Introduction of Starch Damage?
Starch damage is one of the important quality parameters in wheat flour milling, primarily in the milling of wheat flour for production of bread, biscuits, cakes, noodles, pasta and other bakery products. Flour milling is a process of reducing the size of wheat kernels and then separating the endosperm from the bran and the germ. During this process, some of the starch granules may be physically damaged by the mechanical forces generated by the grinding rolls.
Starch damage is natural in flour milling, but when it exceeds a certain level it can negatively affect flour quality, water absorption, dough handling, fermentation, baking and the quality of the end products. On the other hand, an adequate level of starch damage can improve the functional properties of flour especially in breadmaking.
The aim of a flour miller is not to completely eliminate starch damage, but to keep it at a level adequate for the desired end use of the flour.
Various factors including characteristics of wheat, milling condition, roll settings, grinding pressure, roll surface condition, feed rate, tempering conditions, quality control, and flour extraction affect the level of starch damage in wheat flour. By controlling these factors appropriately, flour millers can produce flour with desirable functional properties for the intended end use, enhance milling efficiency and reduce needless energy wastage in the process.
This paper comprehensively discusses starch damage including the causes, methods of measurement, effects on flour quality, factors that influence it and strategies to control it in a flour mill.
What is Starch Damage?
Starch damage is the degree of disruption of starch granules in cereal grains (usually in wheat) during milling.
Starch is the major component of wheat flour, composing about 65-75% of wheat flour by dry weight, based on the wheat and flour variety.
In an intact wheat kernel, starch is embedded in a protein-rich matrix in the endosperm. The main objective of the milling process is to remove the bran and germ layers from the kernel and reduce the endosperm to a flour of desired fineness. This is achieved using break rolls, reduction rolls, sifters and a purification system. During this process, some starch granules become damaged, whereas some remain intact.
Starch damage is due to the intensity of the mechanical forces used during milling. Other factors include the wheat variety, moisture content, and condition of the milling equipment.
Intact starch granules
Intact starch granules exhibit a high degree of internal organization, and retain their granular structure when mixed with water. The degree of organization and retention of structure is termed “granularity”. The granularity of starch and wheat flour affect the functionality of flour during the formation of dough and baking, in that a highly organized structure may affect the stiffness and elasticity of the dough during the mixing process, and the volume of the baked product.
Damaged starch granules
Starch granules become damaged during milling. The internal structure of the granule is disrupted and becomes more accessible to water. Some organized molecular structure may also become disrupted.
As a result, damaged starch granules will absorb more water than untarnished starch and will be more prone to hydrolysis by amylase.
If there is a moderate level of starch damage, water absorption and fermentation will be improved during the production of bread. If there is an extreme level of starch damage, the dough will be sticky, require more water to reach a given consistency and the final product will be of poor quality.
Starch damage is not the same as starch gelatinization
While starch damage occurs during the grinding process of flour, starch gelatinization occurs when granules are swelled and the organized molecular structure is disrupted during the cooking or baking process.
It’s important to distinguish between the two as they happen at completely different points in a flour products life cycle.
Why is Starch Damage important in flour milling?
One of the key factors of functional flour characteristics is starch damage. This is because it alters how flour interacts with other dough ingredients.
The importance of starch damage can be shown by the following points.
Water absorption
Damaged starch will absorb a greater amount of water than intact starch. This means that flour with a high starch damage will require a greater amount of water to reach a desired dough consistency.
Flour variation is a challenge for industrial-scale dough production. Dough preparation often requires frequent adjustments to the mixing water. This is because starch damage can vary from batch to batch.
Dough consistency
How extensible, elastic, and stiff the dough is depending on the level of starch damage. Excessively damaged starch makes dough sticky and difficult to handle. Improper starch damage can negatively affect the quality of the finished product.
Fermentation
Damaged starch is more susceptible to enzymatic activity. Starch is the main substrate for yeast. Thus, accessible starch is important for fermentation.
Baking quality
Damaged starch influences baking quality in a variety of ways. Starch damage can positively impact the volume and texture of the finished product.
Milling efficiency
Excessive starch damage can be due to milling systems applying too much mechanical energy. Improper roll settings can result in excessive mechanical energy being applied to the wheat.
Customer satisfaction
Understanding the balance between starch damage and the other components of flour allows the production of a desired final product. For example, flour with higher starch damage is preferred by some manufacturers of biscuits, cakes and pastries compared to bread manufacturers.
Types and Levels of Starch Damage
Starch damage is generally described in terms of the amount of damaged starch present in the flour. The importance of damage level will depend upon the degree of granule disruption, the method of analysis used, and the purpose for which the flour is used.
Low starch damage
Low starch damage is caused by conditions during milling that preserve the integrity of starch granules in their original form.
Effects may include:
- Decreased water absorption in many flour systems.
- Diminished sensitivity of starch to amylases.
