regular staple in the world diet and can be consumed by all ages. The popularity of RTE cereals is based on the fact that cereals products are ease to consume and are quickly prepared for people on the go. The objective of this report is to outline the process technologies currently used in the manufacturing of cereals and to update you on the new techniques that are being used to increase the yield of cereal production.
Cereal Composition Breakdown
Ready-to-eat cereals are composed of two parts: the first part is the cereal grain which can be from corn, wheat, oat, or rice. The second part of cereal is comprised of additives, sweeteners, vitamins, minerals, and preservatives.
The cereal grains are milled (ground to a powder) before they are cooked. The grain is comprised of three parts: the endosperm, the bran, and the germ. The endosperm, which is the larger portion of the grain, is used to make cereal, corn meal, and corn flour in the same facility. The bran is used as animal feed. And lastly the germ is extracted for the oil it contains.
Some of the various additives, sweeteners, vitamins, minerals, and preservatives that are used in RTE cereal are listed in Table 1 (Tribelhom, 1991). These additives are added to the cereal after the cooking process.
Table 1: Additives Commonly Used in Ready-to-eat Cereals
Usage in cereals
Add butter or peach colour to cereal products
Barley Malt Flour
Add to raw material, source for Î±, Î²-amylase
Enhance starch hydrolysis
Prevention of oxidative rancidity of oils
Added to food directly and/or to packaging materials
Same as Butylated hydroxyanisole
Flavor enhancer added at raw material stage or a part of coating for cereal
Effective in control of gelatinization of starch
Helps control puffing and cooking
Thickening or gelling agent
Used in cereal coatings
Cereal Malt Syrup
Used as a sweetener and flavor additive for raw materials and sugar coatings
Adds brownish colour to cereal
Sweetener and flavor additive for raw materials or sugar coatings
Inhibit crystallization of sucrose during coating
Adds brownish colour to cereal
Adhesive for coatings and glazes on cereal products
Flavor enhancer added to raw material or used in coating for cereal
Flavor enhancer and sweetener
Control of expansion in puffed cereal products
Solvent for sugar and other cereal coatings
The Malting Process
Malting is the process of converting germinated barley grains to brown syrup. The brown syrup (Malt) is a viscous flavoring liquid. Malt provides RTE cereals with a sweet flavor and tints the colour of the cereal to a light brown. The malt is diluted with water before being added to the cereal after cooking. Note, if too much malt is added to the cereal then a bitter taste will develop.
Vitamins and Minerals
Vitamins and minerals are added to RTE cereals for nutritional value. Some of the vitamins that are added to cereal are:
Vitamins A, D, B1, B2, B6, and B12
Some of the Minerals which are added to cereals are:
Sweeteners are additives to RTE cereals that give them a sweet taste. Granulated sugar and/or liquid sugar are regularly used as a convenient way to add a sweet taste to cereal. Liquid sugar is comprised of part granulated sugar (sucrose) and water. Other sweeteners that are also used in the industry are: honey, icing, brown sugar or liquid brown sugar, corn syrup, and non-caloric sweeteners like aspartame. (Caldwell & Fast, 1990)
Preservatives are added to the cereal products to retard rancidity and maintain of the cereals product for a set amount of time. The preservatives are not directly added to the cereals products but are sprayed onto the interiors of the cereal packaging material. Butylated hydroxyanisole (BHA) and Butylated hydroxytoluene (BHT) are two of the most commonly used preservatives used in the industry.
The Manufacturing Process of RTE Cereals
A general process over view for the manufacturing of cereals involves cooking of the raw materials (flour from grains) into a gelatinized mass, then comes the shaping of the mass into individual cereal bits, then the cereal bits are dried, coated and packaged into finished cereal products. To give the cereal character, texturization process are added, they are: Flaking, toasting, baking, puffing, extruding and coating. The idea here is to give the cereal structure, flavor, and to improve digestibility for costumer. (Clark P. J., 1986)
The different manufacturing processes that are used in the production of cereals are flaking, oven-puffing, and extrusion. These methods are described below.
