Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

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Raw Material for Processing

0002 Potato processing requires special quality profiles of

the raw material. Therefore, the number of varieties

suitable for processing is limited. Plant breeders make

a great effort to select new varieties with better agro-

nomical behavior and with higher internal quality.

Today, only a few varieties with a known processing

quality are grown worldwide.

0003 Several specific aspects must be observed:

0004 The oxidative potential of the harvested tubers

should be low to prevent discoloration during the

first processing steps.

0005 Key constituents like protein, minerals, and vita-

mins should be high because of inevitable losses

during processing. Also the total content of solids

should be high to make processing more worth-

while, by achieving an higher input-to-output ratio.

Typical nutrient contents of selected potato products

are listed in Table 1.

Storage Aspects

0006 Storage has to provide high-quality potato tubers to

the processing plant for several months. Good storage

management should prevent excessive loss of

moisture, development of rot, and excessive sprout

growth. Also the loss of potato constituents (e.g.,

ascorbic acid) should be low. Finally, the accumula-

tion of reducing sugars must be controlled to prevent

dark-colored products.

0007 After harvest, potatoes pass through a period of

several months without any sprouting. The following

sprout growth is little or absent at temperatures

below 5



C. However, reducing sugars accumulate in

excessive quantities, resulting in undesirable dark-

brown colored products. Therefore, potatoes for pro-

cessing are stored at 8–10



C. At that temperature

sprouting must be controlled. Experiments have

been conducted with gamma-irradiation of tubers

or treatment of vines with maleic hydrazide. Also

several chemical compounds are known to inhibit

sprouting (e.g., ethylene, volatile monoterpenes, etha-

nol, abscisic acid, and naphthalene), but in practice

isopropyl-N-(3-chlorophenyl) carbamate (CIPC) is

preferred. To prevent the use of antisprouting agents,

potato breeders have created cold storable varieties

without cold sweetening to avoid the problem

described.

0008Immature tubers with considerable amounts of free

sugars will not reduce that level during storage. Also

storehouses with little gas exchange capacity lead to

high contents of reducing sugars inside tubers. This is

because of the increasing carbon dioxide concentra-

tion in the storage atmosphere. An increase of carbon

dioxide towards 0.5% in the atmosphere already

shows a significant deterioration of baking color.

Depending on the geographical region, storehouse

equipment must include a mechanical ventilation

and cooler. The relative humidity should be 90%

minimum.

Potato Processing for Food

0009A schematic overview of several processing tech-

niques is given in Figure 1.

Common Techniques

0010Washing Washing systems remove dirt and attached

soil from the tubers. Stones can be removed through

stone catchers or through exploitation of the different

specific density in comparison to tubers. Often the

washing water is taken from the overflow of waste

water decanters within processing.

0011Peeling With few exceptions, all tubers are peeled

before processing. Abrasive and steam peelers are

common, while in the past lye (caustic) peelers were

also used. Because of concern about environmental

effects, the importance of that type of peeler has

decreased. In abrasive peelers, designed with carbor-

und layers or knives or combinations thereof, the

amount of peeling waste can be up to 60% of tuber

weight, if the tuber shape is irregular or deep potato

eyes exist. Most single cells are disrupted and oxida-

tive enzymatic discoloration must be avoided (often

by additives, such as sulfur). Steam peelers remove

much less of the tuber cortex. Most of the tissue with

high nutritive value achieves further processing.

tbl0001 Table 1 Typical nutrient concentrations of processed potatoes

Per100 g edible material

Energy (kJ) (kcal) Main constituents

Protein (g) Fat (g) Carbohydrates (g) Potassium (mg) Ascorbic acid (mg)

Cooked potatoes 298 (71) 2.0 traces 14.8 443 14

Potato chips (crisps) 2241 (536) 5.5 39.4 40.6 1000 8

French fries 1215 (290) 4.2 14.5 35.7 926 28

Mashed potatoes 314 (75) 2.0 1.9 12.2 259 9

POTATOES AND RELATED CROPS/Processing Potato Tubers 4675

Tubers come into contact with high-pressure steam

(up to 15 bar) for a few seconds. The outer cell layers

are softened, and the cohesion of the tissue is

loosened. Through fast relaxation of the vessel the

peel is separated from the tubers and can be removed

by brushes or by a hard water jet. The main disadvan-

tage is the layer of cooked tissue. Therefore, the pre-

ferred application is when making French fries and

dehydrated products, whereas potato chips (crisps)

are predominantly produced using abrasive peelers.

0012 Cutting Several types of cutting exist depending on

final use. The stream of tubers is directed to specially

designed knives with the help of centrifugal power or

water pressure. High-pressure water knives need pre-

cise positioning of tubers. Except for when slicing

potato chips (crisps), using smooth and crinkled

knives, cutting is carried out in several steps. Disks

are cut first. Then, stripes are cut with rotating

knives. If cubes are to be manufactured, a third cut-

ting is done using chopping knives. Stripes for French

fries production can be cut using water knives. High-

pressure water leads tubers through a pipe with a

cutting fence at its end.

