The reason for uneven coloring of cookies processed by the cookie production line

The phenomenon of uneven coloring of biscuits processed by the biscuit production line (one side deep and one side light, partial burnt, partial whitening):
1. Color difference between the front and back sides (one side deep and one side shallow) may be due to factors such as hot air direction, temperature difference between upper and lower flames, heat absorption by the mesh belt, and issues with the green body sticking to the bottom
2. Some of the same piece or plate may be burnt black or white, which may be due to uneven wind speed in the furnace, differences in thickness and weight of biscuits, fabric density, or local overheating of the heat source

1、 The color depth of the front and back of the cookies is inconsistent (darker on the top and lighter on the bottom/darker on the bottom and lighter on the top)
1. The heating power/temperature of the upper and lower parts of the oven do not match
The tunnel furnace has independent temperature control for upper and lower heating: a high upper heating temperature will cause heavy coloring on the top surface of the cookies; Setting the fire too high will cause the bottom to burn.
Operation method: Set the temperature difference between the upper and lower zones; Crispy cookies are generally heated slightly lower than the upper heat to ensure that the bottom is set and the top is gently colored; The temperature difference between the upper and lower parts of the resilient biscuit is reduced.

2. The direction and angle of the hot air circulation are incorrect
The upper air nozzle blows directly onto the surface of the biscuit, locally impacting the high-temperature airflow and quickly coloring one side; The lower air volume is insufficient.
The air nozzle is blocked, and some air holes have oil and carbon deposits, resulting in weak air flow. Some areas have strong air flow, while others have no air flow.
Adjust the angle of the air nozzle and avoid blowing cookies vertically; Clean the air duct and filter screen; Ensure a balanced flow of hot air above and below each zone.

3. The conveyor belt affects the bottom coloring
1. The mesh belt steel wire is thick and the weaving gap is uneven: the position where the steel wire is pressed is not heated enough to turn white, and the gap position is heated more and darker, resulting in grid printing color difference;
2. The mesh belt has accumulated carbon, turned black and stored heat over a long period of use, and the bottom of the cookies it has passed through has become partially burnt;
3. The tension of the mesh belt is uneven, and the cookies are suspended or attached to different degrees.
Regularly clean and burn the mesh belt to remove carbon deposits; Select food grade stainless steel mesh belt with low heat storage capacity; Correct the mesh belt to be flat and not shake.

4. The problem with the biscuit raw material itself causes color difference in the base
Uneven thickness and uneven weight of the dough (previous fault point): There is a lot of moisture inside the thick billet, and the surface color is slow and turns white; Thin blanks dehydrate quickly and are prone to burning.
There is too much oil on the bottom of the green body, which makes it easier for the bottom to be heavily colored.

2、 Horizontal color difference inside the furnace (furnace width direction: left coke, normal in the middle, white on the right)
1. Uneven lateral temperature of the combustion tube or heating tube, damage to individual heating elements, blockage of the fire discharge, and inconsistent temperature on the left and right sides of the furnace chamber;
2. Dust accumulation on the impeller of the circulating fan and uneven air pressure on both sides; The air duct partition is deformed, and the hot air is biased;
Detect the actual temperature at the left, middle, and right points of the furnace; Repair damaged heating elements; Adjust the air duct guide plate and control the horizontal temperature difference to ≤ 3-5 ℃.

3、 Some cookies on the same website are burnt and some turn white (due to the density effect)
1. Inconsistent arrangement of raw materials: When cookies are stacked together, the moisture cannot dissipate, resulting in insufficient whitening and coloring; The distance between cookies is open and unobstructed, and direct hot air blowing can easily cause them to burn.
Improve the arrangement of plates/blanks, leaving even gaps between cookies; The forming arrangement should be as evenly spaced as possible, avoiding a pile of dense and sparse materials.
2. Before entering the furnace, the hardness of the dough varies: the soft billet has a large extension and high moisture content, making it difficult to color; The low moisture content of the hard dough makes it easy to burn (a chain problem, with the dough fluctuating in the front).


4、 Unreasonable segmented baking curve (tunnel furnace with multiple temperature zones)
1. High temperature in the front section: The surface instantly freezes and locks in water, preventing internal moisture from coming out. The surface is partially charred, and the interior turns white;
2. The temperature in the early stage is too low: insufficient dehydration for a long time, and the surface coking is directly caused by high temperature in the later stage;
Standard three-stage logic:
① Forming zone (front section): medium low temperature, slow expansion and dehydration to prevent premature surface crust formation
② Coloring area (middle section): Increase the temperature for Maillard reaction coloring
③ Dehydration fixation zone (later stage): Reduce the temperature, dry the remaining moisture, and avoid burning the edges


5、 Other easily overlooked triggers
1. Negative pressure/pressure imbalance inside the furnace
The opening of the dehumidification damper at the head and tail of the furnace is incorrect, causing moisture to accumulate in a localized area of the furnace. At this location, the biscuits turn white and do not color properly; Excessive dehumidification leads to heat loss and low local temperature.
Adjust the dehumidification dampers in each area; Ensure smooth discharge of water vapor and do not reflux inside the furnace.


2. Fluctuations in oils and formula reducing sugars
The content of formula sugar and conversion sugar is unstable; The spraying of demoulding oil varies, and areas with more oil are more prone to discoloration and coking.


3. Fluctuations in baking speed
The speed of the mesh belt fluctuates between fast and slow; Fast speed, insufficient baking, whitening; Slow speed and long heating time can easily cause burning.