Latin name: Carya illinoensis/Carya illinoinensis
Source material: Shelled & heated nuts
Common names: Pecan nut, Hickory nut
The large Pecan tree, native to and very common in southern and especially south-eastern North America, is important for its timber and edible nuts. The Pecan nut was a major food source for the American Indians for thousands of years.
The US South yields 250 million pounds of the nuts in an average year, Texas being the largest producer. The tree is planted far beyond this range, however, covering several of the world's warmer temperate zones and subtropical areas.
The Pecan tree bears sweet edible nuts, deep brown in color, that range from 2.5 to 5 cm in length. The nuts have smooth shells and a mild, Walnut-type flavor. Pecan nuts are not produced until trees are 5 or 6 years old. The nuts ripen from mid-September until December in the Northern Hemisphere. They are harvested after they fall to the ground. Pecans can last up to 6 months in their shells if refrigerated.
Over 100 varieties are commercially available, e.g., Shagbark hickory nuts and Mockernut hickory nuts. They are used whole or crushed. Oil from the nuts is an ingredient in processed foods, is used in the manufacture of cosmetics and soap, and is a drying agent in paints.
Only 1 Pecan allergen has been characterised to date:
- Car i 1, a 2S Albumin commonly called napin (1-2).
The allergen profile may change as the Pecan matures, is stored or heated. One study demonstrated that IgE antibodies to allergens present in aged or heated Pecan nuts could be detected, but no IgE to any in fresh Pecans. These neoallergenic determinants were located on protein(s) with a molecular weight of approximately 15 kDa (3). Maillard-type reactions may be responsible for the change in allergenicity of the protein (4). Pecan nut allergens in small quantities may result in severe reactions.
Furthermore, analysis of the native and heat-denatured proteins that were previously subjected to in vitro simulated gastric fluid digestions indicates that stable antigenic peptides are produced (5).
Pecan appears to be highly cross-reactive with Walnut at even the lowest concentrations used, and this correlates with clinical reports of cross-reactivity in the patients in question (6-7). Some degree of cross-reactivity between Peanut and Pecan has been observed (8).
Conformational analysis of the linear IgE-binding epitopes mapped on the molecular surface of Ara h 2 showed no structural homology with the corresponding regions of the Walnut Jug r 1, the Pecan nut Car i 1 or the Brazil nut Ber e 1 allergens. The absence of epitopic community does not support the allergenic cross-reactivity observed between Peanut and Walnut or Brazil nut, which presumably depends on other ubiquitous seed storage protein allergens, namely the vicilins. However, the major IgE-binding epitope identified on the molecular surface of the Walnut Jug r 1 allergen shared a pronounced structural homology with the corresponding region of the Pecan nut Car i 1 allergen. With the exception of Peanut, 2S albumins could therefore account for the IgE-binding cross-reactivity observed among some other dietary nuts, e.g. Walnut and Pecan nut (1).
IgE-mediated allergy to foods containing Pecan nuts is common. Pecan nut allergy frequently has an onset in the first few years of life and generally persists, accounting for severe and potentially fatal allergic reactions (9-12).
The Food Allergy and Anaphylaxis Network (FAAN) Peanut and Tree Nut Allergy Registry in the USA, covering 5149 patients (mainly children), reports Walnut as the first cause of allergic reactions to tree nuts in 34%, followed by Cashew (20%) and Almond (15%). Nine percent of self-reporting tree nut-allergic patients list Pecan as an allergen (13).
Pecan nut allergens may be absent in fresh Pecan nuts, which become allergenic as the Pecan nut protein matures during aging (2-3). Neoallergens may result from the heating process for roasting Pecan nuts, and have been identified as the cause in anaphylaxis in a child who experienced this condition after eating cookies containing these nuts (2).
In a group of 122 children who had experienced acute reactions to nuts, 68 reacted to Peanut alone, 20 only to tree nuts, and 34 to both. Of those reacting to tree nuts, 34 had reactions to 1 type, 12 to 2, and 8 to 3 or more, the most common being Walnut, Almond, and Pecan. Initial reactions usually occurred at home and were thought to result from a first exposure in 72% of cases. Eighty-nine percent of the reactions involved the skin (urticaria, angioedema), 52% the respiratory tract (wheezing, throat tightness, repetitive coughing, dyspnoea), and 32% the gastrointestinal tract (vomiting, diarrhoea). Two organ systems were affected in 31% of initial reactions, and all 3 in 21% of reactions (14).
IgE antibodies would appear to be most useful in confirming the diagnosis of nut hypersensitivity in children or in particularly highly allergic patients for whom skin testing poses a risk of anaphylaxis (5).
Commercial products may contain Peanuts that have been deflavoured and reflavoured and coloured to resemble Walnuts, Pecans, or Almonds. This may result in anaphylaxis in Peanut-sensitive individuals (15).
Protein contact dermatitis from Pecan nut has been reported (16).
A patient is described who experienced an acute vesicular cutaneous reaction after prolonged contact with pecans (17).
Compiled by Dr Harris Steinman, firstname.lastname@example.org
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