- Possibly enhanced functionality for certain biscuits, cakes and other products that are dependent on controlled water absorption.
- Decreased potential for dough stickiness because of the lower level of starch damage.
However, in some cases, excessively low starch damage may be unfavorable for breadmaking as it may not provide sufficient water absorption or release fermentable carbohydrates.
Moderate starch damage
Moderate starch damage is desirable for most breadmaking flours.
It may lead to:
- Improved water absorption
- Improved utilization of starch by amylase
- Sufficient availability of fermentable sugar
- Proper consistency of dough
- Improved loaf volume and crumb characteristics of bread
If wheat is of correct hardness, with adequate protein quality and enzyme activity and in the presence of optimum processing conditions, moderate starch damage may be achieved.
High starch damage
High starch damage occurs when a higher proportion of starch granules are physically disrupted during the milling process.
It may cause:
- High water absorption
- Sticking of dough to the mixing machine
- Greater sensitivity toward variations in amylase activity
- Increased tendency toward gummy or tacky crumb
- Poor control on dough development
- Less suitability for use in low water absorption applications
Excessive starch damage is not always indicative of poor flour quality.
Major Causes of Starch Damage
Excessive Grinding Pressure
Grinding pressure is one of the key factors that determine level of starch damage. More starch granules are fractured when excessive mechanical force is applied.
A roll gap that is set too small will result in excessive grinding intensity, especially on the reduction passages where the objective is to reduce endosperm particles to fine flour.
Control measure: For specific particle-size reduction, adjust roll gap considering the characteristics of wheat, passage, and feed rate. Do not use excess pressure to compensate for poor feeding, worn rolls, or flow condition.
Incorrect Roll Gap
Roll gap determines how strongly the stock or grain is sheared and compressed between the rolls.
Increasing the roll gap leads to increased particle size, and a smaller roll gap results in a smaller particle size.
Control measure: It is necessary to experiment with roll gap and set it appropriately. It is important to inspect the rolls to ensure the roll gap is set as required, using approved procedures. One should not rely on the indicated roll gap.
Excessive Differential between Roll Speeds
In various milling systems, the two rolls are driven by differently speeded drives. The speed differential contributes to the shearing effect between the rolls.
A higher speed differential increases shearing effect, but there are design limitations and other factors that make this effect non-uniform in various mills.
Control measure: Use the differential recommended by the design. Measure the effect of increasing the roll differential on flour yield and particle size and starch damage.
Incorrect Roll Surface Conditions
Corrugated and smooth rolls are used in milling systems for different purposes.
Break rolls are corrugated and aid in opening the kernels and releasing endosperm. In Reduction rolls, smooth surfaces aid in breaking endosperm into smaller particles and flour.
Rewriters Note: I had to condense a lot in the Rewrite, since the Input was already pretty concise.
Worn or damaged roll surfaces can increase starch damage and affect milling efficiency.
Control measure: Inspect roll surfaces regularly. Correct roll alignment, and follow recommendations of the equipment manufacturer regarding the condition of rolls.
Inadequate Wheat Tempering
During tempering, kernels are conditioned with the addition of small quantities of water. Tempering time is sufficient to allow water to move throughout the kernel.
During the tempering process, the bran becomes more pliable and the endosperm becomes easier to mill. Insufficient tempering leaves the bran brittle and endosperm difficult to remove.
The effect that moisture has on starch damage is still not fully understood. Bean, et al. (1994) concluded that additional time to achieve adequate tempering was sometimes as effective as altering the moisture level.
Control measure: Set appropriate target moistures and tempering times, and verify actual moisture.
Hard Wheat
There is a general assumption that hard wheats require more energy for grinding than soft wheats. The fracture behavior of hard wheat during milling is different from soft wheat, which can lead to greater starch damage.
Control measure: Use different or optimized milling settings for different wheat classes. Monitor starch damage when milling different wheat classes.
Excessive reduction grinding
Excessive fine grinding can increase mechanical energy applied to starch, and therefore, increase the starch damage.
Particle size and starch damage are related, but not identical. Flour with the same particle size can have different levels of starch damage and flour with high starch damage is not necessarily fine.
Control measure: Set minimum particle size for each product and avoid unnecessarily fine grinding. Evaluate particle size and starch damage separately.
How to Control Starch Damage Better in a Flour Mill
A combination of raw-material management, setting of the milling equipment, equipment maintenance, laboratory analysis, and process management is the best way to reduce starch damage.
Optimize Wheat Conditioning
Conditioning is one of the best improvements to milling performance.
The goal of conditioning is to reduce the moisture of the kernel to an optimal level for efficient and maximum bran flexibility to allow easy separation of endosperm and bran.
Some recommended practices include:
- Determine the average moisture of each major wheat lot/blend.
- Classify wheat based on milling characteristics.
