Pectins can be recovered from the fibrous matter of fruits and vegetables by extrusion cooking.
DURING THE WINTER, THE SPECTER OF A JAM that fails to set haunts cooks who make orange marmalade. Sometimes, instead of forming the expected semisolid mass, the juice remains juice. How can this culinary debacle be avoided? Jean-François Thibault, Catherine Renard, Monique Axelos, and Marie-Christine Ralet at the Institut National de la Recherche Agronomique station in Nantes have found an answer to this question, although the purpose of their research was mainly commercial. They showed that the technique of extrusion cooking, used especially in fabricating cocktail crackers, makes it possible to extract large quantities of pectin, which is to say gelatinizing molecules, from the pulp of lemons, oranges, and beets.
The occasional failure of jams to set seems paradoxical. Pectins are molecules that, together with cellulose and other polysaccharides (long molecules formed by the linking of simple sugars), form the cell walls of most plants; only cereals do not contain them. But because the method of moderate heating used to prevent volatile aromas from evaporating does not efficiently break down the cell walls, the pectins are unable to come together to create a network, or gel, that fills up the entire volume of the solution.
In the case of commercial production a different problem arises: The residue from pressing apples to make apple juice and cider, the pulp of lemons and oranges left over after juicing, and the pulp of beets that remains after their sugar has been extracted all contain large quantities of plant fiber that producers would like to find a use for. Traditionally pectins are recovered from these fibers through heating in an acid solution. In this way the pectins are detached and then, in a further step, purified. The resulting material is used as a gelatinizing or thickening agent or to coat potato chips. In the future it may find a new role as a detergent for cleansing polluted water (because pectins bond strongly with the metallic ions that are the source of water impurities).
Nonetheless, the acid extraction process has several disadvantages. In particular, it threatens to degrade pectin molecules by breaking the bonds between their constituent sugars and altering the chemical groups carried by these sugars that are responsible for gelatinization.
From Polymers to Dietary Polymers
The Nantes chemists had studied another method known as extrusion cooking for several years in connection with the processing of meats and starches, which gave them the idea of using it to extract pectins from fruit pulp. The apparatus for this technique (adapted from devices used to extract polymers) has one or more Archimedean screws that rotate at variable speeds, shearing off the fibrous material and injecting it into a tube, where it undergoes rapid expansion. The extracted material then passes through a series of chambers in which it is heated at controlled temperatures.
Not all pectins promote gelatinization. Their properties depend on their chemical composition, which varies according to the fruit or vegetable from which it is derived: Whereas pectins from limes, lemons, oranges, and apples are efficient agents, the ones extracted from beet pulp do not gelatinize (although beet pectins, because they chelate heavy ions, produce superabsorbent gels after chemical modification). The differences in the properties of pectins are a consequence mainly of esterification. The sugar chains that form their molecular skeleton carry carboxylic acid (–COOH) groups, which are esterified to varying degrees according to the fruit. As a result, these groups now display the form –COOCH3. Depending on the extent of esterification, the pectins associate more or less easily with one another. It is this ease of association that ensures gelatinization. Extraction for the purpose of producing gelatinizing or thickening agents therefore must respect the integrity of the pectin lateral chains.
The extrusion cooking method has several advantages. Not only is the extractive apparatus much simpler and cheaper than that of other techniques (the screws used at Nantes are only a meter long), but the processing is rapid and can be automated. Additionally, it yields as many pectins as the traditional acid treatment and preserves their molecular structure. The Nantes researchers found that the shearing off of plant matter during extrusion cooking is the critical step. Indeed, if this material is not heated, the molecular integrity of the pectins is preserved despite compression.
Culinary Moral
How can you take advantage of these results cooking at home? At room temperature, squeeze the juice from the fruit and set it aside, finely grind up the remaining fibrous matter in a food processor, then add the juice back to the ground-up material and cook over low heat. But because the pectin molecules are apt to bond with the copper atoms of your pan instead of with one another, don’t leave your jam to cool in the pan. Let it set in glass containers instead.