Early Detection of Changes During Heat Processing and Storage of

Chapter 4

Early Detection of Changes During Heat Processing and Storage of Tomato Products K. Eichner, I. Schräder, and M. Lange

Downloaded by UNIV OF ARIZONA on March 27, 2017 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0631.ch004

Institut für Lebensmittelchemie der Universität Münster, Piusallee 7, D-48147 Münster, Germany

During heat processing of tomatoes dependent on the reaction conditions different chemical reactions take place. Pyrrolidone-carboxylic acid formed by cyclization of glutamine arises already during the break process. In the course of concentration and pasteurization processes several Amadori compounds occur. They represent chemical markers for the onset of the Maillard reaction. Very high concentrations of Amadori compounds could be found in dried tomato products, especiallyfructose-glutamicacid and fructose-pyrrolidone-carboxylic acid. They are also formed during storage at elevated temperatures. Amadori compounds were analyzed by gas chromatography after proper derivatization. During tomato drying also several heterocyclic compounds and Strecker aldehydes having a low sensory threshold are formed; they can be used as chemical markers for the onset of undesirable changes in flavor. These compounds can be analyzed by headspace gas chromatography and simultaneous destillation-extraction.

Tomato processing involves several heat processing steps for obtaining a shelf-stable product having the desired physical and sensory properties. For inactivation of enzymes the tomatoes are crushed at 60 - 70°C (cold break (CB) process) or at 90 - 95°C (hot break (HB) process) (7). During the CB process pectolytic enzymes are inactivated slowly, therefore pectin is hydrolyzed during the heating process to some extent and galacturonic acid (GalA) is set free (7); GalA may participate in the Maillard reaction being a very reactive reducing sugar (2). In the HB process the enzymes are inactivated quickly and pectin is preserved resulting in a higher viscosity and textural stability of the product. After removing seeds and skins the remaining tomato juice is pasteurized or concentrated under reduced pressure at about 80°C yielding tomato paste with different

0097-6156/96/0631-0032$15.00/0 © 1996 American Chemical Society Lee and Kim; Chemical Markers for Processed and Stored Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1996.

4. EICHNER ET AL.

33 Heat Processing and Storage of Tomato Products


dry matter (DM) contents (double concentrate: 28-30 % DM; triple concentrate: 36 40% DM) (5). Tomato powder or tomato flakes are produced by spray-drying or drum-drying of tomato paste (dry matter content: 30-40 %) (4)\ starch may be added before drying in order to facilitate the drying process and to increase product stability. In order to evaluate the intensity of thermal treatment during heat processing of tomatoes, it seems desirable to characterize the extent of thermal impact during different steps of heat processing by using chemical markers formed by heat induced chemical reactions of reducing sugars and amino acids in the product. Of primary interest in this connection are early Maillard products (Amadori compounds, cf. (5) ) and pyrrolidonecarboxylic acid (PCA) formed by cyclization of glutamine under acidic conditions. Chemical Markers for Early Detection of Chemical Changes during Tomato Processing Methodology. Reducing sugars, organic acids and Amadori compounds were determined by gas chromatographic separation on capillary columns. First Amadori compounds were extracted from the tomato products with water and bound to a cation exchange resin (Lewatit S1080 column); sugars, organic acids and fructose-pyrrolidone-carboxylic acid (Fru-PC A, which is less basic than the other Amadori compounds and not bound to the resin) are elutedfromthe cation exchange column with water. Thereafter, the Ama dori compounds are eluted with 0.5 M trichloroacetic acid (TCA); the TCA is removed by extraction with diethylether. Both eluates are concentrated and freeze-dried. Prior togas chromatographic analysis reducing sugars and Amadori compounds were converted into the corresponding oximes (syn- and anti-form) with hydroxylammonium chloride in pyridine (30 min, 70°C) (5). The oximes and the organic acids present are treated with N,0-bis-(trimethylsilyl)-acetamide and trimethylchlorsilane (30 min, 70°C) leading to the trimethylsilyl derivatives (6). The gas chromatographic separation of thereducingsugars, organic acids and Fru-PC A was carried out on a fused silica capillary column covered with methyl silicone (OV-101). Temperature program: 120-280°C with a heating rate of 6°C/min. Figure 1 shows a standard chromatogram of reducing sugars, saccharose, organic acids, PCA and Fru-PCA. Xylitol and trehalose were used as internal standards. Reducing sugars and the Amadori compound appear as characteristic double peaks, corresponding to the syn- and anti-form of the oximes; the first peak represents the syn-form, the second one the anti-form (6). The gas chromatographic separation of the remaining Amadori compounds was performed using a fused silica capillary column covered with methylpolysiloxane(DB-5). Temperature program: 140-300°C with a heating rate of 6°C/min. Figure 2 shows a standard chromatogram of Amadori compounds occurring in tomato products (xylitol and trehalose as internal standards). Again, each compound appears as a double peak, corresponding to the syn- and anti-form of the oximes.

