Inﬂuence of pH, Ca concentration, temperature and amidation on the gelation of low methoxyl pectin Didier Lootens a,1 , Franc¸ois Capel a , Dominique Durand a , Taco Nicolai a, * , Patrick Boulenguer b , Virginie Langendorﬀ b a Polyme ` res, Colloı ¨ des, Interfaces, UMR CNRS, Universite ´ du Maine, 72085 Le Mans Cedex 9, France b Degussa biosystems SAS Research Center Baupte R50500, France Received 3 January 2002; revised 10 April 2002; accepted 22 May 2002 Abstract We have studied the inﬂuence of the calcium ion concentration, [Ca 2þ ], and the pH on the storage (G 0 ) and loss (G 00 ) shear modulus at 1 Hz of low methoxyl pectin solutions and gels. Upon lowering the temperature in the presence of Ca 2þ , G 0 and G 00 increase immediately followed by a further slow logarithmic increase with time. The immediate response increases with increasing [Ca 2þ ] and decreasing temperature. The ‘gel’ temperature (T g ) where G 0 and G 00 cross increases with increasing [Ca 2þ ]. However, G 0 and G 00 have a universal temperature dependence at all [Ca 2þ ] if plotted as a function of T 2 T g . The pH was reduced slowly by addition of GDL. Reduction of the pH to below 3 leads to the formation of strong gels for amid and much weaker gels for non-amidated pectin. For non-amidated pectin lowering the pH below 3 leads to a weakening of the Ca 2þ , while for amidated pectin the gel is reinforced. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Pectin; Gel; Shear modulus 1. Introduction Pectin is a polysaccharide extracted from plant cell walls and is used in many industrial applications as thickener or gelling agent. Pectin consists essentially of linear chains of a-D galacturonic acid residues with a smallfraction of rhamnose and small side chains formed by other sugars. Pectin is usually characterized by its degree of methylation (DM) of the carboxyl groups (Thakur, Singh,& Handa, 1997). Aqueous solutions of pectin with DM less than 50%, so-called low-methoxyl pectin,gelin the presence of Ca ions.It is wellknown that the gel strength increases with decreasing temperature and increasing Ca concentration, see Clark, Evans,and Farrer (1994) and Clark and Farrer (1996), but as far as we are aware a systematic investigation of the temperature dependence as a function of [Ca] has not yet been reported. Recently, Gilsenan,Richardson, and Morris (2000) reported that low-methoxyl pectin gels in the absence of Ca at low pH. The authors conclude that below a certain pH that depends on the temperature, pectin has a conformation- al transitionwhich in turn inducesaggregationand eventually gelation. The eﬀect of the pH on the Ca induced gelation has, however, not yet been investigated. The aim of the present study wasto investigate the inﬂuence of pH, Ca concentration, and temperature on the gel modulus. Usually the DM ofnaturalhigh-methoxyl pectin is reduced by converting the methoxyl groups to carboxyl groups. Alternatively the methoxyl groups can be converted to amide groups which modiﬁes some properties of the pectin gels that are useful in certain applications (Racape & Thibault, 1989).We looked atthe eﬀectof amidation by comparing two pectin samples with almost the sameDM, but one of the two sampleswas partially amidated. We will ﬁrst show the eﬀect of cooling on the gel modulus of pectin solutions at a ﬁxed pH 6.5 and varying concentrations of Ca. Then we will show the eﬀect of pH at 20 8C by addition of glucono-delta-lactone (GDL). This molecule slowly hydrolyzes in time which decreases the pH. We exploited the slow decrease of the pH to induce the slow 0268-005X/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. PII: S 0 2 6 8 - 0 0 5 X ( 0 2 ) 0 0 0 5 6 - 5 Food Hydrocolloids 17 (2003) 237–244 www.elsevier.com/locate/foodhyd 1 Present address: ESPCI, Paris. * Corresponding author. Fax: þ 33-2-43-83-35-58. E-mail address: taco.nicolai@univ-lemans.fr (T. Nicolai).