Food Biochemistry and Food Processing (2 edition)

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42 Food Allergens 805

(MW∼300 kDa) has a hexameric structure with each monomer

comprising an acidic and basic subunit linked by disulfide bonds.

β-conglycinin (MW∼150 kDa) has a trimeric structure with

the three major subunitsα,α′andβhaving MWs of 76, 72

and 53 kDa, respectively (Liu 1997, Krishnan 2000). Various

studies have shown that all glycinin andβ-conglycinin subunits

bind IgE from soya bean-allergic patients although a few studies

have found an absence of binding to some subunits (Pedersen

and Djurtoft 1989, Krishnan et al. 2009, Boye et al. 2010).

The soya bean vacuolar protein (P34; MW of 34 kDa), which

is an oil body-associated protein, is also a major soya allergen.

Over 65% of soya bean-allergic patients reportedly react to it

(Ogawa et al. 1991, 1993). The KTI, which is a 21 kDa belonging

to the plant defence system, is another important allergen. KTI

exerts its effect by inhibiting the activity of proteases such as

trypsin. Although the major allergenic epitopes have not been

identified, reports suggest that it may be a culprit in soya bean-

induced occupational respiratory disorders (Baur et al. 1996)

along with the soya bean hydrophobic lipid transfer proteins

identified in soya bean hulls (Gonzalez et al. 1992, 1995).

Prevalence, Symptoms and Threshold

Prevalence rates of soya allergy range between 0.3% and 0.4%

(Becker et al. 2004, Sicherer and Sampson 2006). The exact

amount of soya proteins required to induce an allergic response

is, however, not known. Reported threshold levels vary signif-

icantly and range between 0.0013 and 500 mg of soya protein

(Bindslev-Jensen et al. 2002, Ballmer-Weber et al. 2007).

Effect of Processing on Soya Allergens

In general, processing treatments such as thermal treatment,

microwaving, irradiation, hydrolysis and fermentation have not

been shown to remove soya bean allergenicity. Some treatments

may cause modifications in proteins structure and hide aller-

genic epitopes reducing but not eliminating their immunogenic-

ity, whereas other treatments cause unfolding of the proteins,

allowing greater exposure of hidden allergenic epitopes and in-

creasing the immunogenic properties of the proteins (Davis and

Williams 1998, Soler-Rivas and Wichers 2001).

Soya bean is processed into products such as soya beverages,

tofu, edamame, miso, natto and tempeh. The beans can also be

processed to obtain soya oil, soya flour, soya lecithin, soya fibre,

soya protein concentrate (>65% protein on dry basis), soya pro-

tein isolate (>90% protein on dry basis), textured soya products

and soya hydrolysates or hydrolysed vegetable protein, which

are used in a wide variety of products. Other novel applications

include soya ice cream and soya yogurt. Allergenicity of soya

foods is linked to the presence of residual soya proteins in these

foods, so unless the soya ingredient used is highly refined (e.g.

refined soya bean oil), it is likely to contain allergenic protein

and must be labelled when used in foods. Studies conducted

on soya lecithin have generally reported little or no allergy risk

to sensitised individuals (Awazuhara et al. 1998); however, hy-

drolysed and fermented soya products tend to retain some of

their allergenic properties (Hefle et al. 2005, L’Hocine and Boye

2007). Appropriate caution therefore needs to be exercised in

the consumption of processed soya foods.

Soya Allergen Cross-Reactivities

Although not confirmed clinically, a high cross-reactivity

has been reported for different legumes (soya bean, peanut,

lentils and beans; Yunginger 1990, Eigenmann et al. 1996,

Kalogeromitros et al. 1996). This may be explained by the

high amino acid sequence homology, which frequently occurs

in plant proteins belonging to the same family. Clinical co-

reactivity rates in peanut-allergic patients for soya using placebo-

controlled challenges range between 1% and 6.5% (Burks et al.

1988). Additionally, coexisting clinical reactivity has been re-

ported between soya bean and cow’s milk for some allergic

patients ranging between 5% and 50% depending on the spe-

cific group of patients studied (Host and Halken 1990, Burks

et al. 1994).

PEANUT AND TREE NUT ALLERGENS

Peanuts and tree nuts contain some of the most potent food

allergens. Prevalence rates of 0.8–1.5% for peanut allergy in

the UK and US population and about 0.6% for tree-nut allergy

in the US population have been reported (Grundy et al. 2002,

Sicherer et al. 2003). Nuts and tree nuts belong to different plant

species; however, they are frequently discussed together as their

presence, handling and use in the food chain as well as the

allergic responses they induce are often similar.

Peanut (Arachis hypogea) is a legume belonging to the fam-

ilyLeguminosae.It grows under the ground in peanut pods

containing the peanut seed. Tree nuts, on the other hand, are

edible seeds that grow on trees. Examples of tree nuts that are

of most concern as allergens are hazelnut (Corylus avellana),

almond (Prunus dulcis), pistachio (Pistachia vera), macadamia

nuts (Macadamia integrifolia), cashew (Anacardium occiden-

tale), walnut (Juglans regia), pine nut (Pinus pinea), pecans

(Carya illinoinensis) and Brazil nut (Bertholetia excelssa).

Prevalence and Threshold

Prevalence rates of peanut and tree nut allergies vary for different

populations but appear to be higher in Western societies such as

in Europe and North America. Reported rates vary from 0.2 to

1.7 for peanut and 0.1 to 1.4 for tree nuts (Sicherer et al. 2003).

Allergic responses following ingestion of peanut and tree nuts

by allergic patients range from oral pruritus, nausea, vomiting,

urticaria, angiodema, bronchospasm, bronchitis, hypotension,

anaphylaxis and death in some instances. The reaction often

occurs within minutes to a few hours after food consumption, and

the severity of the response may be exacerbated by preexisting

asthma (Sampson 2002, Sicherer et al. 2003).

Threshold doses for clinical reactivity reported in the literature

range from 100μg to 10 mg for peanut and 20μg to 7.5 mg for

tree nuts (Hourihane et al. 1997, FDA 2006). Cross-reactivities

between peanut, soya bean as well as other tree nuts have been

reported (de Leon et al. 2003).