Lee and Kim; Chemical Markers for Processed and Stored Foods ACS Symposium Series; American Chemical Society: Washington, DC, 1996.


34

CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS

Figure 1. Separation of reducing sugars, saccharose, and organic acids by capillary gas chromatography (standard chromatogram). F = fructose; G = glucose; GA = galacturonic acid; I = inositol; S = saccharose; C = citric acid; M = malic acid; PCA = pyrrolidone-carboxylic acid; FPCA =fructose-pyrrolidone-carboxylicacid. Internal standards. X = xylitol; T = trehalose.

X

1

10

T

20

min

Figure 2. Separation of Amadori compounds by capillary gas chromatography (standard chromatogram) 1 Fru-Ala; 2 Fru-Gly; 3 Fru-Val;4 Fru-Leu; 5 Fru-Gaba; 6 Fru-Ser; 7 Fru-Thr; 8 Fru-Asp; 9 Fru-Glu (Gaba = y-aminobutyric acid). Internal standards: X = xylitol; T = trehalose.


4. EICHNER ET AL.

Heat Processing and Storage of Tomato Products 35

Analytical Control of Tomato Juice Processing. In Table I characteristic chemical changes during tomato juice processing are shown. The raw material is subjected to a HB process. After a certain holding time at elevated temperatures the product is sieved in order to remove seeds and skins and pasteurized after addition of sodium chloride. Table I. Chemical Changes during Processing of Tomato Juice (Concentrations in mmol/100 g dry matter)


Processing step:

Dry matter (g/lOOg) PCA Glutamine Fru-PCA

Fresh product

HB process

Holding period

Pasteurization

6.82 n.d. 14.4 n.d.

7.17 3.2 9.5 n.d.

6.24 4.1 5.0 0.55

6.54 4.4 2.0 0.68

(PCA = Pyrrolidone-carboxylic acid; n.d. = non detectable)

As shown in Table I, thefirstnoticeable chemical change occurring during the HB process is the formation of pyrrolidone-carboxylic acid (PCA) by cyclization of gluta mine; this reaction is promoted under acidic conditions existing in tomatoes (pH 4.2). At a concentration of 0.05 % in tomato juice PCA creates a tart off-flavor, occasionally described as bitter; however, this concentration limit is not yet reached during tomato juice processing as indicated in Table! Fructose-pyrrolidone-carboxylic acid (Fru-PCA) being the first noticeable Amadori compound appears not till the holding period and increases during the pasteurization process. Apart from the decrease of glutamine no considerable changes in the concentrations of amino acids could be observed. Therefore it can be concluded that the Maillard reaction plays a minor role under the relatively mild conditions of tomato juice processing. Tomato Paste and Tomato Powder Processing Control. Evaporation of water in the concentration plant, which operates under reduced pressures, brings about a progres sive increase in solids content of the tomato pulp until a paste of the desired density is produced (3). Concentration to afinalsolids content of 28 - 30 % is the most common practice in Europe, and this paste forms the "double concentrate" of commerce. The temperature of the paste must be raised to at least 90°C before it is filled into cans in order to prevent the survival of microorganisms. Tomato powder usually is produced by spray-drying of tomato paste, whereas tomato flakes are obtained by drum drying of tomato paste, usually after addition of starch.


