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

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release of immune mediators, such as histamine,

which actually cause the symptoms associated with

allergic reactions. (See Food Intolerance: Food Aller-

gies for more information.)

0003 The sites on a protein that are recognized by an

antibody are known as epitopes and can be either

linear or conformational. In the former, only the pri-

mary sequence of a polypeptide is involved in anti-

body recognition, but in conformational epitopes, the

three-dimensional structure of a protein is important.

Thus, a number of segments of the polypeptide chain

that may be quite distant in the amino acid sequence

of a protein are brought together spatially as a conse-

quence of its tertiary and quaternary structure, to

form an epitope. Most epitopes are thought to be

conformational in nature and are particularly diffi-

cult to define in relation to food allergens where

processing can have such a disruptive effect on native

protein structure (see below).

Allergen Nomenclature

0004 The World Health Organization and the Inter-

national Union of Immunological Societies produce

an official list of allergens, which are designated by

the Allergen Nomenclature subcommittee. Allergens

included in this listing must induce IgE-mediated

(atopic) allergy in humans with a prevalence of IgE

reactivity above 5%. An allergen is termed major if it

is recognized by IgE from at least 50% of a cohort of

allergic individuals but does not carry any connota-

tion of allergenic potency; allergens are otherwise

termed ‘minor.’ The allergen designation is then

based on the Latin name of the species from which

it originates and is composed of the first three letters

of the genus, followed by the first letter of the species

finishing with an arabic number, e.g., Ara h 1 relates

to an allergen from Arachis hypogea (peanuts).

Types of Food Allergens

0005 There appear to be around eight foods, including

those of both plant and animal origin, that are re-

sponsible for causing the majority of food allergies,

namely peanuts and tree nuts, wheat, soya, cows’

milk, egg, shellfish, and fish. The geographical distri-

bution of these allergies reflects patterns of food con-

sumption with, for example, buckwheat allergy being

much more common in the Far East, where they are

staple foods, than is the case in Western countries.

The distribution of allergies also differs across

Europe, with reactions to fish and shellfish being

higher in Mediterranean countries, where fish con-

sumption is higher.

0006 In addition to foods that sensitize individuals dir-

ectly (probably via the gastrointestinal tract), there is

a group of fruit and vegetable allergies that arise

from the presence of cross-reactive epitopes in the

allergens. Thus, individuals who become sensitized

to pollen are more likely to develop IgE, which recog-

nizes homologous proteins present in fresh fruits and

vegetables. Around 70% of individuals who develop

birch pollen allergy with IgE towards the pollen pro-

tein Bet v 1, develop allergies to apples and other

fruits of the Rosacea because the anti-Bet v 1 IgE

reacts with fruit homologs such as the apple protein

Mal d 1. Similarly, many of those who develop allergy

to latex have IgE, which recognizes the chitin-binding

domain of the hevein polypeptides that make up the

rubber latex network. An almost identical domain

is found in a group of enzymes, the class I chitinases,

found in a number of fruits including avocado, chest-

nut, and banana, which the anti-latex IgE recognizes.

As a consequence of this cross-reactivity, individuals

with latex allergy frequently suffer from allergic

reactions on consuming these fruits (Figure 1.)

Protein Properties and Allergenicity

0007Two factors that are thought to contribute to the

allergenic potential of food proteins are their resist-

ance to the effects of thermal processing used in food

preparation (discussed below) and breakdown by

digestive enzymes in the gastrointestinal tract. The

stability of a number of allergenic proteins to pepsin,

including those from peanuts, soya, and cows’ milk,

has been compared with that of nonallergenic

proteins. It was found that all the allergens either

remained undigested or gave stable fragments that

persisted for 8–60 min (depending upon the allergen),

whereas the nonallergens were completely digested

after less than 15 s. As peptides are required to have

a molecular weight of greater than 3000 Da in order

to stimulate an immune response, large stable

The major latex allergen prohevein (Hev b 6.01)

Latex-fruit syndrome allergen class I Chitinases

1 43 187

Hevein Carboxy-terminal domain

1 43 328

Hevein

Catalytic domain

Hev b 6.02, chitin-binding domain

Chitin-binding domain

NPT

N-glycosylation site

fig0001Figure 1 Presence of conserved hevein domains in latex and

the class I chitinases of fruits responsible for many latex–fruit

cross-reactive allergies.

144 ALLERGENS

fragments, as well as intact proteins, have the poten-

tial to act as sensitizers. Similarly, such large frag-

ments may be able to cross-link mast cell IgE,

leading to histamine release. Whilst the bulk of food

proteins are broken down into immunologically in-

active fragments, very small proportions (milligram

amounts) of material, which are still immunologically

important, may escape digestion. Circumstantial evi-

dence that food proteins can resist digestion and cross

the gut barrier is offered by the presence of antibodies

to a wide range of food proteins in the circulation of

normal individuals. The presence of proteins, such

as ovalbumin in the blood of individuals after con-

sumption of egg, also supports the premise that intact

proteins, albeit in small quantities, can enter the body.

However, although the nature of the transported pro-

teins has not been clearly defined, a large proportion

of cow’s milk allergic patients have specific IgE, which

recognizes proteolyzed b-Lg, some of which reacts

better with the hydrolyzed protein than the intact

form. Thus, the resistance of allergens to the effects

of the digestive process plays an important role in

determining the allergenic potency of food proteins.