0013Sorting Irregular cuts must be removed, if the shape

of single pieces is a quality criterion, e.g., when manu-

facturing French fries, and thus is done before and

after the main processing. Separation takes place by

screening or by roller sorters. Optoelectronic sorters

allow an additional sorting according to discolor-

ation. Single pieces are visually inspected by charge-

coupled device (CCD) cameras. Next to light and

dark differentiation, state-of-the-art systems also

offer the possibility of color differentiation. Dis-

colored pieces are removed from the main stream by

an ejection system using compressed air.

0014Blanching Blanching means a short-term heat treat-

ment (70–100



C) of raw potatoes or potato pieces.

Several reactions occur. Enzymes are inactivated (e.g.,

polyphenoloxidase) to prevent discoloration. Often

the peroxidase system is regarded as a key enzyme

mirroring the efficiency of enzyme inhibition. Redu-

cing sugars are leached off, to minimize the Maillard

reaction, as well as starch pastes, to reduce fat

uptake during frying. Blanching for about 20 min at

70–75



C results in a firm structure. Native pectin

methyl esterase is activated to reduce cross-linking

Trash remover

Washer

Peeler (steam-heated or abrasive)

Washer

French fries (prefried) Potato chips (crisps) Dehydrated potatoes

Diced ones Flakes Granules

Trimming belt

Size grader

Strip cutter

Sliver eliminator

Inspection belt

Blancher (up to three)

Dewatering screen

Drier

Frier

Defatter

Cooler

Freezer

Packa

e line

Cutter

Washer

Blancher

Slicer

Washer

Blancher

Washer

Drier

Optoelectronic sorter

Package linePackage line

Cooler

Cooker

Mashing unit

Roller drier

Grinder

Optoelectronic sorter

Mixing (optional)

Package line

Trimming belt

Chip slicer

Washer

Frier

Defatter (optional)

Spicer

Slicer

Washer

Blancher

Cooler

Cooker

Mashing unit

Conditioner

Pneumatic drier

Add

back

Optoelectronic sorter

Mixing (optional)

Packa

e line

Screening unit

Fluidizing drier (I and II)

fig0001 Figure 1 Schematic description of main processing lines.

4676 POTATOES AND RELATED CROPS/Processing Potato Tubers

of pectin, and free carboxyl groups may react with

calcium or magnesium liberated from starch granules

after gelatinization to form a thermostable pectin

network. The addition of calcium to the blanching

water intensifies that reaction.

0015 Blanching takes place predominantly in a continu-

ous screw motion through a water bath or through a

steam chamber. Depending on the raw material and

the shape of single pieces, blanching parameters vary

widely.

Dehydrated Potato Products

0016 Dehydrated diced potatoes Dehydrated diced pota-

toes have been produced since the beginning of the

twentieth century. Their first use was as a staple food.

Today, snack industry and other food manufacturers

(e.g., for dumplings, hash brown) are the main users

of dehydrated diced potatoes. Because of different

outlets, no uniform quality level can be addressed,

but discoloration is to be prevented, flavor should

be fairly typical of the varietal characteristics, and

texture of the reconstituted product should be similar

to that of fresh prepared potatoes.

0017 Processing Preliminary inspection by hand or by

optoelectronic sorters immediately after washing

removes potatoes that are unfit for processing.

0018 Standard peeling equipment uses steam or abrasive

peeling. The expense of the inspection and trimming

of peeled tubers can be reduced by optoelectronic

inspection belts.

0019 Mechanical dicers which can be adjusted to various

cutting sizes (e.g., cubes, disks) cut the tubers. Irregu-

lar cuts depend on the sharpness of the cutter blades

and on the mass flow of potatoes. Released solids (up

to 15%) cannot be avoided but should be reduced as

far as possible. Often, free starch is separated from

the wash water and is used for special purposes.

0020 Blanching for 2–10 min at 90–100



C reduces ad-

verse enzymatic reactions. Under- and overblanching

negatively affects the texture and appearance of the

finished product. Therefore it is to be controlled

periodically. After blanching, gelatinized starch must

be removed from the surface by a further water bath

or a rinsing spray.

0021 Sulfiting during or just after blanching protects

against nonenzymatic browning reaction (darkening

after cooking) during drying and reduces adverse

oxidation during storage. Chemicals to be used are

sodium sulfite, sodium bisulfite, sodium metabisulfite

or combinations thereof. The European Community

tolerates up to 400 mg sulfite as sulfur dioxide per kg

ground dehydrated potatoes.

0022 Potato drying is a two-stage process by migration

of water to the surface and by removal of surface

water by evaporation. Slow dehydration at a low

temperature (e.g., 30



C) gives a hard dense product,

while a higher temperature results in a more porous

material with better rehydration capacity. Hot air

driers, constructed as hordes drier with a batch

charging, or as a continuous-belt drier (conveyor

drier) are used in most cases. During the first stage

of drying moisture is readily removed and the drying

rate is high. Therefore, the temperature can be high

(70–90



C). Within the second stage of drying, pro-

gress is much less and temperature is reduced to

50–70



C. Potato size, shape, and raw material char-

acteristics influence the efficiency of water removal.

Microwave-assisted drying reduces the drying time

drastically. The rapid diffusion of moisture through

the surface will tend to eliminate surface hardening.