36



Table II: Chemical Composition of Industrially Processed Tomato Paste and Tomato Flakes (g/100 g DM)

Dry matter (DM) Fructose (Fru) Glucose (Glu) Galacturonic acid (GalA) Pyrrolidone-carboxylic acid (PCA) Glutamic acid (Glu) y-Aminobutyric acid (Gaba) Alanine (Ala) Aspartic acid (Asp) Serine (Ser) Threonine (Thr) Fru-Glu Fru-PCA Fru-Gaba Fru-Ala Fru-Asp Fru-Ser Fru-Thr A (420 nm)

Tomato paste

Tomato flakes

CB

HB

CB

HB

30.69 27.8 25.8 0.819 1.63 3.46 1.06 0.353 1.21 0.205 0.172 0.06 0.47 0.11 n.d. n.d. n.d. n.d. 0.061

30.95 24.8 20.7 0.171 1.65 3.64 1.20 0.347 1.15 0.192 0.163 0.03 n.d. 0.19 n.d. 0.03 n.d. n.d 0.071

96.15 22.6 12.5 0.240 1.53 0.71 0.21 0.110 0.37 0.079 0.119 1.7 2.8 2.4 0.226 2.0 0.241 0.141 0.196

95.95 20.4 10.1 n.d. 1.69 1.15 0.40 0.139 0.49 0.102 0.081 1.4 2.3 2.3 0.226 1.7 0.241 0.113 0.161

CB: Cold Break; HB: Hot Break; n.d. non detectable A (420 nm): 0.50 g DM/100 ml solution

Table H shows the chemical composition of tomato paste produced by the cold break (CB) and the hot break (HB) process as well as the composition of tomato flakes produced from CB- and HB-tomato pastes. It can be seen that the CB-tomato paste contains much more GalA than the HB product, caused by the delayed inactivation of pectolytic enzymes during the CB process. Glutamine is not detectable in both products because of its complete cyclization to PCA during the heating process. On the other side, only a relatively small portion of the amino acids present has been converted to the corresponding Amadori com pounds. Figure 3 summarizes the conversion of PCA and the most important amino acids of tomato to Amadori compounds in CB- and HB-tomato paste on a molar basis. Using Amadori compounds as chemical markers for the onset of the Maillard reaction, it can be concluded that during production of tomato paste the Maillard reaction still has a minor importance.