Allergens of Plant Origin

0008 Many plant food allergens are pathogen-related (PR)

proteins, which are produced by the plant as a defense

against pathogenic microorganisms, insects, and

invertebrate pests. They have been classified into a

number of families on the basis of sequence similar-

ities or enzymic properties and are receiving attention

from plant breeders, as a tool for GM crops, because

of their protective activities against agricultural pests.

However, they are cross-reactive allergens in fruits,

vegetables, and pollen. Allergens, other than PR

proteins, are inhibitors of proteases and a-amylases,

profilins (actin-binding proteins), seed-storage pro-

teins (albumins, globulins, and prolamins), proteases

and lectins (agglutinins). These are summarized in

Table 1.

Fruit and Vegetables

0009 Fruit allergy is increasing and is often associated with

birch- and grass pollinosis, as well as latex–fruit syn-

drome. Oral allergy syndrome is a manifestation of

food allergy, where certain foods cause itching or

hives when they touch the lips and mouth. Fruit and

vegetables are often responsible for this reaction,

owing to pollen cross-reactivity, and many fruit

allergens are PR proteins.

0010 Allergens homologous to the major birch pollen

allergen Bet v 1 (PR group 10), are found in apple

Mal d 1, cherry Pru av 1, apricot Pru ar 1, pear Pyr

c1, carrot Dau c 1, and celery Api g 1, but may trigger

responses to many other fruits and vegetables. By

contrast, in areas free of birch, the major allergens

in patients sensitized to Rosaceae fruits are lipid

transfer proteins (LTP, PR group 14), 9-kDa proteins

that often have antifungal and antibacterial activities.

LTP allergens characterized include apple Mal d 3 and

peach Pru p 3, but homologous proteins are found in

a range of plant-derived foods (vegetables, nuts, and

seeds), and as a consequence, LTPs have been de-

scribed as panallergens.

0011Class-I chitinases (PR group 3) are a group of car-

bohydrases active against the exoskeletons of insects

and the cell walls of fungi. They are widely distrib-

uted in plants and are also termed panallergens, with

characterized allergens including those from avocado

(Prs a 1) and banana (Mus p 1.2). Wound-induced

chitinases (Win) have been extracted from virus-

infected leaves (PR group 4) and have been reported

in turnip, tomato, and potato. Another group of

carbohydrases, the glucanases (PR group 2), are

active against fungal cell walls, have been isolated

from latex, and are recognized by IgE from food-

allergic patients with hypersensitivity to banana,

potato, and tomato. Lastly, thaumatin-like proteins

(PR group 5) are another group of allergens that have

antifungal and/or antibacterial activity, allergens

having been characterized in apple (Mal d 2)and

cherry (Pru a 2).

0012NonPR protein allergens include the profilins, ori-

ginally identified from birch pollen, Bet v2, which are

now recognized as cross-reactive plant allergens in a

wide range of fruits, vegetables, nuts, and seeds. Thiol

proteases (proteolytic enzymes) have also been iden-

tified as allergens in kiwi Act c 1, papaya, fig, pine-

apple, and soybean. Patatin, a storage protein from

potato, has also been shown to be an allergen in raw

potato and cross-react with the latex allergen, Hev v 7.

Tree Nut and Peanuts

0013Nut allergy is the most common form of food allergy

and the most common cause of death by anaphylaxis.

The major allergens are 2S albumins and the 7S and

11/12S globulins. In peanut, three of the major

allergens are major storage proteins: Ara h 1 is the

7S globulin, conarachin, a vicilin-like glycoprotein;

Ara h 2 is a 2S albumin; and Ara h 3, is a subunit of

the legumin-like 11S globulin, arachin. The IgE bind-

ing epitopes of Ara h 1 and 2 have been determined

and mutagenized to alter their activity. Brazil nut

contains a 2S storage protein allergen, Ber e 1;a

major allergen is a 2S albumin with a high methionine

content. Related 2S albumins are found in walnut,

Jug r 1, which also contains a 7S globulin allergen,

Jug r 2. In addition to storage protein allergens, chest-

nut contains an endochitinase allergen Cas s 1,and

ALLERGENS 145

tbl0001 Table 1 Major food allergens of plant origin

Food Allergen Allergen designation Molecular weight Functionand stability

Fruit

Apple Hom: Bet v 1 Mal d 1 17 700 Possible sterol binding protein (PR-10)

Hom: thaumatin Mal d 2 31 000 Antifungal activity (PR-5)

LTP Mal d 3 9 000 Antifungal and antimicrobial activity, heat-stable

(PR-14)

Apricot Hom: Bet v 1 Pru ar 1 17 700 Possible sterol binding protein (PR-10)

LTP Pru ar 3 9 000 Antifungal and antimicrobial activity, heat-stable

(PR-14)

Avocado Endochitinase Prs a 1 32 000 Degrade chitin (PR-3)

Banana Chitinase Mus p1.1 32 000 Degrade chitin (PR-3)

Chitinase Mus p1.2 34 000 Degrade chitin (PR-3)

Cherry Hom: Bet v 1 Pru av 1 17 700 Possible sterol binding protein (PR-10)

Hom: thaumatin Pru av 2 23 300 Antifungal activity (PR-5)

Profilin Pru av 4 15 000 Actin (cytoskeleton)-binding protein

Kiwi Cysteine protease Act c 1 30 000 Proteolytic enzyme

Peach LTP Pru p 3 9 000 Antifungal and antimicrobial activity, heat-stable

(PR-14)

