One mystery about allergies is how food allergens can survive the
transit through the stomach to cause trouble when they meet the immune
system in the intestines.  Many previous studies have suggested that
allergens can't survive the harsh mixture of acid and pepsin in the
stomach, however this new study explains how they can do it though both
in vitro and in vivo human studies.  The pectin in fruit appears to
protect allergens against the stomach's corrosive environment.

This may explain the advice given to people with yeast problems to eat
fruits separately from other foods like meats.  The traditional reason
for this advice is that meats and fruits have a different pH but I think
the protective effect of pectin may play a bigger role.

Clin Exp Allergy. 2007 May;37(5):764-71. Links

A matrix effect in pectin-rich fruits hampers digestion of allergen by
pepsin in vivo and in vitro.
Polovic N, Blanusa M, Gavrovic-Jankulovic M, Atanaskovic-Markovic M,
Burazer L, Jankov R, Cirkovic Velickovic T.

Department of Biochemistry, Faculty of Chemistry, University of
Belgrade, Belgrade, Serbia.

BACKGROUND: It is a general belief that a food allergen should be stable
to gastric digestion. Various acidic plant polysaccharides, including
pectin, are ubiquitous in fruit matrixes and can form hydrogels under
low-pH conditions. OBJECTIVE: The purpose of this study was to
investigate the effect of hydrogel forming polysaccharide-rich fruit
matrixes on in vivo gastric and in vitro pepsic digestion of fruit
allergens. METHODS: Fruit extract proteins (kiwi, banana, apple and
cherry) and a purified major kiwi allergen Act c 2 were digested with
simulated gastric fluid in accordance with the US Pharmacopeia. In vivo
experiments on kiwi fruit digestion were performed on four healthy
non-atopic volunteers by examining the gastric content 1 h after
ingestion of kiwi fruit. The Act c 2 and kiwi proteins were detected in
immunoblots using monoclonal anti-Act c 2 antibodies and rabbit
polyclonal antisera. RESULTS: Crude fruit extracts were resistant to
digestion by pepsin when compared with commonly prepared extracts. In
the gastric content of all volunteers, following kiwi fruit ingestion
and immunoblotting, intact Act c 2 was detected with anti-Act c 2
monoclonal antibodies, while kiwi proteins of higher molecular weights
were detected using rabbit polyclonal antisera. Addition of apple fruit
pectin (1.5% and 3%) to the purified kiwi allergen was able to protect
it from pepsin digestion in vitro. CONCLUSION: The matrix effect in
pectin-rich fruits can influence the digestibility of food proteins and
thereby the process of allergic sensitization in atopic individuals.

PMID: 17456224 [PubMed - in process]