Scald Efficiently without Damaging the Product

The feathers grow from follicles in the skin. The follicle is a complex tubular structure that prevents the feather from falling out. Inside the follicle, the feather is stuck to the papilla where it was created, but connective tissue, muscles and other structures also hold the feather firmly inside the follicle. The force needed to remove the feather is called the ‘Feather Release Force’ (FRF), and this force must be reduced in order to effectively pick the birds.

Heat treatment of the skin immediately after killing and bleeding is the most effective way to reduce the FRF, and hot water has the ability to penetrate deep into the follicles, bringing the heat to the feathers’ surrounding structure thereby loosening the feather. However, hot water can be harsh on product quality. If the temperature is too high for too long, there is a risk of denaturation of the underlying muscle proteins, producing the so-called cooked-fillet appearance, and the removal of the epidermis can result in discoloration of the skin during subsequent air chilling. Thus, finding the right balance between loosening the feathers and damaging the product quality becomes paramount in daily poultry processing.

Several physiological and environmental factors influence the ability to remove the feathers from the follicles.

Bird age: The age of the bird determines the feather development stage. Growing feathers have a blood supply, with an artery entering and a vein leaving the feather base. This increases the FRF and pulling out developing feathers may result in red spots on the skin caused by blood in the follicles. Fully developed feathers have no blood supply.

Rigor mortis and stress conditions: The postmortem chemical change called rigor mortis results in a stiffening of the body muscles, including the muscles that help to hold the feathers in place. Rigor mortis onset is relatively fast in broilers. The earlier the onset of rigor mortis, the more difficult the picking process will be. Therefore, the time between killing and scalding should be kept to a minimum. Broilers are homoeothermic animals meaning their body temperatures range from 40.5¬41.5°C and they do not have sweat glands to regulate body temperature. In a high-temperature environment, broilers find it difficult to release their body heat, triggering acute heat stress that causes glycogen breakdown of the muscles. The total energy content left in the bird (ATP) and the accumulation of lactic acid (stress conditions) affect the rigor mortis development – faster development complicates the picking process. Therefore, broilers must be maintained in a thermo-comfortable zone that does not disturb the physiological processes of the birds. Good ventilation in the lairage area eliminates excessive heat and ensures a proper supply of oxygen to the animals.

Stunning method: The bird’s neurological system is involved in holding the feathers and any neurological manipulation will affect the FRF and thus the picking process. Therefore, defeathering is more critical with CO₂ stunning as the birds are brain dead when leaving the stunner and no nerve impulses can be sent from the brain. When applying CO₂ stunning with soft scalded products, temperatures might need to be increased slightly to be able to reach the same defeathering result. The main reason is the rigor mortis development, which is different in electrical stunning than CO₂ stunning. When applying CO₂ stunning the challenging areas are the tail feathers, the tail feather shafts, shoulder areas and sometimes the wing tip feathers. Consequently, BAADER recommends implementing a tail feather puller immediately after the scalding process when stunning with CO₂.

Scalding time and temperature: Scalding time and temperature determine the degree of protein denaturation of the skin and muscle proteins, including the proteins in the feather follicle muscles. The higher the temperature and the longer the scalding time, the higher the degree of protein denaturation. If proteins are fully denaturated, they cannot return to their original structure. Irreversible scalding is usually the case with hard scalding (>56°C) and therefore the picking process is less critical. With lower temperatures like medium scalding (53¬57 °C) and soft scalding (50¬54°C) the picking process often becomes more complicated, due to the lower degree of protein denaturation. On the other hand, the benefit of lower scalding temperatures is a reduced risk of whitening the meat.

Feather location: Not all feathers are subject to the same FRF. While wing tips are thin and easily penetrated with hot water, the wing feathers are easily removed. The tail feather follicles are deeply embedded in fat tissue and consequently, the hot water takes a long time to penetrate and has limited effect. Tail feathers are therefore difficult to pick, especially when applying soft or medium scalding.

Water movement method: When the bird enters the scalder, it should be fully immersed in the hot water, and the water movement inside the scalder should help open up the feather package and give the hot water access to the skin. Different methods can facilitate this ruffling of feathers: air bubbles that break the surface of the water and agitates the water, or a powerful water flow that opens up the feather package.

Water viscosity: Among other factors, the hardness of the water inside the scalding tank determines the viscosity and thereby the immersion time. Soft water has a lower density and birds will immerse faster. The water contamination level, e.g. faeces and blood, affects water density and viscosity.

The time between scalding and feather removal: The heat absorbed by the birds during scalding must be pre-served as much as possible during the transfer from the last scalder to the first picking machine or the follicles can start to close. This evaporative temperature decline is most critical for low-scalded products.

Cooked breast: The breast meat represents approximately 19% of the whole bird weight and is normally well paid for, so any damages to the breast meat will cause severe economic losses. When scalding temperatures are too high it leads to denaturation of the meat protein and a ‘cooked’ appearance, which in retail is not accepted.

Broken wings: Each wing represents approximately 4.5% of the whole bird weight and broken or dislocated wings have no or limited value to the poultry processor. If scalding is not effective, more aggressive picking is needed to remove the feathers which increases possible wing damages.

Skin damages: As in the case of wing damages, aggressive picking can cause skin damages, which can be a challenge when producing skin-on products. Furthermore, the risk of contamination increases when the skin is torn.

Unwanted epidermis removal or unwanted epidermis retention: High scalding temperatures effectively loosen the feathers from their follicles, but hard scalding is also harsh on the skin as the outer layer, the epidermis, becomes loose and is later removed during picking. Too high scalding temperatures in combination with harsh picking can result in unwanted epidermis removal. The missing epidermis results in a brown discoloration of the skin if it is dehydrated during subsequent air chilling. Water spraying in the chilling area can (partly) reduce the brown patches. In other cases, the epidermis has to be completely removed. Complicated areas for epidermis removal are the bird’s sides, hocks and shoulder area. Incomplete epidermis removal sometimes results in an unwanted product appearance, as is the case for the Latin American ‘pollo pintado’ which requires full removal of the epidermis for effective attachment of the yellow colour. This is also the case when marinating birds.

Product contamination: The water temperature inside the scalders highly determines the degree of cross-contamination during the process. A common perception is that scalding is a dirty process; however, the water temperature will kill most of the microbes and the bacterial load. If the processor needs to pick the birds aggressively, the extra pressure on the bird can cause faecal contamination, which can lead to condemnation with associated economic losses. Scalding temperatures below 50°C are a microbiological threat to the products as pathogenic bacteria like Salmonella spp., Campylobacter jejuni and Escherichia coli in the scalding water will survive the scalding process. Daily pre-production pasteurization will eliminate any risk of cross-contamination from the previous day.



Growing conditions for salmonella, campylobacter, E.coli

To best facilitate feather removal and avoid the above-mentioned quality issues, the scalding process must be controlled at all times. As effective heat transfer to the skin is the purpose of scalding, controlling the water temperature and receiving notifications about unintentional temperature deviations are paramount in any modern poultry slaughterhouse. Advanced vision technologies can furthermore monitor product appearance after feather removal and alert about upstream processing irregularities affecting the quality.