Rehydration potential rises. It is most suitable at

moisture levels between 6 and 30%, but often ener-

getic and economic disadvantages prevent utilization.

0023Screening and inspection by pneumatic separators

follows drying, to remove small pieces, discolored

pieces or peeling left on potato dices, and black spots.

0024Dehydrated mashed potatoes

Dehydrated flakes Potato flakes are dehydrated

mashed potatoes made by drum drying a thin layer

of cooked and mashed potatoes, and breaking the

sheet of dehydrated solids into a suitable size for

packaging. Next to retail consumption in a pure

state or prepared with milk solids and other constitu-

ents, flakes are taken as a food ingredient for potato

snacks (e.g., potato chips made from a dough with

uniform size and curvature, and for dehydrated

French fries). A finely ground potato flour for soups

and baked goods is also available.

0025Processing Raw potatoes should be washed to

remove adhering soil, and sorted (by hand or by

optoelectronic sorters) to remove defective tubers.

0026Peeling may be done by any process described

above. In most cases steam peeling is used. Trimming

may ignore some peel and defects, because of a final

product control by optoelectronic sorters.

0027The tubers are sliced into disks of 1 cm thickness to

obtain a more uniform heat treatment during pre-

cooking and cooling. Thinner slicing is possible, but

flavor deteriorates and loss of solids clearly increases.

0028Precooking leads to gelatinization of starch and

enzyme inactivation. Cooling below 20



C for about

20 min reduces the level of free starch in the final

flakes. For a long time that effect was described as

retrogradation of gelatinized starch, but that reaction

needs several hours to become visible. More evidence

is given in toughing cell wall components (e.g.,

POTATOES AND RELATED CROPS/Processing Potato Tubers 4677

reduced pectin esterification) for an increased resist-

ance against shear forces during drying.

0029 For cooking, steam of atmospheric pressure is

injected into a screw conveyor. To determine the

proper degree of cooking, time must be evaluated

periodically. Overcooking results in a poorer texture

of the finished product, whereas undercooking causes

excessive loss of mash from the applicator rolls and a

high level of defects.

0030 Mashing immediately follows cooking to avoid cell

rupture of cooled and firmed potatoes. Also mixing

with additives is easier in the hot stage. Next to

emulsifiers (0.5% monoglyceride, to complex free

gelatinized starch), chelating agents like sodium acid

pyrophosphate (which reacts with iron, preventing

darkening after cooking) and antioxidants like

butylated hydroxyanisole (which extends storage life

and provides reduced rancidity) are used widely. Also

sulfur can be added to prevent nonenzymatic

browning. The proper dosage of additives is difficult

because of high losses during drying (up to 75% of

antioxidants is lost and 50% of sulfur dioxide).

0031 Drying takes place on a single-drum drier fitted out

with four to six applicator rolls. The drum (e.g., 1.5 m

diameter and 5 m length) is steam-heated. The tem-

perature on the surface is between 140 and 160



The mash is applied above the top roll while the

direction of drum rotation is downward. Therefore,

the distance between the top roll and the drum

surface determines the rate of flow of mash to the

rolls below. Each applicator roll applies a layer of

potato cells to the drum. The distance of the applica-

tor rolls to the surface of the drum must be controlled

very carefully, because cell rupture is influenced by

distance. A wider distance may result in a nonuniform

deposition of potato cells and lumps from larger

agglomerates can be in the product. Finally, a knife

removes the dry sheet. It drops down into a screw

conveyor which transfers the roughly broken sheet to

a collecting tank. Then it is ground with hammer

mills, creating a product with acceptable texture.

High amounts of fines is necessary to prevent a sticky

reconstituted product. The shelf-life depends on the

atmosphere in the final products packages. Autoxida-

tion of free fatty acids remains a problem.

0032 Dehydrated granules Potato granules are dehy-

drated single cells or aggregates of cells dried to

about 6–7% moisture after prior cooking. The prin-

cipal process used is the ‘add-back’ process. Potatoes

are partially dried by adding back previously dried

granules to give a moist mix, which is air-dried after

conditioning. The result is potato flakes, but the

product is much less sticky, because most cells remain

intact.

0033Processing The first steps of processing are as for

flake processing. Raw potatoes are washed and sorted

(by hand or by optoelectronic sorters) before steam

peeling. Trimming removes the rest of peel and the

defects, and slicing results in disks with 1–2 cm thick-

ness to obtain a uniform heat treatment.

0034During precooking sulfur dioxide is added to

prevent nonenzymatic browning.

0035Water cooling below room temperature for about

20 min reduces the level of broken cells in the final

product.

0036Steam cooking at atmospheric pressure in a screw

conveyor for about 30–50 min gives fully cooked

potatoes ready for mashing and mixing with dry

add-back granules. Repeated mild shearing and press-

ing of the added granules against the cooked mash

causes separation of the latter into single cells with a

low content of broken cells.

0037The moist mix is cooled to approximately

15–25



C and held for about 1 h at that temperature

(conditioning) by moving the moist mix on rubber

belts with a depth of product of 10–20 cm, to avoid

excessive compression. Since some agglomeration

does occur during conditioning, gentle mixing is ad-

vised after that. Very large agglomerates and bruised

tissue can be separated by a scalping reed. Poor

granulation is a self-perpetuating process. Therefore,

the amount of large granules will increase progres-

sively with continued recycling. Because the larger

particles do not absorb moisture rapidly enough, the

process is to be controlled periodically.

