Freshness and Shelf Life of Foods - American Chemical Society

Chapter 10

Effect of Pre- and Post-Harvest Treatments on Fresh Tomato Quality

Downloaded by UNIV LAVAL on May 17, 2016 | http://pubs.acs.org Publication Date: October 10, 2002 | doi: 10.1021/bk-2003-0836.ch010

Fabienne Boukobza and Andrew J. Taylor Samworth Flavor Laboratory, Division of Food Sciences, University of Nottingham, Sutton Bonington Campus, Loughborough LE12 5RD, United Kingdom

To measure pre- and post-harvest effects on the flavor of individual tomato fruits, a controlled maceration device was coupled directly to an Atmospheric Pressure Chemical Ionization Mass Spectrometer to follow the release of nine volatiles associated with fresh tomato flavor. Throughput was rapid (100 fruit per day) and reproducibility ranged between 1.7 and 28%. The effect of pre-harvest treatments (variety, nutrients, season) on volatile content showed both varietal and seasonal effects on the amounts of some volatiles but the nutrient treatments used had no significant effect. Typical post-harvest factors (storage temperature and atmospheric composition) were applied to shop-bought fruits. Refrigerated storage caused an irreversible decrease in most volatiles tested as did short term high temperature storage (45°C for 15h). For lipid-derived volatiles, the decrease was due to lower enzyme activity within the lipid oxidation pathway.

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© 2003 American Chemical Society

Cadwallader and Weenen; Freshness and Shelf Life of Foods ACS Symposium Series; American Chemical Society: Washington, DC, 2002.


133 Tomatoes are second only to potato in terms of world tonnage produced per year. Fresh tomato production has become highly organized involving the breeding of appropriate varieties, intensive production and refrigerated transport to the point of sale. These developments have increased yield per hectare as well as producing fruits which are of uniform color and size and have better resistance to softening. However, there have also been complaints about the lack of flavor in tomatoes. Previous work has identified the key aroma compounds in fresh tomatoes (1, 2) along with their biochemical origins. The precursors can be amino acids, carotenoids, sugars or lipids. It is generally true that aroma compounds from the first three classes are formed during the ripening period and are present in the fruit prior to eating. The lipid derived volatile compounds are actually generated when the fruit is macerated allowing enzyme and substrate to mix. The reaction is rapid with detectable amounts of various C6 compounds formed in a period of 10 to 30 seconds. In the analysis of tomato flavor, there is usually a maceration step followed by a set time period (to allow generation of the lipid-derived volatiles) before the flavor compounds are sampled and analyzed. Tomato flavor analysis is characterized by other problems. One is that these climacteric fruit show rapid changes in their biochemistry during ripening and there are significant day to day changes. The other problem is that there appears to be considerable fruit to fruit variation (3). While this is often overcome analytically by macerating several fruits together and measuring the mean flavor composition, in practice, it can mean that the consumer is faced with variable quality in a batch of tomatoes even though the average quality is acceptable. One of the limiting factors is the speed at which analysis of tomato flavor can be carried out. Typically, extraction is followed by G C - M S and a sample takes around 1 h to analyze. Thus daily throughput is limited to 6-8 fruit per G C - M S . Given the fruit to fruit variation, this throughput means that two different samples can be compared in one day (assuming 3 to 4 replicates of each sample). This analytical constraint has hampered work on fruit quality and limited the types of experiment that could be carried out and successfully analyzed. To address this problem, our laboratory has developed real time analyses using direct M S with no chromatography (4) and our method is based on Atmospheric Pressure Chemical Ionization Mass Spectrometry- (APCI-MS; (J)). A P C I - M S has been combined with a controlled maceration device so that tomato analysis can be carried out rapidly with a throughput of around 100 fruits per day per M S . This opens new opportunities for studying the tomato crop preand post-harvest. This paper describes the technique and how it has been used to study both pre-harvest factors (effect of variety, growing conditions, seasonal effects) and some post-harvest factors (storage temperature and modified atmospheres) on the flavor volatiles of tomato fruits.