- Calculate the desired conditioning amount of water based on the initial and final moisture of the wheat and the actual mass of the wheat.
- Allow sufficient time for conditioning for moisture to equilibrate through the kernel.
- Confirm conditioned moisture of the wheat with a validated test method.
- Adjust the parameters of conditioning with changes in wheat varieties or blends.
- Record moisture, conditioning time, temperature and starch damage.
Important: Do not increase the tempering moisture to reduce starch damage, unnecessarily. Excess moisture can lead to handling issues, affect milling performance, and increase storage issues. It must be appropriate to the wheat and the mill.
Adjust break-roll settings
The objective of the break system is to progressively open the wheat kernel and separate the endosperm from the bran.
To achieve this objective:
- Proper roll gaps should be utilized for each break passage.
- Excessive grinding in the first break passage should be avoided.
- Proper roll alignment and condition of the roll surfaces should be maintained.
- The amount of bran entering the downstream passages should be evaluated.
- The yield of break flour and the condition of stock exiting each passage should be monitored.
- Stock should not be forced into later passages because of poor performance in the earlier passages.
- Adjusting a brake system properly eliminates unnecessary grinding and helps downstream passages to function better.
Optimize reduction passages
Reduction passages are essentially starching damage control because their purpose is to grind the endosperm into flour.
If the reduction rolls are too tight, starch damage can increase significantly.
In order to improve things:
- The roll gap for each reduction passage should be reviewed.
- The reduction sequence can be changed based on the size of the stock and the requirements of the flour.
- Passages should not be overloaded.
- Excessive stock should not be passing through the same reduction roll.
- Starch damage and the extent of particle size reduction should be evaluated for each flour stream and not for the flour blend.
The aim is to achieve the required flour fineness with the least mechanical damage.
Maintain stable feed rates
Variable feeding increases the amount of stock passing through the roll gap and causes inconsistency in grinding.
When the feed is too low, the grinding behavior may become unstable. When the feed is too high, the passage may become overloaded and result in an inconsistent particle reduction.
Effects on the mill vary, and actual impacts on the mill may not be as predicted.
Stable feeding can be achieved by eliminating variations in the feed flow and controlling the feeder performance. Blockages can also result in variations in feed distribution to the different passages.
Control roll speed and differential
Roll speed and differential for each passage are set according to its function.
When making changes, evaluate the effects on starch damage, particle size, flour yield and loading of the equipment.
When changes are necessary, record the setting being changed and test the resulting flour.
Balance the milling diagram
A milling diagram illustrates the flow of the stock through the various breaks, rolls, and sifters and shows how the flour is collected.
An unbalanced diagram may lead to excessive use of certain passages or unnecessary repeated grinding.
- To improve a milling diagram:
- Check the performance of sifters and purifiers.
- Avoid routing material to a passage that is not suited for that purpose.
- Always evaluate flour yield and starch damage for the flour streams and examine passages that result in excessive starch damage.
- Reassess the milling diagram when there are changes in the type of wheat being processed or when there is a change in the volume being processed.
Maintain sifters and purifiers
Although sifters and purifiers typically do not harm starch like grinding rolls do, they still affect the performance of the entire milling system.
Sifters can malfunction in a number of ways, and the most detrimental of these is when unscreened material makes it through the sifting process. This results in particles of the wrong size being directed to downstream passages where they are further reduced in size by grinding rolls.
Recommended practices include checking sifter screens for wear and damage, inspecting airflow if there is any obstruction, and checking stock distribution to ensure it remains stable.
Use blending to achieve target quality
Due to the heterogeneous nature of cereal grains, different passageways of the milling system can cause different levels of starch damage.
Blending different passageways can produce a blend of flour which meets functional requirements.
Blending should not conceal the effects of a persistent mechanical issue. For instance, if one of the passageways continuously causes high levels of starch damage, the cause should be traced and resolved.
There are many parameters to consider when starch damage is analyzed. These include the level of starch damage, as well as protein, ash, moisture content, particle size, and enzyme activity, in addition to the requirements of the customer.
Instead of focusing on individual test results, the mill should analyze trends.
Some important trends include:
- The moisture and hardness of the wheat being delivered to the mill.
- The composition of the wheat blend.
- The moisture and conditioning time of the temper.
- The roll gap and differential speed.
- The feed rate and load of the passage.
- The starch damage caused by the passage.
- The starch damage in the flour.
- The water absorption of the flour.
- The extraction rate of the flour.
- The energy consumption, where reliable measurements are available.
- Customer complaints and the results of baking trials.
Starch damage can be stable, gradually increasing, or suddenly changing, as evidenced by a simple trend chart.
Unusual variation may be identified by statistical process control. Warning limits should be based on what the mill can control, not what’s generally accepted. If the wheat composition changes and starch damage gradually increases, the mill may need to make adjustments to the conditioning or roll settings.