4. EICHNER ET AL.


Compared to these results it was of great interest to see how far the Maillard reaction will proceed under drying conditions taking into account that this reaction has a maximum rate at certain moisture contents which must be crossed during the drying process (7). As Table II shows, the concentrations of amino acids have been greatiy diminis hed during drum-drying of CB- and HB-tomato paste; at the same time there is a strong increase in the concentration of the corresponding Amadori compounds and of visible browning (A (420 nm) = absorbance value at 420 nm). Figure 4 again summarizes the conversion of the most abundant amino acids of tomato to Amadori compounds in CB- and HB-tomato flakes on a molar basis. The Amadori compounds as such do not contribute to sensory changes; however, they generally can be used as chemical markers for an early detection of the Maillard reaction in foods (5, 6, 9). By decomposition of Amadori compounds melanoidins and volatile Strecker aldehydes having a very low sensory threshold value are formed. The ratio between the concentration of Amadori compounds and the intensity of sensory changes depends on the reaction conditions. The CB- and HB-tomato flakes under investigation show only a slight burnt and adstringent off-flavor. Fructose does not form the corresponding Heyns rearrangement products in detectable amounts; its contribution to browning is comparatively low. Comparing Figure 4 with Figure 3 it becomes clear that the molar sum of amino acid and Amadori compound remains constant for y-amino butyric acid (Gaba), whereas in the case of glutamic acid (Glu) this sum is greatly diminished. But it also can be seen in Figure 4 that a great deal of the Glu initially present has been converted to Fru-PCA by cyclization of Fru-Glu, the concentration of PCAremainingabout the same in tomato paste and tomato flakes. Chemical Changes during Storage of Tomato Powder. In order to elucidate storage changes of dried tomato products by chemical markers tomato paste was freeze-dried, adjusted to a water activity (a ) of 0.35 (8) and stored at 40°C. As shown in Figure 5, the concentration of fructose-glutamic acid (Fru-Glu) increases sharply at the beginning of storage and decreases in the course of time after reaching a maximum. On the other hand, the proportion of the relatively stable fructose-pyrrolidone-carboxylic acid (FruPCA), the cyclization product of Fru-Glu, increases slowly, whereas the concentrations of the other Amadori compounds listed in Figure 5 after an initial increase remain almost constant. Contrary to the formation of Amadori compounds the onset of visible browning shows an induction period. Figure 5 shows clearly that, for evaluating the course of Maillard reaction due to thermal treatment or storage of tomato powder at elevated temperatures, the sum of the marker substances Fru-Glu and Fru-PCA must be considered (9). w


38

CHEMICAL MARKERS FOR PROCESSED AND STORED FOODS 25

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s Downloaded by UNIV OF ARIZONA on March 27, 2017 | http://pubs.acs.org Publication Date: May 5, 1996 | doi: 10.1021/bk-1996-0631.ch004

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fWMW/M^

CB HB BGalA SGaba

CB HB

CB HB

CB HB

• PCA • Amadori comp.

— — I CB HB

0

H Glutamic acid • A (420 nm)

Figure 3. Important constituents of industrially produced tomato paste (CB,HB cold break resp. hot break process) (cf. Table II for further data).

-T=l—i-O

CB HB BGalA EGaba

CB HB

CB HB

CB HB

• PCA • Amadori comp.

CB HB O Glutamic acid 5 A (420 nm)

Figure 4. Important constituents of industrially produced tomato flakes (CB, HB = produced from cold break resp. hot break tomato paste) (cf. Table H for further data).


4. EICHNER ET AL.



Chemical Markers for Early Detection of Sensory Changes during Tomato Processing. The Maillard reaction which becomes predominant during thermal treatments in food processing (70) may create desirable aroma components like in baking and roasting processes (77-75), but on the other hand it often causes detrimental sensory changes (14,15). During further progress of the Maillard reaction Amadori compounds being the first stable intermediates of this reaction decompose via 1,2- or 2,3-enolization to yield 3-deoxyosones or 1-deoxyosones, respectively (5). 3-Deoxyosones cyclize to form hydroxymethylfurfural (HMF) which can be used as a marker for detecting Maillard reactions in acid foods (15). At higher temperatures the "1-deoxyosone-pathway" prevails (16). Figure 6 shows the decomposition pathways of the 1-deoxyosone which is formed by 2,3-enolization of Amadori compounds followed by G-elimination of the amino acid residue (16). By retro-aldol cleavages of the 1-deoxyosone short-chain dicarbonyls like diacetyl, methylglyoxal or hydroxy di acetyl are formed which - like the deoxyosones - may participate in the Strecker degradation of amino acids, thus creating volatile flavor components (17,18). On the other hand, the 1-deoxyosone undergoes several cyclization reactions yielding e.g. acetylfuran, maltol, isomaltol, furaneol and 5-hydroxy-5,6dihydromaltol (DHM) (76). In order to demonstrate the effect of heat impact during drum-drying of tomato paste on the formation of volatile compounds, their concentrations in drum-dried tomato flakes and in tomato paste, from which the flakes were produced, were compared. Table HI shows some highly volatile compounds analyzed in tomato paste and tomato flakes by head-space-gas chromatography. The gas chromatographic separation was carried out on a 60 m fused silica capillary, covered with Stabilwax. Temperature program :40-220°C with a heating rate of 4°C/min, holding time: 20 min. The aldehydes shown in Table IE were formed by Strecker degradation of the amino acids alanine, valine, isoleucine and leu cine; dimethyl sulfide may be formed by thermal degradation of S-methyl-methionine. Table III shows that the concentrations of all listed volatile compounds are far above the sensory threshold values (determined in water) thus strongly contributing to flavor, they may contribute to off-flavor, if they exceed certain concentrations. Therefore the Strecker aldehydes and dimethyl sulfide listed in Table III can be used as very sensitive marker substances for sensory changes during heat processing of tomatoes, especially 2- and 3-methylbutanal, since their concentrations are more than hundredfold higher in tomato flakes than in tomato paste. In Table IV thermally generated Maillard reaction products having a medium vola tility are shown; they were isolatedfromthe tomato products by simultaneous distilla tion-extraction (SDE) using methylene chloride as a solvent and separated by gas chroma tography on a60 mfused silica capillary, covered with Stabilwax. Temperature program: 40°C for 5 min; 40-220°C with a heating rate of 5°C/min, holdingtime:30 min.



