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Vitamins are preserved in processed green vegetables

Most of the folates (vitamin B9) present in processed vegetables are preserved.  The greater the thermal intensity of the treatment they receive, the presence or absence of oxygen is a determinant parameter in the integrity of this vitamin.

Etalage de haricots verts
Updated on 05/14/2014
Published on 05/14/2014

Folates are vitamins that play a crucial role in human health.  It is advised that adults should consume approximately 300 µg/day, children 150 to 250 µg/day (depending on the age group), and pregnant women 400 µg/day.  Fruits, and particularly green vegetables, are good sources of folates, but most of them are consumed in a processed form such as frozen or canned products, or more generally they are cooked.  In the context of a thesis project, scientists focused on the impact of transformation processes on the preservation of this vitamin, taking two products as their examples: canned green beans and frozen spinach.

The astonishing stability of folates

Folates exist in different chemical forms, some of which appear to be particularly stable.  The research team thus demonstrated the maintenance in finished products of a significant proportion of the folates present in the raw material: 62% of folates (relative to fresh product) were preserved during the freezing of spinach, and 70% during the canning of green beans.  Most of the losses observed were attributed to leaching phenomena as soon as the vegetables were in close contact with water, rather than to the thermal treatments.  The washing of spinach is particularly critical from this point of view, as is the presence of liquid with canned green beans.  As for blanching, this did not cause a significant loss of folates, either during the freezing of spinach or the canning of green beans.
A study of domestic cooking produced similar findings: purely thermal treatments such as steaming did not cause a loss of folates, unlike boiling that placed the vegetables in contact with water, where losses ranged from 20% for green beans to 50% for spinach.  At the same time, an analysis of the cooking water showed that it contained most of the folates.

An absence of oxygen to preserve vitamins

The research team also studied what happens when plant matrices are broken down, as is the case with puréed spinach or beans.  Contact with the air proved to be another critical factor in folate loss, apart from diffusion phenomena.  Thus on the processing chain for pasteurised spinach purée, most folate losses occurred during cooking of the spinach and during mixing.  Losses during pasteurisation were less marked.  Folate contents remained stable during cold storage in vacuum-packed trays.    In the absence of oxygen, no reduction in the folate content was observed during thermal treatments.

During a thermal treatment in the presence of air, folate levels fell, and more rapidly as the temperature rose, but a residual folate content was present whatever the treatment temperature.  This residual level corresponded to the stable chemical forms of folates.  The level of folate losses was therefore more influenced by the matrix (content in stable forms) than by temperature or pH.  Oxygen therefore plays a central role in degradation, but only relative to the folate fraction in the plant matrix.
During industrial and domestic cooking processes, folate losses are therefore greater when the vegetables are in contact with water than during steps which involve thermal treatments.  An improvement in the nutritional quality of finished products could thus be achieved by rationalising processes in order to reduce contact with water; for example, blanching vegetables by steaming rather than in water.  But whatever the case, processed vegetables remain a good source of folates.

Find out more

ANR Ribenut 2010 – 2014 project:
"New approaches to evaluating the compromise between the microbiological risk and nutritional benefits of thermally-treated vegetables"

The Ribenut project focused on the preservation of vitamins, and particularly folates, in terms of nutritional benefits versus a reduction in pathogenic bacteria and spores, in terms of the microbiological risk.
A mathematical model representing the entire transformation process made it possible to determine parameters with the most interesting impacts.  Based on the model, a computerised decision support tool was developed so that professionals can apply this approach to their own processes.
This generic approach could also be transferred to the study of other compromises, such as energy savings - in terms of economic and environmental benefits - versus a reduction in pathogenic bacteria and spores - in terms of the microbial risk.
Contact: Christophe NGUYEN THE  - christophe.nguyen-the@avignon.inra.fr

Partners in the ANR RIBENUT project

  • UMR-SQPOV/INRA and Université d’Avignon
  • UMR Genial/Agroparistech
  • AERIAL
  • CTCPA
  • ADRIA
  • Mét@risk/INRA Paris
  • LUBEM/Université de Bretagne Occidentale
  • Bonduelle
  • Créaline SA

For further information

These results were obtained in the context of the thesis project carried out by Nicolas Delchier: "Devenir des folates au cours de la transformation des végétaux verts : identification des points clés et des mécanismes.". (Fate of folates during the processing of green vegetables: identification of key points and mechanisms, available online).

 

This thesis work formed part of the ANR Ribenut 2010 – 2014 project: "New approaches to evaluating the compromise between the microbiological risk and nutritional benefits of thermally-treated vegetables"