Pear Hom: Bet v 1 Pyr c 1 18 000 Possible sterol binding protein (PR-10)

Profilin Pyr c 4 14 000 Actin (cytoskeleton) binding protein

Isoflavone reductase Pyr c 5 33 500 Enzyme involved in secondary metabolism

Plum LTP Pru d 3 9 000 Antifungal and antibacterial activity, heat-stable

(PR-14)

Vegetables

Carrot Hom: Bet v 1 Dau c 1 16 000 Possible sterol binding protein (PR-10)

Celery Hom: Bet v 1 Api g 1 16 000 Possible sterol binding protein (PR-10)

Profilin Api g 4 14 300 Actin (cytoskeleton)-binding protein

Unknown Api g 5 55/58 000 Unknown

Potato Patatin Sola t 1 43 000 Storage protein

Turnip Hom: prohevein Bra r 2 25 000 Defense protein

Nuts

Brazil nut 2S albumin Ber e 1 9 000 Storage protein

Cashew nut Anacardien Ana o 50,0000 Storage protein

Chestnut Chitinase Cas s 1 30 000 Degrade chitin (PR-3)

Hazelnut Hom: Bet v 1 Cor a 1.0401 17 000 Possible sterol binding protein (PR-10)

Peanut Cor Arachin Ara h 1 63 500 Storage protein

Conglutin Ara h 2 17 000 Storage protein

Arachin Ara h 3 60 000 Storage protein

Arachin Ara h 4 37 000 Storage protein

Profilin Ara h 5 15 000 Actin (cytoskeleton)-binding protein

2S albumin Ara h 6 15 000 Storage protein

2S albumin Ara h 7 15 000 Storage protein

Walnut 2S albumin Jug r 1 15 000 Storage protein

Vicilin Jug r 2 44 000 Storage protein

Seeds

Barley Inhibitor Hor v 1 15 000 Glycosylated a-amylase inhibitor

Inhibitor CMb 16 000 Glycosylated subunit of barley tetrameric

a-amylase inhibitor

Castor bean 2S albumin Ric c 1 10 000 Storage protein

Lentil Vicilin Len c 1 45-50,0000 Storage protein

Maize LTP Zea m 14 9 000 Antifungal and antibacterial activity (PR-14)

Mustard 2S albumin Bra j 1 14 000 Storage protein

2S albumin Sin a 1 14 000 Storage protein

Rice RAP 14 800 Rice allergenic protein

Rye Inhibitor Sec c 1 14 000 a-Amylase/trypsin inhibitor

Sesame 2S albumin Ses i 1 10 000 Storage protein

Vicilin Ses i 3 60,0000 Storage protein

Soybean Cys protease

(inactive)

Gly m 1 7 000 Function

Gly m 2 8 000 Unknown

Profilin Gly m 3 14 000 Actin (cytoskeleton)-binding protein

Wheat Inhibitor CM16* 16 000 Glycosylated subunit of wheat tetrameric

a-amylase inhibitor

Prolamin g-gliadin 30-50,0000 Storage protein of glutenin

146 ALLERGENS

peanuts a profilin, Ara h 5. Other nuts, such as

cashew, hazelnut, pecan, and pine nuts are known to

be allergenic, and contain storage globulin, 2S albu-

min and LTP allergens.

Seeds

0014 Seed allergens are predominantly nonPR-type pro-

teins, comprising storage proteins, proteases and a-

amylase inhibitors. Wheat and related cereals, barley

and rye, contain a range of allergens including the

prolamins (alcohol soluble storage proteins), which

are responsible for food-dependent exercise-induced

anaphylaxis and atopic dermatitis; the same proteins

are active in celiac disease and dermatitis herpetifor-

mis. Inhibitors of proteases and a-amylases protect

the seed from microorganisms and insect pests, char-

acterized allergens including the 12–15-kDa proteins

and inhibit a-amylases or trypsin and are both

inhalant allergens (e.g., baker’s asthma) and food

allergens. Those with the highest activity are glycosy-

lated subunits of tetrameric a-amylase inhibitors

from wheat (CM16) and its homologs from barley

(CMb) and BMAI-1 and rye Sec c 1, similar proteins

being found in rice. The major allergens in rice are a-

amylase and trypsin inhibitors, and in buckwheat, the

major allergen is another trypsin inhibitor. Peroxid-

ases in wheat and barley (PR group 9) of 36 kDa have

been identified as allergens in baker’s asthma.

0015 The major allergens in yellow mustard Sin a 1 and

oriental mustard Bra j 1, castor bean, cottonseed,

chickpea, oilseed rape BnIII, and sunflower SFA8

are 2S albumins (apart from sunflower, which con-

sists of a single polypeptide chain), and they consist

of two polypeptide chains linked via disulfide bonds.

In soybean, the two major storage globulins,

b-conglycinin and glycinin, are major allergens;

other allergens are profilin Gly m 3 and a papain-

related thiol protease Gly m Bd28.