0038Flash drying with a heated air stream, an inlet

temperature of 170



C, and an outlet temperature of



C reduces the moisture content to about 12%.

Material coarser than 60–80 mesh is screened off

and returns to the process as add-back. Further drying

takes place in a two-chamber fluidizing drier with

alternate hot- and cold-air streams towards a final

moisture content of about 6%.

0039For retail purposes the granules can be agglomer-

ated with fat, milk solids, and flavors to give a ready-

to-eat product. In that case only rehydration with hot

water is necessary.

French Fries

0040French fries are deep-fat-fried potato strips. Commer-

cially distributed French fries (UK: chips) are semifin-

ished, and must be fried or heated before consumption

(par-fried ¼partially fried). Shelf-life depends on the kind

of distribution, either fresh (cooled) or deep-frozen. With

respect to the desirable quality profile, large and long

oval-shaped tubers (50 mm up) are preferred.

0041Processing Sorted tubers are cleaned. Subsequent

peeling is with steam peelers. After removal of the

4678 POTATOES AND RELATED CROPS/Processing Potato Tubers

skin, strips are cut either by mechanical knife systems

or by water knives (e.g., 6 6, 9 9, 11 11,

13 13 mm). In an inspection unit small pieces and

defects are removed. The total amount of waste can

be up to 50%, but most rejected material is processed

to other products, e.g., pommes croquettes. In a

washing unit free starch and other solids are washed

off. Blanching reduces enzyme activity. A layer of

pasted starch reduces fat uptake during frying. To

prevent nonenzymatic discoloration, sulfur is often

added to the blanching water. After washing, some-

times a second blancher with a low concentration of

glucose is implemented to adjust the content of redu-

cing sugars for a more stable frying color. Also dip-

ping in a water bath containing a coating substance

occurs, to enhance the texture stability for about 5–

10 min after final frying. Predriers remove free water

from the surface and the outer layer. Fat uptake will

be reduced by the higher dry-matter content. Frying

units can have a capacity of 4 t h

1

of product, or

more. The fat content of such units can be more

than 5000 l of fat. Temperature is between 140 and

180



C. The potato strips are moved through the oil

bath by conveyor belts. To enhance texture behavior

and capacity, two frying units can work in a line.

Normally, the total frying time is between 3 and

5 min. After frying a defatting unit removes free fat

from the surface by vibration or by hot-air streams.

Mechanical cooling reduces the temperature of pre-

fried strips to 4



C. That product can be packaged and

distributed as par-fried fresh French fries. Alterna-

tively, the French fries are frozen inside tunnel

freezers, packaged, and distributed as par-fried

deep-frozen French fries.

Potato Chips

0042 Potato chips (UK: crisps) are thin slices (1.1–1.6 mm)

of fresh potatoes which have been normally peeled

and fried in hot oil to reduce the water content to less

than 2%. For processing, special varieties with a very

low content of reducing sugars (< 0.15% fresh

weight) are required.

0043 Processing After cleaning and visual inspection, the

tubers are peeled abrasively with losses up to 25%.

Thin slices are cut with smooth or crinkled knives. In

a washing unit free starch and other solids of dis-

rupted cells are washed off. Optionally, blanching

may reduce the content of reducing sugars. Predriers

remove free water from the surface and the outer

layer. Frying units can have a capacity of 4 t h

1

product or greater. The oil content of such units can

be more than 5000 l of oil. Temperature is up to

190



C. The potato slices are of lower specific density

than the oil. Therefore specially designed rotating

paddles dip and move the slices forward. Total frying

time 2–4 min. The slices absorb up to 50% oil. That

high amount of oil must be continuously replaced.

After frying, a defatting unit removes free fat from

the surface by vibration or by hot-air streams. The hot

slices are seasoned and packaged.

Others

0044Peeled potatoes Peeled table potatoes are produced

with abrasive peelers, either carborund or knives

or combinations thereof. Shelf-life depends on the

hygienic status. Preservation with chemicals (e.g.,

sulfur) leads to a shelf-life of 8 days, whereas a sub-

stitution of air by nitrogen or carbon dioxide or com-

binations thereof leads to a shelf-life of 10 days.

0045Potato dough Several potato products prepared

of dough are available, either fresh or deep-frozen

(e.g. pommes croquettes, pommes duchesses). Dough

preparation of cooked potatoes and several other

ingredients can be from fresh potatoes (e.g., small

pieces from French fries production) or from dehy-

drated potatoes. A final frying or heating is necessary.

0046Sterilized potato products Peeled potatoes are avail-

able as canned potatoes. After peeling, diced or whole

tubers are blanched with water at 85



C, filled in cans

and covered with brine, sealed, and sterilized at

121



C. In addition to table potatoes, other potato

products are also sealed in plastic bags and sterilized.

Potato Processing for Industrial Purposes

Ethanol

0047Ethanol originates from sugar fermentation of yeast

(Saccharomyces cerevisiae). Processing of ethanol

production integrates these steps: first starch is

broken down by steaming the potatoes. After mash-

ing the potatoes, yeast fermentation runs in a batch

process for up to 3 days. Finally the crude spirit is

distilled and rectified.

