Technological advances in egg product processing with reference to allergenicity
Résumé
Egg is a widely used ingredient in many food products all around the world. It is indeed a major source of high quality proteins and essential nutrients and provides many desirable functional attributes such as foaming, emulsifying, gelling, colouring, and flavouring. However, hen’s egg is also one of the most frequent allergenic foods, particularly in childhood, affecting 1.6 to 3.2 % of young children in Europe.
Egg products represent 15 to 50% of egg consumption in Europe depending on the country. For microbiological safety reasons, thermal treatments are applied to egg products. Liquid egg products are thus pasteurized for 2 to 6 min between 56°C (for egg white) and 68°C (for whole egg and yolk), whereas egg white powders are dry-heated up to several weeks between 60 and 80°C. The effects of these treatments on egg functional properties have been widely studied; this is not the case for egg protein digestibility or allergenicity. The aim of our study was thus to investigate the effect of a wide range of pasteurization rates (on liquid whole egg) or dry heating rates (on egg white powder) on the in vitro digestibility, antigenicity and allergenicity of egg proteins.
Liquid whole egg was pasteurized from 2 to 6 min between 60°C and 66°C, and egg white powder was dry heated from 1 to 10 days between 60°C and 90°C. In vitro digestion of samples was performed using a model that mimics successive gastric and duodenal stages of digestion in the adult human. The degree of hydrolysis was quantified by OPA method at the end of each stage, and the SDS PAGE pattern was followed throughout the in vitro digestion. Ovomucoïd, ovotransferrin and lysozyme antigenicity was tested by inhibition ELISA with specific monoclonal antibodies. Liquid whole egg allergenicity was investigated in 54 egg-allergic children by skin prick testing and IgE-binding pattern to egg proteins was determined by indirect ELISA. Egg white powder allergenicity was tested on mice sensitized and challenged through oral route by measuring plasmatic histamine concentration, specific and total IgE and IgG levels, and cytokine concentration in spleen cell culture supernatants.
Pasteurization of liquid whole egg, from 4 to 10 min at 60°C, improved protein in vitro digestibility and decreased ovomucoïd antigenicity, but did not significantly change ovotransferrin and lysozyme antigenicity. Conversely, pasteurization from 4 to 10 min at 66°C decreased protein in vitro digestibility and lysozyme antigenicity, but increased drastically ovotransferrin antigenicity. Pasteurization (6 min at 66°C) had a limited impact on the prick test (PT) reactivity of most children. However, two main classes of patients were obtained from PT diameter variations: for 34 children, pasteurization had no impact on PT reactivity, whereas for 20 children, pasteurization significantly decreased reactivity. A large effect was even found for 4 children who had null PT with pasteurized liquid whole egg. No correlation was found between patterns of IgE-binding to egg proteins and patterns of PT-reactivity as a function of pasteurization.
Dry heating of egg white powder, from 1 to 10 days at 60°C, improved protein in vitro digestibility and slightly decreased lysozyme antigenicity. On the other hand, dry heating over 1 day at 80°C or 90°C decreased protein in vitro digestibility and lysozyme antigenicity, but drastically increased ovotransferrin antigenicity. Dry heating of egg white powder up to 7 days at 80°C had a limited impact on egg allergy tested in mice. No significant effect was found on histamine concentration, or specific and total IgE and IgG levels. Only a significant decrease of IL4 cytokine concentration was observed for mice groups fed with dry heated powders.
Heat treatments applied to egg products thus have, in most cases, a rather low impact on in vivo allergenicity, despite significant changes in protein digestibility and antigenicity.