Allergens of Animal Origin

Egg

0016 The allergens primarily originate from egg white, the

major allergens being ovomucoid (Gal d 1) and oval-

bumin (Gal d 2), which constitute 10 and 50% of

white proteins, respectively. Both proteins are heavily

glycosylated with 25% of the mass of ovomucoid

comprising carbohydrate. However, in ovomucoid,

the IgE epitopes are clustered in seven regions of the

protein sequence and do not encompass glycosylation

sites. Whilst IgE epitopes of ovomucoid appear to be

conformational in newly acquired sensitivities to egg,

linear epitopes are more important in long-standing

egg allergy. The IgE epitopes of ovalbumin are also

resistant to enzymic digestion and denaturation, and

seven IgE-binding regions have been identified, which

tend to cluster at the N- and C-terminal regions of

the protein. Ovalbumin has also been found in an

immunologically active form in the blood of human

subjects following consumption of raw egg. Two

minor allergens, ovotransferrin (Gal d 3) and lyso-

zyme (Gal d 4), have also been identified in egg white.

In general, cooking has been found to reduce the

allergenic activity of egg.

Cows’ Milk

0017Allergens are found in both the whey and casein

fractions, the most common allergens being b-lacto-

globulin and a-casein, although other IgE-reactive

proteins have been identified (Table 2). b-Lactoglo-

bulin is highly resistant to proteolysis and is taken up

in an intact form by the gut in experimental systems,

with the major IgE epitopes having been in four main

regions located on the more mobile surface loops

of the protein. The caseins also appear to contain

thermostable epitopes, with IgE binding being located

in around seven different regions of the protein.

Fish

0018The major allergen in fish is the muscle protein parv-

albumin (Gad c 1), which belongs to a group of such

proteins unique to fish and amphibians. Whilst it has

only been characterized in depth for cod and salmon,

parvalbumin is conserved across fish species, a factor

responsible for the cross-reactive nature of allergens

in cod, salmon, mackerel, herring, and plaice, amongst

many other fish species. It acts as a calcium buffer

protein in fast muscle, binding the Ca

2þ

ions in a

calcium-binding motif known as an EF-hand. Gad

c1has been shown to bind IgE in five regions evenly

distributed along the length of the protein, one of

which encompassed one of the Ca

2þ

-binding sites.

Like other calcium-binding proteins, Gad c 1 is

heat-stable, with the holo-form being both more

IgE-reactive and more heat-stable than the apo

form.

Shellfish and Seafood

0019Tropomyosin, a heat-stable muscle protein, is the

major allergen in shellfish and seafood, with highly

homologous proteins being found in the commonly

edible crustaceans. These homologies are responsible

for the cross-reactive allergies observed between vari-

ous types of seafood including shrimps, lobsters, crab,

squib, and abalone, and inhalant insect allergens,

such as those from cockroaches. Two main linear

IgE-binding sites have been identified in the shrimp

allergen, Pen i 1, one in the N-terminus, which

ALLERGENS 147

showed no homology with vertebrate tropomyosin,

and another in the C-terminal region. The first two

residues of the C-terminal epitope appear to be cru-

cial for IgE binding and are not found in vertebrate

tropomyosin. As a consequence of the lack of hom-

ology in the IgE epitopes, there is no cross-reactivity

between IgE from shell-fish allergic individuals and

animal muscle tropomyosins. In addition to being

found in cooked meat, the allergen also leaches into

cooking water.

Meat

0020 Meat allergy is rare, the major allergens having been

identified as serum albumins and IgG, both of which

are also minor milk allergens. However, unlike shell-

fish, muscle proteins such as tropomyosin do not

appear to be involved. Processing has been found to

reduce the allergenic activity of meat allergens, with

more severe processes, such as sterilization, causing

the greatest reduction.

Factors Altering the IgE reactivity of Food

Allergens

0021 Postharvest treatments and processing can potentially

alter the IgE reactivity of allergens by altering the

levels of allergens present in raw materials and

altering IgE-epitope presentation.

Effects of Raw Material and Postharvest Handling

0022A number of plant protein allergens belong to multi-

gene families, where a number of isoforms differing in

only a few amino acid residues may be present. In

addition, the levels of different isoforms may vary

between cultivars. These changes have been demon-

strated to make a difference particularly with regard

to the apple allergen Mal d 1, where the IgE reactivity

of certain cultivars is demonstrably higher than

others. Furthermore, one variant, Mal d 1a, has a

higher IgE-binding capacity than another variant

Mal d 1b, which differs by only 15 amino acid

residues. Allergen levels appear to increase in apples

following harvest and during storage at 4



C over 130

days, probably because of their association with the

ripening process, and the fact that they are ‘PR’ pro-

teins and hence synthesized in response to environ-

mental stress. Storage under modified atmospheres

can reduce these increases in allergen levels, as

might be anticipated from its ability to delay fruit

ripening.

Effects of Processing

0023There are several food proteins that do not adopt a

compact globular three-dimensional structure, but

are highly mobile, adopting an ensemble of conform-

ations. These include the g-gliadin from wheat and

caseins from milk. The IgE epitopes on such proteins

tbl0002 Table 2 Major food allergens of animal origin

Food Allergen Allergen designation Molecular weight Functionand stability

Hen’s egg Ovomucoid Gal d 1 28 000 Protein source with protease inhibitor

Ovalbumin Gal d 2 42 000 Serpin family member

Ovotransferrin Gal d 3 70 000 Iron-binding protein and bacteriocide

Lysozyme Gal d 4 14 300 Carbohydrase and bacteriocide

Cows’ milk b-Lactoglobulin Bos d 5 14 000 (monomer) Dimeric protein belonging to the lipocalin

superfamily that binds a number of small

lipophiles such as retinol

a-Lactalbumin Bos d 4 14 200 Calcium-binding protein

Serum albumin Bos d 6 66 000 Fatty-acid-binding protein

IgG Bos d 7 150 000 Immunoglobulin

a-Casein Bos d 8 32 400 Phosphoprotein able to chelate calcium

b-Casein Bos 26 600 Phosphoprotein able to chelate calcium

Fish Parvalbumin Gad c 1 (cod) 12 500 Calcium-binding muscle protein that is

pH- and heat-stableSal s 1 (salmon)