Starch

0048Starch is the main component of solids in potato

tubers. In comparision with other types of starch,

isolated potato starch offers several technological

advantages for applications in the food and nonfood

industry. Also plastic substitutes (e.g., shopping

bags) have become of strong interest. The main dis-

advantage of potato starch separation is the campaign

separation outside the winter season: destroying the

cells by rasping liberates the starch granules. After

POTATOES AND RELATED CROPS/Processing Potato Tubers 4679

fruit water separation, starch is washed out in jet

extractors of various types. In addition to welded

sieves, plate sieves improve the efficiency of this

step. After desanding, crude starch milk is purified

by countercurrent washing with demineralized fresh

water, dewatered, and dried. Optimum engineering

results in starch recovery rates of at least 97–98%.

0049 Fruit water is separated and concentrated by

decanting for subsequent protein recovery by iso-

electric precipitation and heat coagulation. The

remaining deproteinized fruit water can be concen-

trated by ultrafiltration or reverse osmosis. Further

evaporation results in a potato protein liquid.

See also: Browning: Nonenzymatic; Enzymatic –

Biochemical Aspects; Toxicology of Nonenzymatic

Browning; Enzymatic – Technical Aspects and Assays;

Chilled Storage: Use of Modified-atmosphere Packaging;

Packaging Under Vacuum; Controlled-atmosphere

Storage: Applications for Bulk Storage of Foodstuffs;

Cooking: Domestic Techniques; Drying: Theory of Air-

drying; Potatoes and Related Crops: The Root Crop and

its Uses

Further Reading

Agblor A and Scanlon MG (1998) Effects of blanching

conditions on the mechanical properties of French

fry strips. American Journal of Potato Research 75:

245–255.

Bergthaller W, Witt W and Goldau H-P (1999) Potato

starch technology. Starch/Sta¨ rke 51: 235–242.

Boskou D and Elmadfa I (1999) Frying of Food. Lancaster,

PN: Technomic.

Lisinska G and Leszczynski W (1989) Potato Science and

Technology. London: Elsevier Applied Science.

McMinn WAM and Magee TRA (1997) Physical character-

istics of dehydrated potatoes – part I and part II. Journal

of Food Engineering 33: 37–48, 49–55.

Mensah-Wilson M and Gierschner K (2000) An improved

method for the production of canned potatoes. Fruit,

Vegetable and Potato Processing 85: 132–139.

Moreira RG, Castell-Perez ME and Barrufet MA (1999)

Deep-Fat Frying. Gaithersberg, MD: Aspen, USA.

Talburt WF and Smith O (eds) (1987) Potato Processing,

4th edn. New York: AVI Book.

Willard M (1993) Potato processing: past, present and

future. American Potato Journal 70: 405–418.

POULTRY

Contents

Chicken

Ducks and Geese

Turkey

Chicken

A J Maurer

, Formerly of University of Wisconsin-

Madison, Madison, MI, USA

This article is reproduced from Encyclopaedia of Food Science,

Introduction

0001 Chicken is one of the most widely accepted muscle

foods in the world. Its high-quality protein, relatively

low fat content, new products, and generally low

selling price because of favorable feed conversion

make chicken a high-demand food in the market-

place. Furthermore, the absence of cultural or reli-

gious taboos allows increased chicken production

and consumption worldwide.

0002This article reviews specific characteristics of

chicken, its chemical composition and nutritive

importance, production advances, slaughter and fur-

ther processing, food uses and products, and micro-

biological problems.

0003The USA leads the world in poultry meat produc-

tion, followed by the former Soviet Union, China,

and Brazil. Consumption of chicken in the USA has

increased dramatically over the last 20 years. Table 1

shows the changes in chicken consumption and

tbl0001Table 1 Meat consumption per capita (kg) in the USA

1994 1995 1996 1997 1998

Beef 30.3 30.5 30.6 30.3 30.5

Pork 24.0 23.8 22.2 22.1 23.6

Poultry 40.4 40.3 40.8 41.1 41.9

Source: http://meat.tamu.edu/consum.html.

Deceased.

4680 POULTRY/Chicken

compares those changes with other meats. Broiler

consumption in the USA continues to increase.

Chemical Composition, Nutrition, and

Dietary Significance

0004 Chicken is a very digestible source of high-quality

proteins. Its muscles are differentiated into light

meat (primarily the breast) and dark meat (the legs).

These muscles vary in myoglobin content as well as in

fat content. Table 2 lists the chemical composition of

selected chicken parts, and shows some of the prox-

imate changes that occur on cooking. (See Protein:

Food Sources.)

0005 Chicken meat is low in total fat and saturated fat,

and light meat is lower in fat than dark meat (Table 2).

Most of the fat in poultry is deposited under the skin

and is easily removed by pulling the skin from car-

casses or parts. Because chicken fat has a high ration

of unsaturated to saturated fatty acid (approximately

30% saturated), the fat melting point is relatively low

and liquefies easily. Chicken fat is also prone to oxi-

dative rancidity. Although most consumers believe

otherwise, sodium and cholesterol contents in

chicken are similar to those in most meats. Light

meat contains more protein, and dark meat contains

higher levels of fat and cholesterol. Older chickens

generally contain more fat and less moisture.

