Interactions Between Pectins and Flavor Compounds in Strawberry Jam

Chapter 11

Interactions Between Pectins and Flavor Compounds in Strawberry Jam Elisabeth Guichard

Downloaded by TUFTS UNIV on July 15, 2016 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0633.ch011

Laboratoire de Recherches sur les Arômes, Institut National de la Recherche Agronomique, 17 rue Sully, 21034 Dijon Cedex, France

Gelling agents are added to commercial products to achieve desired firmness or consistency. These agents should not interfere with the aroma, flavor or taste of the product to which they are added. Among them, pectic substances find many applications, particularly in jam manufacturing. Composition of headspace, consistency, taste and flavor characteristics were determined in jam made with different pectins. At the usual concentrations, high methoxylated pectin induced an undesirable modification of typical flavor and intensity of flavor and taste, whereas low methoxylated pectin induced few alterations. At fixed concentration and molecular weight, a decrease in degree of esterification produced a significant decrease in consistency and noticeable modifications of the flavor perception and headspace composition, but no taste alteration. Mechanical reduction of pectin molecular weight significantly modified only the consistency. Texture characteristics, and particularly consistency, are important factors in the overall acceptability of jam. Pectin is a naturally occurring polysaccharide, mainly extracted from citrus peel and apple pomade. High methoxylated pectins (HMP) are used to form gels in acidic media of high sugar content (7), and low methoxylated pectins (LMP) are used in products of lower sugar content. The strength of gels obtained with L M P varies essentially with concentration of calcium ions in the medium but also with the molecular characteristics of the polysaccharide. At a specific degree of methylation, the physical properties of a H M P are modified by the distribution and location of the remaining free carboxylic groups (2). The molecular weight of pectin can also influence some gel strength characteristics. Crandall and Wicker (2) found that the elasticity modulus was influenced primarily by the short, rigid chains and was independent of pectin molecular weight (MW). On the contrary, they also concluded that breaking

0097-6156/96/0633-0118$15.00/0 © 1996 American Chemical Society

McGorrin and Leland; Flavor-Food Interactions ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


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strength was influenced primarily by the longer, more flexible chains which remained cross-linked after the shorter, rigid chains had ruptured, (which were related to MW). Panchev et al. (3) tested different pectins and found that the optimal strength of the gel corresponded to degree of esterification (DE) values between 57-58 %. However, they also tested pectins in which the M W was decreasing with degree of esterification, thus precluding a definitive conclusion about the relation between gel strength and DE. Many studies have demonstrated that hydrocolloids not only modified viscosity, but often reduced intensities of odor, taste and flavor (4, 5). Some evidence indicated this masking effect varied with the type and concentration of hydrocolloid used. Most of these studies, such as the one by Marshall and Vaisey (6), concerned the effect of hydrocolloids on taste qualities in model solutions. Lundgren et al. (7) investigated the effect of pectin on odor, taste and flavor intensities in jams, but at concentrations 10-times higher than those used in jam manufacturing. The objective of our study was to clarify the influence of the amount of pectin added, and the D E and M W of that pectin on sensory characteristics (such as consistency of the gel, typical flavor character and intensity of flavor), and on amounts of volatile compounds in headspace. Experimental Procedures Pectin Preparation. One rapid-set H M P and one L M P from Mero Rousselot Satia (France) were used. Both were non-standardized citrus pectins (245° S A G for the HMP), currently recommended for standard jam manufacturing (60° Brix, 45% fruit). Experimental Samples. Four experiments were carried out using the following design, as shown in Table I: 1. Jams with increasing amounts of H M P at 83% D E (0, 0.05, 0.1, 0.2 and 0.4 %) 2. Jams with increasing levels of L M P at 37% D E (0, 0.1, 0.2, 0.4 and 0.6 %). 3. Jams with 0.2% of pectin at varying degrees of esterification (HM pectin was de-esterified according to Guichard et al. (δ)), giving three pectins with degrees of esterification of 83, 66 and 54%). 4. Jams with 0.2% of pectin with varying molecular weights (molecular weight of H M pectin was reduced to 86.000,75.000, 59.000 and 32.000). In each, a control sample without added pectin was included. The jam preparation has been previously described (8). Chemical Analysis Isolation of Volatiles. A headspace analysis was used in order to avoid gel disruption. Four hundred grams of jam were introduced into a 1-L flask and extracted according to the method described by Guichard and Ducruet (9): the vapor phase was stripped for 19 hr by a stream of 110 mL/min nitrogen, and the volatile


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Table I. Characteristics of Pectins Used in Different Experiments


Experiment No.