Shellfish Tropomyosin Pen a 1 (northern brown shrimp) 36 000 Contractile muscle protein that is heat-stable

Pan i 1 (Indian white shrimp) 38 000

Met e 1 (greasy black shrimp) 34 000

Pan s 1 (spiny lobster)

Hom a 1 (American lobster) 34 000

Cha f 1 (crab) 34 000

To d p 1 (squid) 34 000

Hal m1 (abalone) 38 000

49 000

Meat Serum albumin Bos d 6 (beef) 68 000 Fatty-acid-binding protein

Bos d 7 (beef) 150 000 Immunoglobulin

148 ALLERGENS

are likely to be linear and hence less affected by

thermal treatments than those on globular proteins.

In contrast, globular proteins undergo a marked de-

naturation at around 50–60



C, which is accompan-

ied by a loss of tertiary and secondary structure and is

usually followed by aggregation.

0024 Cross-reactive allergens, such as those involved in

the pollen–fruit and latex–fruit allergy syndromes,

are generally destroyed by cooking, and hence the

allergic reactions are confined to raw produce. In

addition, heating has been found to reduce the

allergenicity of beef and purified bovine allergens.

However, many other food allergens are remarkably

heat-stable, particularly those originating from plant

seeds, which are involved in sensitization via the

gastrointestinal tract. This is illustrated by Figure 2,

which shows the differential scanning calorimetry

(DSC) thermogram for glycinin, indicating that the

main thermal transition for this protein is around



C. There is also evidence that, whilst many of

highly disulfide bonded plant proteins unfold on

heating, they are able to refold on cooling to an

almost native structure. Hence, many allergenic

proteins may be able to retain both linear and con-

formational epitopes following processing, allowing

an IgE-mediated reaction towards the native and pro-

cessed protein. However, the unfolding and subse-

quent aggregation processes that occur in many food

proteins offer the possibility of introducing new epi-

tope sites. This applies to many allergens, including

those from soya, egg, and shellfish meat, which form

aggregates on heating that become incorporated into

heat-set gel networks. Thus, allergic IgE binding to

roasted nuts has been shown to be 90 times greater

than that observed towards raw peanuts, indicating

that processing does introduce additional IgE binding

sites. The technical difficulties of working with large

aggregated protein systems has meant that the char-

acterization of such processing-induced epitopes has

been difficult.

0025Carbohydrate residues on glycoproteins have also

been implicated as thermostable IgE epitopes, al-

though some workers have shown that their removal

does not affect IgE reactivity. Furthermore, because of

their sparse nature in some allergens, they do not

offer the polyvalency necessary IgE cross-linking on

mast cells and subsequent histamine release. Whilst

they have not been well characterized, there is now

evidence that Maillard browning adducts formed

following severe heat treatments contribute to the

allergenic activity of roasted nuts such as peanuts

and pecan nuts. It appears that Maillard modified

peanut allergens Ara h 1 and Ara h 2 become cross-

linked to form high-molecular-weight aggregates that

bind IgE more effectively than unmodified allergens

and are also more resistant to gastric digestion. The

role that other thermally induced modification, such

as lactosylation of milk proteins, may play in altering

the allergenic activity of products, including severely

heat-treated milks, has yet to be characterized in

detail.

Problems of ‘Hidden’ Allergens

0026Those individuals who suffer from severe forms of

food allergy, such as anaphylaxis, can have reactions

triggered by very small amounts (as little as 100 mg) of

an allergen present in a food. Indeed, traces of nuts

found in processed oils and carry-over of allergens on

utensils used for serving foods can be enough to cause

an allergic reaction. Proteins are widely used as func-

tional ingredients in foods, with soya flours being

routinely included in bread and bakery products as

an improver, whereas other soya ingredients are used

in soups and sauces as well as meat products such as

sausages. Similarly, caseinates and whey protein isol-

ates are used in a range of products such as sauces,

cakes, and confectionery.

0027Another difficulty is presented by the uncertainty

relating to accidental contamination, where process-

ing lines are shared between products utilizing differ-

ent ingredients. This is particularly problematic for

confectionery, where tree nuts and peanuts are a

favored ingredient and has led to labeling of products

with the warning ‘may contain nuts.’ Indeed, surveys

have shown that a large proportion of chocolate

confectionery is contaminated with hazelnut pro-

teins, probably as a result of cross-contamination in

−0.25

−0.20

−0.15

−0.10

−0.05

0.0

0.05

0.10

0.15

0.20

0.25

0.30

70.0 80.0 90.0 100.0

Temperature (⬚C)

Heat flow (mW)

fig0002 Figure 2 Thermostability of the soya allergen, glycinin, as

determined by DSC. Native protein (——); after treatment at



C, 10 min (–); 80



C, 10 min (---) and 90



C, 10 min (---).