Cooking tends to concentrate the level of cholesterol

and protein because water is lost in the heating pro-

cess. (See Cholesterol: Properties and Determination;

Fatty Acids: Properties; Sodium: Properties and De-

termination.)

0006 Chicken contains all the essential amino acids, and

is a good source of B vitamins and minerals such as

iron and phosphorus. Poultry meat also provides po-

tassium, calcium, magnesium, and copper, but carbo-

hydrates and fiber are negligible. Light meat contains

more nicotinic acid than dark meat, although both

are good sources. Conversely, dark meat contains

more riboflavin, iron, and zinc than light meat. (See

Amino Acids: Properties and Occurrence; Refer to

individual vitamins and minerals.)

Economics of Fast-Growing Broiler

Chickens

0007Chicken broilers have been bred to a uniform size

specifically for meat yield. A common broiler strain

combines Cornish genes for conformation and flesh-

ing with the White Plymouth Rock for white feathers

and faster growth. White-feathered birds are the

norm for commercial broiler production because of

their cleaner-looking defeathered carcasses.

0008Many factors have contributed to the efficiency

and economics of broiler production. Advances in

breeding, nutrition, disease control, and management

practices have enabled the broiler industry to produce

a chicken weighing 1.8 kg in 6–7 weeks. Vaccines,

antibiotics, confinement rearing, and computer-

balanced rations assist in producing broilers with a

feed conversion ratio of less than 2 kg of feed per kg

gain. Vertical integration in the broiler industry has

increased efficiency even more by combining most of

the activities in producing, processing, and marketing

broilers under the same ownership and management.

Chicken Processing

0009Chicken slaughter and processing have evolved into

a sophisticated and automated procedure whereby a

processing plant can handle as many as 20 000

broilers per h. Transforming live chickens into a

ready-to-cook form involves live bird catching, crating

and hauling, unloading, hanging on a conveyor line,

stunning, slaughtering and bleeding, scalding,

tbl0002 Table 2 Proximate analysis of chicken broilers (per 100 g edible portion, wet-weight basis)

Nutrient Breast meat

with skin (raw)

Breast meat

without skin (raw)

Breast meat

with skin (roasted)

Legmeat

with skin (raw)

Legmeat

with skin (roasted)

Water (g) 69.46 74.76 62.44 69.91 60.92

Protein (g) 20.85 23.09 29.80 18.15 25.96

Energy (kJ) 772 462 827 785 974

Lipid

Total (g) 9.25 1.24 7.78 12.12 13.46

Saturated (g) 2.66 0.33 2.19 3.41 3.72

Monounsaturated (g) 3.82 0.30 3.03 4.89 5.24

Polyunsaturated (g) 1.96 0.28 1.66 2.65 3.00

Cholesterol (mg) 64 58 84 83 92

Ash (g) 1.01 1.02 0.99 0.85 0.92

Sodium (mg) 63 65 71 79 87

Reproduced from Posati LP (1979) Composition of Foods. Poultry Products, Raw, Processed, Prepared. Agriculture Handbook 8–5. Washington, DC: Science

and Education Administration, US Department of Agriculture.

POULTRY/Chicken 4681

defeathering, eviscerating, inspecting, chilling,

grading, packing, and shipping. (See Meat: Slaughter.)

Assembly of Live Birds

0010 Broilers should be taken off feed 8–12 h before the

time of slaughter to reduce fecal contamination

during processing. Bruising in chickens can be min-

imized by removing feeders and waterers prior to the

arrival of the catching crew, and by careful handling

of the birds as they are caught and placed into coops.

Novel mechanical methods (e.g., herding, sweeping,

and vacuum systems) for catching chickens are also

available and commercially used by a few companies

in Europe.

Hauling and Unloading

0011 After the birds are loaded into wooden or plastic

crates or specially built compartments, they are trans-

ported via open-sided trucks to the slaughter plant.

Large-volume, slow-speed fans or evaporative cooling

provides good ventilation and comfort for the birds in

the holding shed.

0012 Trailer loads move into the plant unloading area as

needed. Various methods (e.g., manual bird removal,

or dumping the birds through side-doors) unload the

crates. Birds are hung by their shanks on shackles

attached to an overhead monorail conveyor for

transfer through the slaughtering operations. The

unloading area should be dimly lit, and a breast rub

bar can be used to calm the birds. According to UK

regulations, birds should be slaughtered within 3 min

after they are suspended from shackles.

Stunning, Killing, and Bleeding

0013 Stunning is used to enhance bleeding and feather

release. The heads of the birds are usually dragged

through an electrically charged water bath. For con-

sistent stunning, the water should contain from 0.1 to

1.0% sodium chloride. A fine mist of water spray

should be directed at the bird’s feet to provide a

positive electrical contact prior to entering the stun-

ner. The shackles complete the circuit and cause an

electric current to run through the body of the bird as

a result of the applied voltage (approximately 50 V).

Overstunning can rupture blood vessels and break

carcass bones due to excess muscle contraction. It is

essential that the electric shock does not kill the bird;

bleeding must be the cause of death.