Cornflakes are flaked cereals that are produced out of milled whole corn kernels. The milling of the corn kernel breaks down the kernel into bits called grits. One grit makes one flake. The major steps in the production of flaked cereals is first the grains are cleaned using water and then milled to obtained the grits. The purpose of milling is to produce uniform grits which will give uniform flakes. The cleaning and milling steps can be removed if the cereal manufacture chooses to buy flour from a supplier. The grits or flour is then cooked in a automated rotary batch pressure cooker. The cooker uses steam at a pressure of 15 - 18 psi (1atm) and is heated to around 122 oC for 2 hours. The rotary pressure cooker rotates one to four revolution(s) per minute to prevent burning and uneven cooking of the flour. The rotary pressure cooker transforms the raw starch to digestible material. A typical rotary pressure cooker is 1.2 m in diameter and 2.4 m in length. The moisture content of the gelatinized product after cooking usually is 32% or less (Caldwell & Fast, 1990). Figure 1 illustrates the flaking process procedure.
The product after cooking is then carried away to the delumper on a conveyor belt. The delumper breaks the cooked product down into smaller chunks (cereal pieces). Then the chunks are sent to a dryer to be dried at a temperature of 121 oC. The moisture content will reduce to 10 - 14% in the dryer. The dried product is then tempered in tempering bins so that the moisture content can reach equilibrium. Moisture equilibration takes about 24 hours for completion. Moisture equilibration is necessary for uniformity and also if there is non-uniformity then the flakes will be over toasted in the later set of toasting.
The cereal flakes are then sent to flaking rolls. These flaking rolls operate at different speeds so that the cereal dough (with 10 - 14% moisture content) will stretch into sheets of cereal flakes. The typical capacities of flaking rolls are about 30 lb/min. These flakes are then toasted for 50 sec at about 300 oC. The toasting step gives the product a toasty flavor and a light brownish colour. The moisture content at the end is 1.5 - 3 %. This small amount of moisture content is necessary so that the product will not be hard, dry and brittle.
Figure 1: The Flaking Process
Rice Krispies is an example of oven-puffed cereals. The manufacturing process for oven-puffed is very similar to that of flaked cereals. The rice grains are cleaned and then cooked in a rotary pressure cooker for 1 1.5 hours at 122 oC. After the cooking cycle the rice grains are dried in a dryer for 30 - 45 min and left to temper in a tempering bin for 1 or 2 days. After this the grains are passed through flaking rolls. This time the rolls operate at the same speed. This is to create a bumping motion instead of a stretching motion. The bumping creates fissures within the rice grains which will allow heat to penetrate the grains when they are heated inside an oven. The grains are then sent to an aerated oven where they are puffed and leave with a moisture content of 9 - 11% (Caldwell & Fast, 1990). Figure 2 illustrates the oven-puff process procedure.
The aerated oven is controlled at a temperature of 288 - 343 oC. When the rice grains are subjected to high temperatures, the water inside the bumped grain will go from the liquid phase to the gaseous phase. The point of vaporization of water in the grains is considered to be the point of flashing. It is this flashing that causes the grain to puff (a significant volume expansion of the plasticized matrix) to occur (Clark P. J., 1986).
Figure 2: Oven-Puffing Process
An example of extruded cereals is Froot Loops or Corn Pops. Cereal extrusion is to apply high heat and high pressure to raw materials to form edible cereal piece. This method of operation has many advantages over traditional ones and this is why this method is mostly used today around the world. The process is very energy efficient because of the high heat transfer and since the mechanical energy produced by turning and conveying motion of the screws can also be used to cook the material. The tempering bins are also removed and therefore a lot of the space need for other methods is not needed for this method. The cereal extrusion method can also be used to cook and make more than one type of cereal product (flaked and oven-puffed cereals) (Harper, 1986). Figure 3 illustrates the extrusion process procedure.