ALLERGENS 149

factories during manufacture. One of the conse-

quences of this for the allergic consumer is that aller-

genic proteins are hidden as ingredients in

manufactured products. Accidental exposure to aller-

gens is a frequent occurrence, and epidemiological

studies have shown that around 50% of peanut-aller-

gic individuals have a reaction at least once a year as a

consequence of accidental exposure.

0028 In order to address concerns over inadvertent

contamination of raw materials by allergenic nuts

and seeds, methodologies have been developed for

detecting allergens in raw materials and finished

products. The favored method of analysis is immuno-

assay, particularly the form known as enzyme-linked

immunosorbent assay (ELISA). In general, the

approach adopted has been to develop either a poly-

clonal or a monoclonal antibody preparation that is

specific for a major allergen present in a food stuff,

such as Ara h 1 from peanuts. Whilst it is possible to

develop sensitive calibration curves for allergens in

buffer, difficulties can be encountered when analyzing

foodstuffs, where the presence of fats and polyphenol

components found in products such as chocolate can

reduce assay sensitivity by 10-fold. Nevertheless, such

assays have been developed for a range of allergenic

proteins, including milk proteins, hazelnuts, and

peanuts, and are available for manufacturers, retail-

ers, and other analysts to detect allergen contamin-

ation. Another exciting new development is the

application of highly sensitive mass spectrometry

methods to allergen analysis, which has the potential

to offer a second orthogonal method of analysis

to immunoassay, as is desirable for purposes of

enforcing labeling regulations and hazard control

procedures.

See also: Food Intolerance: Types; Food Allergies; Milk

Allergy; Lactose Intolerance; Elimination Diets; Food

Labeling (Labelling): Applications; Immunoassays:

Principles; Radioimmunoassay and Enzyme

Immunoassay; Protein: Chemistry; Food Sources;

Determination and Characterization; Interactions and

Reactions Involved in Food Processing; Heat Treatment

for Food Proteins

Further Reading

Bousquet J, Bjorksten B, Brujijnzeel-Koomen CAFM et al.

(1998) Scientific criteria and the selection of allergenic

foods for product labelling. Allergy 53: 3–21.

Breiteneder H and Ebner C (2000) Molecular and bio-

chemical classification of plant-derived food allergens.

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ALMONDS

D E Kester and A A Kader, University of California

Davis, Davis, CA, USA

S Cunningham, Blue Diamond Almond Growers,

Sacramento, CA, USA

Introduction

0001 The almond is the major commercial tree nut crop

of the world. This importance has been achieved by

the very large increase in acreage and production in

California during the past 25 years. This article

reviews the areas of production, principal cultivars,

important uses, and methods of handling and storage.

The Crop and its Importance

Global Distribution

0002The cultivated sweet almond (Prunus dulcis Miller

(D. A. Webb) syn. P. amygdalus Batsch) originated

from within the wild species known originally as

150 ALMONDS

Amygdalus communis L. which grew on the lower

slopes of mountains in central Asia. About 30 related

almond species have been described occupying spe-

cific ecological niches in the arid steppes, mountains,

and deserts of central and south-western Asia and

southern Europe.

0003 The geographical range of the cultivated almond

corresponds to the three stages of cultural evolution:

(1) Asiatic (south-west and central Asia); (2) Mediter-

ranean (countries bordering both sides of the Medi-

terranean sea); and (3) Californian (central valleys of

California, parts of Australia, central Chile, and areas

of South Africa).

0004 Almonds are adapted to Mediterranean, steppe,

and desert climates characterized by mild, rainy

winters and hot, dry summers. Although they have

traditionally been grown with other arid tree and vine

crops, such as olive, pistachio, and grape, almond

trees respond so well to supplementary irrigation,

fertilizers, good soil, disease, and insect control and

other intensive culture methods that yields can be

increased five-to 10-fold over the traditional culture

practiced for centuries.

Commercial Importance

0005 Since 1950, almonds have become the most import-

ant world tree nut crop, with present annual

production of approximately 350–400 10

(800–900 10

lb). In any given year, California

produces about 70% of the world’s supply of

almonds (Figure 1), with most of the balance

coming from Mediterranean countries (Spain, Italy,

Greece, Portugal, Morocco, Tunisia, Canary Islands,

France). Almonds are also grown, often in primitive

culture, as local crops in Iran, Israel, Afghanistan,

Pakistan, south-eastern Russia, north-west India,

Syria, Turkey, and Iraq.

Cultivars

0006 Older botanical literature described two botanical

varieties as Prunus amygdalus var. dulcis, the sweet

almond, and P. amygdalus var. amara, the bitter

almond. In reality, bitter and sweet kerneled trees

exist side by side in seedling populations with the

same parents; the difference is apparently due to a

single dominant gene. Some of the more primitive

almond-growing areas (e.g., in Asia and Morocco)

still grow almond trees as seedling populations (wild

or cultivated).

0007Modern orchards grow selected cultivars, which

are clones that are vegetatively propagated by bud-

ding or grafting upon specific rootstocks. Rootstocks

include almond and peach seedlings, as well as certain

plums and peach  almond hybrids. Historically,

many cultivars were apparently identified when su-

perior individual trees were selected to replace ‘bitter’

trees within seedling orchards. In recent years, con-

trolled breeding has created cultivars with special

qualities. Use of genetic engineering methods will

augment genetic improvement.