0014 Killing can be either manual or mechanical. In the

manual operation, a skilled worker with a sharp knife

severs the jugular vein and carotid artery by cutting

across the side of the neck at the base of the bird’s

head. In mechanical neck cutting, a guide bar with

grooved rollers holds the head rigid and extends the

neck in preparation for the cuts. The bird’s head is

guided across a single, revolving, circular blade or

between a pair of revolving blades.

0015Bleed time should be 1.5–2 min, and the blood lost

accounts for approximately 4% of the live weight.

The shackles convey the birds through a blood tunnel,

so that the blood can be collected and disposed of

properly.

Scalding

0016After bleeding, the birds are scalded to loosen the

feathers by immersion in agitated hot water. Wing

flapping and struggling should have ceased by the

time birds enter the scalder. In general, a soft- or

semiscald temperature of 50–54



C is used, with an

immersion time of 1.5–2.5 min. For the fresh chilled

broiler market, low-temperature scalding permits re-

tention of the yellow cuticle on the chicken skin and

retards skin drying.

0017Chickens are subscalded at 59–60



C for cuticle

removal if there is a preference for lighter-skinned

birds, if the carcasses are to be frozen, or if the parts

are to be battered and breaded. Higher temperatures

greatly facilitate feather removal, and necks and

wings of semiscalded birds are often scalded at the

subscald conditions to achieve cleaner carcasses. In

the USA, the US Department of Agriculture (USDA)

recommends a minimum overflow from the scalder of

11 per bird to reduce build-up of contamination.

Various chemicals added to scald tanks assist feather

removal by reducing surface tension and enhancing

wetting of the feathers. An alternative to immersion

scalding with separate defeathering machines is a

combined spray-scalding and defeathering system.

This type of scalding may reduce contamination

spread from bird to bird.

Defeathering

0018After the carcasses leave the scalder, they enter a series

of online defeathering machines. The picking ma-

chines consist of banks of counter rotating stainless-

steel disks or drums, with rubber ‘fingers’ mounted

on them. As the birds are conveyed through the rotat-

ing picker fingers, the feathers are rubbed or plucked

from the carcass. Continuous water sprays flush away

the feathers.

0019Remaining pinfeathers are removed by hand. In the

USA, the birds pass through a gas flame to singe the

fine hairs. A final step in the defeathering area is an

outside spray wash.

0020The head removal operation may take place in the

defeathering area between the picker and the outside

bird washer, or in the eviscerating room to facilitate

crop removal. The bird’s neck passes through a device

4682 POULTRY/Chicken

which restrains the head as the overhead conveyor

pulls the body forward, separating the neck vertebrae

at the base of the skull. Some head removal devices

can include a set of rollers to separate the neck bones

and a rotating knife to sever the neck skin instead of

tearing it; yield is improved with this system.

0021 The birds pass through an automatic hock cutter

which severs the shanks at the hock joint and causes

the carcass to drop onto a conveyor for transfer to the

evisceration area. In the UK, the feet of the broilers

are cut off just above the spur by means of a rotating

knife. The severed feet remain on the shackles and are

removed mechanically on the return line.

0022 The scalding, defeathering, and other operations

occur in a separate portion of the plant, apart from

the evisceration and final processed bird area. The

transfer conveyor takes the defeathered carcasses

into the evisceration area, where the carcasses are

rehung on the clean evisceration line.

Evisceration

0023 The first cutting operation in the evisceration area

involves removing the preen or oil gland, located on

the top side of the tail next to its base. A sharp, short-

bladed knife is used to remove the gland manually, or

a machine with a cutting blade can be used online.

0024 An opening cut into the body cavity is then made

using a knife, a special drill-like vent cutter with

a rotary blade, or an online automated machine.

Making an incision in the abdomen and removing

the vent can be performed manually or by machine

to complete the opening process for evisceration.

0025 Removing or drawing the viscera from inside the

bird can be performed by hand or by machine. To

retain the identity of the viscera within the carcass

and be in a position for easy inspection, the viscera

are draped uniformly to one side of the tail but remain

attached to the carcass until the carcasses have passed

the inspection for wholesomeness.

0026 Mechanized removal of the viscera is very common

in large processing plants (Figure 1). An eviscerating

spoon is mechanically inserted into the cavity and

withdraws the viscera. Back-up workers may be ne-

cessary to remove viscera missed by the machine.

0027 Mandatory USDA inspection of every bird for

wholesomeness is performed under the supervision

of an inspector employed and trained by the govern-

ment. If evidence of unwholesomeness is detected, the

carcass or affected parts are trimmed or condemned.

0028 Workers remove the giblets (heart, liver, and giz-

zard) from the viscera of inspected birds and clean

and chill them. Then the lungs are removed with a

vacuum lung gun, or they can be withdrawn with the

viscera on a mechanical eviscerator. The neck is gen-

erally cracked with shears, the crop and windpipe are

removed, and the neck is then pulled off by stationary

guide bars.

0029Finally, the shackles convey the carcasses through a

bird washer where spray nozzles rinse the inside and

outside of the carcasses.