Figure 3: Extrusion Process
New Developments in Cereal Processing
Twin Screw Extruders
Twin Screw Cooking Extruder (TSE) is a common food processing unit, vastly used in the baking industry. It consists of fast-speed bioreactor with heating, cooling, compressing, mixing, evaporating, cutting, and aerating confined one unit operation. Twin-screw extruders are now becoming more popular in the industry due to their ability to manipulate a number of parameters easily. (Schlosburg, 2005)
The first commercial use of extruders was found to be in the rubber industry in the late 1870s. These extruders comprised of ram extruders and screw extruders of short length to diameter ratios. In 1935, the basic principles of twin screw extrusion were considered and implemented in to the thermoplastics industry. In 1960s, the first commercial production of dry ready to eat cereals using the single screw cooking extruder was introduced to the food industry. With time developments in areas of process control, equipment design, such as development of twin screw extruders, and sound understanding of the extrusion process kept researchers motivated. (Riaz, 2000)
Single screw extruders frequently experienced slippage and surging due to high pressure in the barrel. As a result the product did not undergo proper cooking or processing. The control over processing parameters such as temperature, pressure, screw speed, moisture content and flow rate was limited. Single screw extruders were incapable of cleaning themselves leading to potential dead spots meaning if conveyed will adhere to the screws. Further, this burned on product was breaking and blocking the die and plugging the barrel. Single Screw extruder allowed only a number of products to be made. In addition, the scale-up, the process of going from laboratory development of a product to full-scale production, was a problem with single screw cooking extruders. (Clextral)
The growing demand of ready to eat cereals and flaws in the single screw extruders served as incentives to the development of more advanced technology. With Twin Screw Extruders in the market, the companies started to show interest in them. In the last few decades, the twin screw extruder with intermeshing, self wiping screws has emerged from the food extrusion laboratory to the production floor quite successfully. As more and more companies started to show concern in regards to the SSEs, the era of TSEs was beginning. Those using the extrusion were afraid that single screw extruders may no longer be the most efficient means of producing their products. Thus, many companies made the decision of replacing their single screw extruders with the twin screw cooking extruders. (Riaz, 2000)
New Developments in Extrusion
The new research has allowed researcher to develop twin screw extruders. Twin screw extruders have great ability and flexibility for controlling both product and process parameters. They are a flexible design, permitting easy cleaning and rapid product changeover. The ability to better match the desired shear gives twin-screw extruder more control over product variability. (Hauck, 1988)
Twin Screw Extruders have many advantages over the traditional means of extrusion cooking. For instance, Twin screw extruders are flexible and allow variety of products to be processed by just varying the ingredients and processing conditions. The yield of TSEs is also high as it acts as a complete processing plant consisting of mixing, cooking, forming and shearing of cereals. Consequently, this makes TSE cost effective equipment. Currently, these extruders are able to process 16 tons per hour of finished product. Moreover, the High Temperature Short Time (HTST) cooking method on a TSE produces better-quality products. In addition, it forces harmful temperature sensitive bacteria to die while reducing the loss of nutrients or flavors in the food being produced. In some processes, twin screw extrusion is energy efficient. This is because the TSEs can cook more quickly and proficiently due to shorter reaction times, control over temperature and moisture. (Clextral)
Screw Extrusion process adds heat to the processing cereal by means of converting mechanical energy to heat. As the screw rotates inside the equipment it produces friction which in turn forms heat. Due to this very reason Single Screw extruders are limited to 12-17% fat level in the formula. As fat content above the specified will lubricate the rotating equipment which will prevent the hardware transform mechanical energy into heat. In contrast, fat level in the recipe for twin-screw extruders can be as high as 18-22% and still maintain the mechanical energy. This is only possible because of more screw configurations options with twin screw extruders. In single screw extruders with the help of steam injection, fat content in a recipe can be achieved as high as 17% however, in the TSE with steam addition will yield better binding of the fat in the product and will decrease the leakage of fat from the products during handling and packaging. Moisture content is very critical during the extrusion process for starch gelatinization and protein denaturizing. An average moisture content in a typical formula ranges from 20-28%. Twin screw extruders have the ability to run under a narrow range or wide ranges of moisture. (Guy, 2001)