0008Although many cultivars exist in the world, a

limited number have dominated industrial and local

use within specific production areas. Cultivars are

selected for performance in the orchard. Flowers on

most current cultivars are self-incompatible, so that

two or more cultivars that bloom at nearly the same

time need to be planted together, preferably in

adjoining rows. Hives of honeybees are placed within

the orchard to effect cross-pollination. Combinations

of cultivars are also chosen to facilitate an economical

harvest. Other cultivar characteristics of importance

include yield, tree habit, disease and insect resistance,

ease of harvest and processing, and other specific

characteristics. Nuts of different cultivars vary from

very hard-shelled to soft- and paper-shelled. Cultivars

are also chosen for their marketing potential, and

specific marketing classes have evolved.

0009California Nonpareil accounts for around 44% of

the production. Kernels are uniform in shape, rela-

tively flat, smooth, attractive in appearance, easy to

blanch, and are versatile in processing and utilization.

The nut has a thin, papery shell which is easy to

remove without damaging the kernel. This shell

is subject to worm and bird damage. The cultivar is

subject to a genetic variant referred to as noninfec-

tious bud failure which is controlled through selec-

tion and management of propagation sources.

0010Mission (Texas) is an old cultivar which produces

small, round, plump nuts. Kernels have a relatively

strong almond flavor owing to higher levels of amyg-

dalin. The pellicle is difficult to remove during

blanching. The shell is hard and protective of worm

damage. Padre is a similar cultivar, the kernels of

which can be combined with Mission. Peerless is

grown to a limited amount as a pollinator of

200

400

600

800

1000

1200

1400

79 80 81 82 83 84

California

Million pounds

All other

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99

fig0001 Figure 1 Almonds produced in California in comparison to

world production.

ALMONDS 151

Nonpareil. The shell is hard and the nuts are sold

primarily in the shell.

0011 Ne Plus Ultra has a large, elongated kernel, blooms

early, and cross-pollinates Nonpareil. The cultivar is

gradually becoming obsolete in California, but orch-

ards still exist and it represents 3% of the total crop.

0012 In the mid-1960s, Merced and Thompson were

planted to replace Ne Plus Ultra, Peerless, and Mis-

sion. Similarly shaped and blanchable kernels of these

and later introduced cultivars have been mixed and

marketed under the name California. Other cultivars

of this group include Price, Fritz, Monterey, and

others.

0013 Carmel became popular in the mid-1970s and is

the second most important cultivar in new plantings

(18% of almond acreage in 1996). Sonora was intro-

duced in the mid-1970s and is becoming popular as

an early-blooming pollinator for Nonpareil. Sonora

has large, elongated nuts and is sometimes marketed

with Carmel.

0014 Additional cultivars that have recently been

planted in California include Butte, Livingston,

LeGrand, Mono, and Ruby. Their nuts are grouped

either as Mission or California marketing classes.

0015 Spain Marcona and Desmayo are the most import-

ant cultivars in Spain, each accounting for about 25%

of Spanish production. Marcona has large, heart-

shaped, flat, high-quality kernels which blanch easily.

Desmayo produces large, elongated, flat kernels, the

pellicle of which is thin and can be readily removed

after roasting by rubbing between the fingers. Many

other cultivars exist in specific production areas of

Spain but their kernels are usually mixed together for

marketing.

0016 Italy Most of the older areas (Puglia, Sicily, and

Sardinia) are reducing production. Kernels of the

many cultivars are mixed together for marketing.

The main cultivars in newer orchards include Tuono,

Felipo Ceo, and Genco, which are self-fertile.

0017 France New, late-blooming, high-yielding, vigorous

cultivars derived from breeding programs are being

planted not only in France but other countries of

the Mediterranean area. These cultivars include Fer-

ragnes and Ferraduel, kernels of which are marketed

together with those from other cultivars.

Morphology and Anatomy

Fruit

0018 The almond fruit is known botanically as a drupe,

which is composed of three basic parts: (1) exocarp

(skin), which in almond is pubescent; (2) mesocarp,

which is fleshy and becomes the hull; (3) endocarp

(shell). This overall structure is the same as stone

fruits, such as peaches, plums, and apricots, but

differs in that the almond mesocarp does not enlarge

into fruit, but dehisces (splits) at maturity and dries.

The hull is an important feed for livestock.

Seed

0019The almond kernel is the seed which develops from

the ovules. Although two ovules are present in the

flower, only one normally develops to produce a

single kernel. The seed is edible because it lacks amyg-

dalin, the bitter compound in many seeds. The seed

consists of the embryo (also referred to as the meat),

surrounded by seed coats or testae (also referred to as

the pellicle and sometimes the skin). The embryo

consists of the hypocotyl root axis with growing

points for roots and shoots which appear upon

germination, and two massive cotyledons, which are

storage organs and contain the high-energy com-

pounds characteristic of almond.

0020Almond kernels of different cultivars have charac-

teristic sizes, shapes, appearances, thickness of pellicle

and, to some extent, flavor. Double kernels result

when two kernels develop within the same nutshell.

Twin kernels result when two or more embryos

develop within the same pellicle.

Harvesting, Handling, and Storage

Harvesting

0021Almond nuts should be harvested as soon as possible

after maturation to avoid quality losses and to min-

imize problems with fungal attack and insect infest-

ation. Indices used to determine maturity stage and

optimum harvest dates for almonds include hull

dehiscence (splitting), separation of the hull from

the shell, decrease of the fruit removal force, and

drying of hulls and kernels. Almond harvesting in

California is by machines. Almonds are shaken from

the tree to the ground by various kinds of shakers.