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Materials and Methods


Maceration device The maceration device and details of its operating characteristics have been described previously (6). In brief, a food blender was adapted so that the headspace could be swept with a carrier gas to transport the volatiles released from the tomato fruit to the sampling port of the API-MS (see Figure 1). A single fruit (about 50-80g) was placed in the blender, the lid sealed and headspace sampled into the API-MS. This gave the background level of volatile compounds above the intact tomato. The fruit was then macerated and the headspace concentration of nine selected compounds (see Table 1) monitored for periods of 2 to 5 min. The APCI-MS was calibrated with standards of known concentration and the headspace concentrations expressed as mg/m . The blender was equipped with a port through which solutions of substrates or enzymes could be added to study their effect on tomato flavor generation in vivo. 3

Figure L Schematic view of the modified blender, gas flows and APCI-MS

Tomato fruits Seven varieties were grown (Plum, Espero, Aranca, Yellow cherry, Nectar, Solairo and Santa) by Horticulture Research International under commercial glasshouse conditions in a hydroponic system. To apply nutrient stress to the


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plants (var. Solairo and Espero), roots were split into two separate hydroponie streams so stress (addition of salt) could be applied to the plant solely on one half of the root (T3) or alternatively to one or the other half (T4). These treatments were compared to control plants with a single root using a control nutrient solution (control E C , T l ) and to plants with a divided root using a control nutrient solution (T2). The plants were grown from May to September.

Table 1. Volatile compounds monitored by A P C I - M S from macerated tomato fruits. " A P C I ion" records which ion was used to monitor each compound. "Minimum amount detected" was calculated assuming a signal to noise ratio of 5:1 was needed for quantification. Comparison of this column with the odor threshold values indicates how well the A P C I - M S analysis correlates with the sensitivity of the human nose. Compound

Hexanal Hexenal Hexenol Methylbutanal Methylbutanol Isobutylthiazole 6-Methyl-5-hepten-2-one Ethanol Acetaldehyde

Odor threshold

(m/z)

Minimum amount detected (ug/m )

101 99 83 87 71 142 127 47 45

420 12.3 16.7 161 1.0 2.7 1.9 1.7 2.0

40 0.09-480 4-16 3-6 100-200 N o data 300-500 20-76000 41

Molecular weight

APCI ion

(Da) 100 98 100 86 88 141 126 46 44

3

(ug/m )

3

Fruit were handpicked at point 4-5 on the HRI color scale (red fruit but still not fully ripe) with a weight range of 60 to 80g. Fruit were transported and stored under ambient conditions (21-22°C). Some fruit were stored at 4-6°C for 3 days and then allowed to recover at ambient temperature for 1 or 3 days prior to analysis. Another batch of fruit was stored at higher temperatures (35°C for 6h and 45°C for 6 or 15 h) and then allowed to recover at 21-22°C for 1 or 3 days prior to analysis. A n atmosphere of nitrogen was used to store fruits for periods of 2, 6, 10, 15 and 35 hours followed by a recovery period between 4 and 6h under ambient conditions.


136

Results and Discussion


Maceration system for real time monitoring of volatile release from tomatoes The A P C I - M S was set to record simultaneously the ions listed in Table 1 and typical traces are shown in Figure 2. Since the technique resolves solely on a mass basis, resolution of isomers is not possible and "hexenal" represents the sum of £-2- and Z-3-hexenals, while methylbutanal(ol) is the sum of the 2- and 3-methyl isomers. For some compounds, there was a rapid rise in signal on maceration which represented release of pre-formed compounds upon maceration of the tomato tissue. For other compounds, the slow rise to a maximum value may be the result of slow, continuing enzyme activity (e.g hexenol) or slow equilibration of a hydrophilic compound between the liquid and gas phases of the system (e.g. ethanol).