Figure 4: Twin Screw Cooking Extruder (Baldwin, 2007)
Table 2: Upgrades in SSE model (Clextral)
Single Screw Cooking Extruder Technology (SSE)
Twin Screw Cooking Extruder (TSE)
Improved Cereal Cooking
In a single screw extruder, friction between the screw and the barrel is used to convey the product. As a result SSE often experience slippage and surging
Eliminated slipping and surging, leading to proper cereal cooking
Limited control over processing factors such as temperature, pressure, screw speed, moisture content and flow rate
Unlimited control over all processing factors
Poor Mixing due to only single screw operation
TSE's uses a wide variety of mixing basics, including intricate mixing discs, reverse screws and compression screws that provide superior mixing
Single screw machines do not have this feature
The dual screws on a twin screw extruder are "self-wiping "
More Flexibility in Production
TSE can produce wide range of products in addition to the ones that get processed in SSE
It can process foods at higher moisture contents, higher fat content and higher sugar content
They can accept dry ingredients at varying particle size whereas, SSEs require uniform particle size for optimal processing
Scale-up on TSE's is easy as long as proper data is recorded during research and development of new products
Drying is the operation that takes place after the coating session. Drying time and temperature varies from one cereal product to another. Cereal dryers use very hot air moving at high velocities to dry cereals. Rotary and flat-bed dryers are used in the drying of cereals.
The rotary dryer is a large horizontal cylinder which can lift and drop flakes through a bed of hot air. These dryers are usually not used for fragile or brittle products.
A flat-bed dryer is better suited for fragile products like cornflakes. It is a large slow moving, porous support, consisting of either segmented perforated plates or a continuous bed. The air flows from top to bottom and even the other way.
Tube-jet and tube -aerated bed toasters are designed for drying, toasting and puffing. Hot air is blown through the vent valve into a pressure plenum which seats at the top and over the length of the dryer. Long tubes then hang down from the plenum towards the conveyor belt below. The equipment is set up so that different temperatures and velocities can be applied to the product during different stages of the cereal development.
Fortification is the addition of vitamins or minerals into cereal which is missing these essential nutrients. Heat stable vitamins (i.e. niacin and riboflavin) are added before cereal processing and heat-sensitive ones (i.e. Vitamin A and thiamin) are added after the heating section (Lee, 1995).
The packaging for cereals must be a strong carton which can be used to protect the cereals from rough handling and breakage. The liners most resist and protect against odours and moisture penetration from outside and in. Also no flavor should escape from packaging. Ideally, liners and cartons should be effective barriers that can minimize odour or flavor between the products and their surroundings (Lee, 1995).
In order to significantly improve RTE cereal manufacturing process, changes ought to be made in its key unit operations - Flaking, Expansion, Shredding, or Extrusion (Clark P. , 2009). During the past two decades, major improvements have been focused in the process of Extrusion. Use of microprocessors, switch to extrusion cooking and humidity control impinging has been used effectively (Fast R. , 2001) (Fast R. , 2001).
Control & Instrumentation Improvements
As the RTE cereal industry has evolved over the past 100 years, so has the need for Control & Instrumentation of the processes involved in RTE cereal manufacturing. Applications of computer control have been implemented since the late 1980's. However, since the computer industry is more rapidly changing than the RTE cereal industry, there is always a significant amount of progress that could be made in the using advanced computerised control in optimizing the continuous or batch processes of RTE cereal manufacturing.