40


Figure 5. Storage of freeze-dried tomato powder at 40°C and a water activity (a ) of 0.35. • Fru-Glu, • Fru-PCA, • Fru-Asn, O Fru-Thr, A Fru-Ser, • Browning (A(420 nm)); the absorbance values refer to an extract of 0.5 g sample material in 25 ml water. w


EICHNER ET AL.


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42

Table IH Determination of Volatile Compounds in Tomato Paste and Tomato Flakes by Static Head-Space-Gas Chromatography


Compound

Acetaldehyde 2-Methylpropanal + Acetone 2-Methylbutanal 3-Methylbutanal Dimethyl sulfide

Tomato paste (mg/kg DM)

Tomato flakes* (mg/kg DM)

Sensory threshold values (mg/L H 0)

6.4

75.1

0.015

18.0 0.21 0.26 10.6

81.9 27.6 31.2 63.9

2xl0" 0.2 xlO" 0.3 x 10"

2

3 3

3

* containing 56 % starch; all data are related to tomato solids

Table IV. Determination of Volatile Compounds in Tomato Paste and Tomato Flakes by SDE with Methylene Chloride Compound

Furfural 5-Methylfiirfural 2-Acetylfuran Phenylacetaldehyde 2-Acetylpyrrole 2-Formyl pyrrole


Tomato flakes (mg/kg DM)

Sensory threshold values (mg/L H Q)

3.34 0.06 n.d. 2.19 n.d. n.d.

51.10 7.95 1.30 48.0 0.31 0.85

3.0 10* 110* 4x 10" 200*

2

n.d. = non detectable. * determined in orange juice

All compounds listed in Table IV show a strong increase due to the drum drying process. The concentrations of phenylacetaldehyde formed by Strecker degradation of phenylalanine lie far above the sensory threshold values; therefore this compound is a useful marker for sensory changes due to heat processing of tomatoes, too. Table V shows thermally generated Maillard reaction products with medium and low volatility isolated from the tomato products by extraction with methylene chloride and cleaned up by gel chromatography. They were separated on a 60 m fused silica capillary (conditions as for the volatile compounds listed in Table IV).


4. EICHNER ET AL.


Table V. Determination of Volatile Compounds in Tomato Paste and Tomato Flakes by Extraction with Methylene Chloride


Compound

Furfural 5-Methylfurfural 2-Acetylfuran Phenylacetaldehyde 2-Acetylpyrrole 2-Formylpyrrole Furaneol HMF DHM


Tomato flakes (mg/kg DM)

Sensory threshold values (mg/L H 0)

Early Detection of Changes During Heat Processing and Storage of

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