Chilling

0030The most common chilling operations in the USA

immerse carcasses in long flowthrough tanks contain-

ing agitated chlorinated water or slush ice. The USDA

regulations require that a chiller overflow rate be

maintained at 2 l of water per chilled bird, in order

to minimize microbial build-up in the chill water.

0031The chickens are first placed in a prechiller, con-

taining water at 10–18



C, and then into a slush ice

chiller at 0–1



C. Chickens must be chilled to 4



Cor

lower within 4 h to meet USDA requirements; the

chilling time generally needed to obtain 4



C carcasses

is about 40–60 min.

0032After chilling, carcasses are hung by one leg on a

drip line for 2.5–4 min for draining, and are then

conveyed to the packing area. Although the meat

absorbs some moisture during washing and chilling,

the drained moisture level is strictly regulated. The

fig0001Figure 1 Automatic chicken eviscerator. Reproduced from

Poultry: Chicken, Encyclopaedia of Food Science, Food Technology

and Nutrition, Macrae R, Robinson RK and Sadler MJ (eds),

1993, Academic Press.

POULTRY/Chicken 4683

drip line shackles are usually weighing devices which

drop the sized birds into appropriate bins.

0033 Air chilling of eviscerated carcasses is used exten-

sively in Europe. Spraying the carcasses with water at

intervals avoids weight loss during evaporative

chilling. (See Meat: Preservation.)

Grading

0034 Carcass grading in the USA is a voluntary practice,

but is often required by purchasers. Government

graders sort the birds into A, B, and C categories

according to body conformation, fleshing, fat cover,

deformities, bruises, and defects such as pinfeathers,

disjointed or broken bones, and missing parts. Europe

uses similar grading standards.

Packing

0035 Giblet packs containing a heart, liver, gizzard, and

neck are stuffed into the body cavity of chilled, sized

carcasses. The carcasses then go into overwrapped

trays which are heat-sealed, or into polyethylene

bags which are clipped or taped shut ready for market.

Another common method in the USA for distributing

broiler carcasses from the processing plant is to put

them into corrugated, wax-impregnated boxes. After

boxing, ice covers the birds to keep the skin moist

during shipment. Nearly half of the broilers in the

USA leave the processing plant as ice-packed birds.

An increasingly popular packing method is chill-

packing, whereby a 6



C air blast lowers carcass

temperatures to between 2



C and 1



C. Refriger-

ation then maintains that temperature during

marketing of the birds.

Waste Products

0036 Poultry processing results in large amounts of waste

waters, semisolids, and solids, which require separ-

ation and treatment before being discharged into the

environment. Waste material that can be reclaimed

and, for example, used for animal feeds or fertilizer

helps to reduce the overall load for disposal.

0037 There are several ways to handle poultry offal

(heads, feet, viscera, inedible parts, condemned

whole birds, feathers, and blood). Usually, all offal

except the blood and condemned birds is floated in

water from the processing areas to an accumulation

area for removal by trucks. Screens and rotating

drums remove water and separate offal components.

Because of its high oxygen demand for decompos-

ition, blood is usually handled separately.

0038 Most of the offal solids goes to a rendering plant,

while liquid generally goes through a primary and

secondary lagoon treatment system. Some plants can

discharge the liquid effluent into a municipal water

system after removing the fat and as many solids as

possible.

0039Another method of waste treatment is spray irriga-

tion or spreading the processing effluent on the land.

This type of waste disposal requires large fields to

prevent overloading. Runoff into streams is a major

concern. (See Effluents from Food Processing: Com-

position and Analysis; Disposal of Waste Water.)

Food Uses and Products

0040Most broilers in the USA are sold as fresh (unfrozen)

carcasses, but frozen deboned meat is used in further

processing. A decline in the sale and consumption of

whole chickens in the USA has been counterbalanced

by a steady increase in the sale of cut-up carcasses and

parts. More than half (53%) of the processed car-

casses in 1990 were marketed as cut-up broilers and

parts.

Cut-Up

0041The cut-up operation is usually mechanized with

motor-driven equipment and shielded circular blades.

Individual cut-up stations or online automated ma-

chines can be used. A popular method of packaging

is to place the cut-up parts in a tray containing an

absorbent pad to collect seepage and then overwrap

the tray. The bagged whole carcasses and tray packs

may be prepriced before delivery to the retail market.

Deboning

0042The spectacular growth in the sale of value-added,

further-processed poultry products in recent years has

placed a heavy demand on the production of deboned

meat. Poultry meat has traditionally been removed

from the carcass by hand with a sharp knife, while

the carcasses hang from special shackles on a slow-

moving line or are positioned on static or moving

cones.

0043Several automatic deboning machines are also

available for removing breast fillets and thigh and

drumstick meat. One system operates by holding a

particular chicken part in position above a contoured

recess in a base plate. The machine forces the meat

from the bone into the depression. Another meat-

deboning system pushes the bone lengthwise out of

the carcass part (e.g., drumstick) and strips off the

meat in the process.

0044Currently, deboning is primarily done on aged

chilled carcasses. Hot-stripping the muscles and skin

from defeathered but uneviscerated carcasses is being

studied. Some researchers are also testing hot debon-

ing, the removal of meat after evisceration but prior

to chilling. Generally, hot deboned meat is tough,

4684 POULTRY/Chicken