Type of Pectin

Degree of Esterification (%)

Molecular Weight

1

HM

83

86.000

2

LM

37

59.000

3

HM

83 66 54

86.000 86.000 86.000

4

HM

83 83 83 83

86.000 75.000 59.000 32.000

compounds were trapped in a liquid-liquid continuous extractor containing 250 mL of a 10% ethanolic solution, and continuously extracted with 100 mL of dichloromethane. Each analysis was performed in duplicate. For quantification of volatiles, n-tridecane (25 pg/g of jam) was added as an internal standard in the solvent extract. Gas Chromatography. Gas-chromatographic analyses of the extracts were performed using a Girdel 300 gas chromatograph equipped with a chemically bonded DB-5 fused silica capillary column (30 m, 0.32 mm i.d., 1 pm, J & W Scientific Inc.). The injection temperature was 220 °C and detector, 250 °C. Extracts (lpL) were injected splitless. After injection, the oven temperature was held at 30 °C for 5 min and then programmed at 2 °C/min to 220 °C. The flow rate of the carrier gas (H ) was 37 cm/s. For quantification, an Enica 10 integrator (Delsi France) was used. Odors of compounds eluting from the column were assessed by three judges (10). 2

Gas Chromatography-Mass Spectrometry. Compound identifications on each extract were performed using a Nermag R 10-10/C mass spectrometer coupled with the gas chromatograph described above, and equipped with a DB-5 column (60 m, 0.32 mm i.d; 1 pm, J & W Scientific Inc.). Ionization was by electronic impact at 70 eV. Sensory Analysis Subjects. Eighteen subjects were selected on their ability to memorize and recognize basic tastes and odors, and to rank jams with different pectin levels on


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oral consistency. Quantitative descriptive analysis was performed on the jams. During four sessions, descriptive terms were generated by the judges from individual evaluation of commercial strawberry jams. During a fifth session, panelists rated the intensity of each term using an unstructured, 13-cm scale. Results were then discussed in order to establish a final list of ten flavor attributes: total intensity, typical flavor, fresh strawberry, unripe strawberry, overripe strawberry, cooked strawberry, candied fruit, caramel, artificial, lemon. Evaluation of Consistency. The most preferred level of jam consistency was estimated by each member of the panel. This was indicated as "just right" in the center, with anchors of "not hard enough" and "too hard" at the opposite ends of the scale. Evaluation of consistency was made separately by the same group of subjects. Oral consistency was rated using an unstructured, 130-mm scale, with a verbal anchor point at each end (left anchor = very soft; right anchor = very hard), as already described (77). Results and Discussion Ideal Consistency. A histogram of the ideal values (Figure 1) showed that only one subject preferred jam with a H M P concentration higher than 0.2%. The mean ideal consistency was calculated to correspond to a H M P concentration near 0.11%. This value seemed low compared to amounts currently used in jams (0.2%) but since the H M P was not standardized, the corresponding amount of standardized pectin (150° SAG, instead of 245°) should be 0.18%. Evaluation of Oral Consistency with Pectin Concentration. At the same concentration, H M pectin gives a harder gel than the L M pectin (Figure 2) and the oral consistency of jam made with 0.6% of L M pectin is equivalent to that made with 0.25% of H M pectin (60° Brix). At a same pectin level, the oral consistency of jam increases with the Brix level. For the same level of pectin (0.2%), the oral consistency increases proportionally with degree of esterification and molecular weight (Figure 3). Volatile Flavor Compound Identifications. Fifteen key volatile compounds of flavor significance were identified in strawberry jam by gas chromatography-mass spectrometry, and their corresponding aroma descriptors are listed in Table Π. Changes in levels of these compounds were compared against sensory differences among the jam samples for the various types of pectins. Influence of Amount of H M Pectin on Jam. Figure 4 shows that the overall intensity and typical flavor notes of the jam decreased with higher amounts of H M pectin. This could be explained by a decrease of strawberry and caramel notes and an increase of the candied note. Headspace analysis showed that only six of the compounds analyzed were significantly affected by an increase in pectin. Figure 5 shows that adding only 0.05% of pectin drastically decreased the amount of ethyl hexanoate, and to a lesser extent the other compounds. A decrease in the headspace


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Interactions Between Pectins and Flavor Compounds in Strawberry Jam

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