After about a week of drying, the nuts are raked into

rows and picked up by mechanical harvesters. They

are then transported in bulk to hulling machines

which separate the hull from the in-shell product.

Postharvest Handling

0022Dehydration of the kernel begins while the nut is still

on the tree. Further drying takes place after the nuts

are knocked to the ground. Under rainy and/or cool

weather conditions, heated-air dehydrators may be

used to reduce moisture content to 7% or less. Most

almonds sold commercially range from 4% to 5% in

152 ALMONDS

moisture. Dried almonds are transported in bulk to

processing plants where they are stored in bins, silos,

or other bulk-storage containers for a few weeks to

several months before final processing and prepar-

ation for market.

0023 In-shell almonds are fumigated with methyl brom-

ide or aluminum or magnesium phosphide, which are

lethal to all insect life stages. Residues are monitored

to insure that levels in finished products are below

legal limits. (See Fumigants.)

0024 Alternatively, almonds can be kept in a controlled

atmosphere of 0.5% oxygen and about 10% carbon

dioxide (balance nitrogen) to control insects. The

nuts can be frozen for insect control at home, or for

small-scale operations. (See Controlled-atmosphere

Storage: Applications for Bulk Storage of Foodstuffs.)

Storage

0025 Compared to other nuts, almonds and almond prod-

ucts have a long life. This is attributable in part to

their low moisture, the presence of relatively low

levels of polyunsaturated fatty acids, and high toco-

pherol levels.

0026 Raw almonds should be stored at 0–5



C, and

65–70% relative humidity (RH), to minimize deteri-

oration during storage. Prolonged exposure to direct

sunlight causes the skins to darken and reduces shelf-

life. Since almonds readily absorb odors, they should

never be exposed to pungent odors from onions, fresh

fruits, fish, cheeses, paint, chemicals, or other

products.

0027 Both the maintenance of quality and safety of

almond and the extension of storage life depend

upon the initial moisture content, the RH and tem-

perature of storage, and the exclusion of oxygen

and insect pests. (See Storage Stability: Parameters

Affecting Storage Stability.)

0028 The Food and Drug Administration (FDA) regula-

tions for tree nuts define a safe moisture level (i.e.,

moisture content which will not support fungal

growth) as a water activity level that does not exceed

0.70 at 25



C. This is equal to a moisture content

of about 7%. The optimum range for unroasted

almonds is 4–6% moisture. The equilibrium moisture

of almonds ranges from 3.8 to 5.0% at 30% RH and

from 8.1 to 10% at 75% RH at 5



0029 The relationship between moisture content and

equilibrium relative humidity (ERH) is temperature-

dependent. From 20% to 80% ERH (for any given

moisture content), ERH rises approximately 3% for

every rise in temperature of 10



C. At a given RH, air

will contain more water vapor at a high temperature

than at a low temperature. Temperatures between 0



and 5



C are recommended for almonds; the lower

temperatures allow longer storage life (up to a year).

0030Low oxygen (0.5% or lower) atmospheres are

beneficial for keeping flavor quality, delaying rancid-

ity, and controlling stored products against insects.

Exclusion of oxygen is usually achieved by vacuum

packaging or replacement with nitrogen in storage

facilities and transport vehicles. The storage life of

raw almonds can be extended up to 2 years under

low-oxygen atmosphere at 0



C. (See Chilled Storage:

Packaging Under Vacuum.)

Quality and Safety Factors

0031Important appearance factors for almonds marketed

in the shell include shell integrity, suture opening,

and shell color. Kernel defects include damage from

insects, mold and mechanical injury, gum, callus

growth, shriveling and doubling. Kernels are graded

for size. Quality criteria used in the grade standards

include freedom from dust particles and other foreign

materials, and uniformity of shape and color.

0032Textural factors, including crispness and firmness,

are influenced by moisture content. Thus, roasted

almonds are usually more crisp than raw almonds.

0033Flavor quality depends upon sweetness, oiliness,

intensity of almond flavor, and absence of off-flavors

resulting from rancidity, staleness, or other causes. A

problem called concealed damage, which appears as

internal darkening and poor flavor after roasting, is

related to wet conditions and high temperatures

during temporary storage following harvest.

0034Safety factors relate primarily to the potential con-

tamination of almonds with mycotoxins, especially

aflatoxin produced by the fungus Aspergillus flavus.

Contamination can occur in the orchard or during

postharvest handling if recommended handling and

storage procedures are not followed. Elimination of

nuts exhibiting any symptoms of fungal contamin-

ation is an integral part of market preparation. (See

Mycotoxins: Occurrence and Determination.)

0035Residues of pesticides used to control the major

insect pest, navel orange worm (Anyelois transitella

Walk), have been reduced by 40% during the last

10 years. This reduction was made possible through

such integrated pest management procedures as orch-

ard sanitation, maintaining natural predator balance,

and prompt harvest. (See Pesticides and Herbicides:

Residue Determination.)

0036Propylene oxide, the only sterilant approved for

nuts, is generally effective against bacteria, but less

effective against yeasts and molds. The nuts are

treated in a specially designed vacuum chamber.

After treatment, a series of air washes under vacuum

removes traces of the remaining gas. Each chamber

load then enters a postconditioning staging area until

cleared for both residue level and microbial load.

ALMONDS 153