Counting the number of birds passing through the poultry processing plant is standard practice, as it reveals any discrepancies between the quantity of birds arriving from the farm and those processed. Yet processing plants sell pounds or kilos of meat, not simply units, and so in addition to simply counting the number of birds, it is important to measure yield at each stage of processing.
Once the necessary equipment and methodologies are in place to monitor yield, measurements can be taken at intervals along the line and, if necessary, operations adjusted should yield be below target.
1. Key yield check points
The volume of meat produced will be the sum of various gains and losses that occur during processing. There are key points along the processing line where losses, often known as hidden losses, occur, and these are the ideal points for monitoring.
When live birds are hung on the overhead conveyor, their weight can be taken as 100 percent.
The first weight loss will occur immediately after slaughter at bleeding. At this point, a chicken’s blood will account for approximately 7 percent of its live weight.
Approximately 45 percent a chicken’s blood will be lost in the bleeding tunnel - the equivalent to 3 percent of its live weight. Therefore, immediately prior to entering the scalder, the carcass should weigh 97 percent of the chicken’s original live weight.
2. Importance of correct bleed time
This is the point where the first yield control should be put in place. If bleeding times are not properly adjusted to local climatic conditions and height above sea level, quality issues may arise, particularly where skin color is concerned.
In warmer climates, blood vessels in stressed chickens dilate. Because of this, the bleed time of stunned birds needs to be between 2 minutes 15 seconds and 2 minutes 45 seconds.
If there is no proper control at this point, yield will be affected, as chickens will lose more blood. Additionally, it is worth remembering that, should birds fall from the shackles at this point, they will be lost, and so drag down final yield.
If growing and processing operations are at altitudes in excess of 1,000 meters above sea level, blood vessels are more constricted and deeper within the body.
When birds are reared at sea level, oxygen enters their lungs under the maximum pressure possible, but as altitude increases, this pressure decreases. It is for this reason that, to maintain homeostasis, more red blood cells are produced to transport oxygen.
Therefore, birds grown at higher altitudes need a greater bleed time – between 3 and 4 minutes.
Should the above not be taken into account, birds will lose less than 3 percent of their blood in the bleed tunnel. However, they will not stop bleeding and, if one looks at the floor where birds exit the plucker, blood can be seen along the line of the overhead conveyor. The problem becomes more noticeable during evisceration, when evisceration trays, connecting trays and the floor can show large quantities of blood if bleed time has been too short.
Another indication of inadequate bleeding will be noticeable on the overhead evisceration conveyor, as at this stage, poor bleeding is highlighted by reddened skin. The problem can even impact the water of the pre-chiller, which will become increasingly red.
Adjusting bleed time will solve these problems. Additionally, at the bleed tunnel exit a counter could be installed to help preserve yield. While this may seem unnecessary, it can help to ensure that all the shackles are full and highlight if losses are occurring.
3. Changes at scalding
Further along the processing line, birds will gain weight, and this occurs during the scalding process, as feathers accumulate water. A bird’s feathers absorb approximately 0.6 liters of water, or some 600 grams.
Under normal circumstances, it can be assumed that if all the shackles enter the scalder full, they will exit the scalder full. However, if shackles exit the scalder empty it means that birds have been lost and this will impact negatively on final yield.
Any birds lost in the scalder will negatively impact the processing plant yield.
In many processing plants, the next operation is to remove the head. If equipment is not properly set, then some additional skin may be removed. While this may be considered only a small detail, any excess skin removal will have an impact on the final yield.
Prior to removing the paws and the transport of birds to evisceration, a system should be installed to automatically count and weigh the birds. At this control point, the headless birds should weigh 88 percent of their liveweight. The separated paws weigh approximately 4 percent of the chicken’s live weight.
4. Monitoring evisceration
At evisceration, various operations are performed to remove organs sold for human consumption from the abdominal cavity, with the remainder removed and sent for rendering. This is again a good point to check and weigh.
At the overhead evisceration conveyor, prior to removal at the pre-chiller, carcasses should be counted and weighed. The average carcass weight at this point, with abdominal fat but without the neck, should be 74.5 percent of the live bird weight. At this point, weight is classified as dry weight.
In plants that conduct automatic evisceration, the giblets are automatically processed and sent to the chillers using a vacuum system or similar. The giblets are stored in batches, and it is at this point that they should be weighed.
The liver, for example should account for 2.5 percent of the bird’s live weight, while the gizzards (without fat or the proventriculus) account for 1.5 percent, and the heart 0.5 percent. This gives a total of 4.5 percent.
These percentages need to be added to the total when calculating the correct dry yield of the processed bird. The carcass – without the neck or paws – will account of 74.5 percent of the total, while the offal should account for 4.5 percent, making a total dry weight of 79.0 percent.
While the above percentages are theoretical, and serve simply as a management parameter, some giblets will always be lost during evisceration, as will whole birds, and this will increase the volumes sent to the rendering plant.
Proper checks and measures installed along processing operations, and the simultaneous monitoring of variables, will help to maintain yield within certain parameters, and should problems be found, they can be rectified immediately.
Careless head removal can result in loss of skin.