In the early days of implementing PLC control systems, automation was desired to achieve a high accuracy in weighing liquids, essential substances such as starch, and ingredients added in small quantities (Fast R. , 2001). Furthermore, various unit operations can be optimized to increase their efficiency. Those unit operations include Cooling, Coating, Toasting. To illustrate further, a combination of Feedback/Feedforward loop controllers could be effectively used to maintain the optimum temperature while performing such unit operations. Likewise, cascade controllers could be utilized to regulate the flow rate of feed while simultaneously maintaining another process variable such as temperature, especially in continuous processes that have very short residence times.
As Fast (2001) discussed, compared to cereal manufacturers two decades ago, they can achieve 'total plant control' very easily because of the integration of personal computers and its high-speed computing power to the unit operations of the manufacturing process line. More importantly, one of the most significant accomplishments of using computer control in cereal industry is "Just-in-time" inventory technology. Large-scale cereal manufacturers have been able to integrate their production with retailers effectively. (Heron & Hayward, 2002)
As shown in the figure below, Process Control implies PLCs (Programmable logic controllers), DSCs (Distributed control systems), Loop controllers, hard-wired systems, dedicated hardware. Likewise, MES implies PC-based software packages, which built upon the PLCs. Similarly, Information system (IS), which includes Networked PCs and terminals, was built upon the advent of softwares.
Figure 5. Control System Platforms at the three levels of Computer Control and Information Gathering. IS = Information System, MES = Manufacturing execution system (Fast & Caldwell, 2000)
Another significant accomplishment is that of electronic upgrades in ingredient weight systems. Bulk dry and liquid ingredients are accurately measured to high levels of microprocessor sophistication (Fast R. , 2001). Equally important, this development in cereal manufacturing needs to be utilized in such a way that it could lead to implementing more number of continuous processes rather than batch processes.
Quality Monitoring Development
Apparently, product quality checks in cereal manufacturing industry are done in three areas - moisture, color and bulk density. Traditionally, techniques such as near infrared (NIR) technology or capacitance measuring have been used to measure moisture content in cereals. Likewise, with the advent of computer control, color measurement is done using on-line color reflectance systems. Similarly, bulk density of cereal, which is also measured on-line, is measured using a computer-driven cup. (Fast R. , 2001)
However, some models of NIR, particularly, have been unreliable and unstable because NIR spectroscopy has been insensitive to mineral content (Huang, Yu, Xu, & Ying, 2008). Interestingly, significant improvements have been made in this area by implementing a combination of techniques such as X-ray fluorescence spectroscopy, UV light and electronic noise technique (Cimander, Carlsson, & Mandenius, 2002) (Navrátil, Cimander, & Mandenius, 2004). Both, Cimander (2002) and Navratil (2004), have been successful in harnessing the best of such a combination.
Improvements in the Process of Cooking
It is in the unit operation of extrusion cooking of RTE cereal that has seen significant developments during the past years. Extrusion cooking, apparently, holds a greater advantage over traditional batch cooking process (Fast R. , 2001). These significant developments have been attributed to various upgrades made in extrusion cooking. Foremost of the these upgrades is the redesign of pressure vessel. Furthermore, the other upgrades include automated hatch cover mechanisms, twin screw extruders and design flexibility of screw elements (Fast R. , 2001). Over the next few years, this already enhanced process of extrusion-cooking might include further developments such as application of everlasting pea wholemeal (Kasprzak & Rzedzick, 2008).
As shown in the figure below, extrusion cooker has five zones - high pressure zone, shear zone, conveying zone, mixing zone, and conveying zone.
Figure 6. Schematic Diagram of Extrusion Cooker, showing its components (Kent & Evers, 1994)
In conclusion, significant changes have been observed over the past couple of decades in the areas of Process control and Instrumentation, Extrusion cooking technology. Likewise, minor developments have been observed in Quality control, drying, blending, tempering.
As computational power of processors increases, microprocessor sophistication will further increase the accuracy of various unit operations. This increase in accuracy of measurements coupled with integrated computer control of RTE cereal manufacturing process, further automation and newer conceptual technologies such as "Just-in-time" inventory systems would be developed.