ioo 1

Isobutylthiazole

100 1

f^Methy!5hepten2one

0^ 100 ίο 1

Hexanal

0^ 100 % 0 100

Hexenal Methylbutanal

0 100 Ί

Hexenol

%1

0" 100 1

-s^Methylbutanol

%1 0" 100 1

%1 0" 100 %

Ethanol

r

Acetaldehyde Time

Figure 2. Relative concentrations of selected volatiles in the headspace above a tomato that was macerated at time Omin. Sampling and analysis conditions as described in Materials and Methods The reproducibility of the device was tested using triplicate fruit portions cut from three large beef tomatoes. Table 2 shows the results with variation


137 ranging from 1.7% to 28% and a mean value of 13%. Previous work has demonstrated large variations in flavor composition between individual fruits. Because tomato fruits are harvested over periods of several months, this variation is not surprising as environmental factors (amount of sun, temperatures) will be different for each time period. In commercial glasshouses, the main stems become longer with each cropping and there may be changes in the amounts of nutrients that reach those fruits that crop late in the season.


Table 2. Variability in amounts of volatiles released from triplicate portions of three beef tomatoes. Values are the percentage coefficient of variation Compound Hexanal Hexenal Methylbutanal Hexenol Methylbutanol 6-Methyl-5hepten-2-one

Tomato 1

Tomato 2

Tomato 3

7.8 4.6 7.7 7.5 1.6 3.3

18 19 13 16 15 13

28 26 9.1 25 12 6.6

The main advantage of the maceration method is that it allows rapid throughput of fruits (100 per day is possible) with variation that is acceptable given the fruit to fruit variation. For comparison of volatile compound release between fruits, it is valid to compare the release traces as the same sampling and analytical conditions were used. The maceration method relies on analysis of volatile compounds in the headspace, so the amounts measured are the net result of volatile generation and/or release as well as dilution in the gas which sweeps the volatiles out of the blender. Although calibration was carried out using solutions of authentic compounds to calculate the gas phase concentrations (in mg/m ), it is not easy to relate these headspace concentrations to the concentrations in the macerate as the mass transfer conditions from tomato tissue to the aqueous phase are not known. To estimate the amount of volatile compound present in the aqueous phase, release of the target volatile compounds could be measured from aqueous solutions of the volatiles under the same conditions as used for tomato fruit. Then, assuming that the tomato slurry-to-air partition coefficient is the same as the air-water partition ( K ) and that all the volatile is in solution, it should be possible to produce an estimate of actual volatile compound in the tissue, a value that is useful when considering biochemical conversion. 3

a w


138 Effects of variety, nutrient stress and season on flavor profiles from tomato fruits


To make comparisons of the amounts of volatile compounds released from the different fruit samples, the maximum concentration of seven compounds was measured. Figures 3a and 3b show the maximum amounts of volatiles in the headspace after maceration, for the low concentration compounds (Fig 3a) and the higher concentration compounds (Fig 3b). These fruits were harvested on the same day from the same glasshouse trial on the same site, thus they should have experienced very similar environmental and nutritional growing conditions and the hypothesis is that the differences are due solely to variety.

Figure 3a. Maximum amounts of volatile compounds releasedfrom different tomato varieties, grown and analyzed under identical conditions. From back to front hexenol, methylbutanol, isobutylthiazole and 6-methyl-5-hepten-2-one.

Figure 4 shows the release of hexanal from fruits (var. Solairo) harvested over the season from May to September as well as the effect of the different nutrient stress treatments. The behavior of hexanal typified the response of the



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Figure 3b. Maximum amounts of hexanal (back row), hexenals (middle row) and methylbutanals (front row) released from different tomato varieties, grown and analyzed under identical conditions

Figure 4. Maximum amount of hexanal released for Solairo grown under four different treatments: Tl (single slab, low control EC), T2 (split root, low control EC), T3 (split root, low/high EC) and T4 (split root, EC pulse).


140 other compounds over the season, namely a steady decrease in volatile release was observed. Since fruit weight varied over the season, the data in Figure 4 are expressed per g of tomato (fresh weight basis). There was no significant effect of nutrient treatment. Post-harvest effects


Anaerobic conditions Tomato fruits were stored under nitrogen for different times and then allowed to recover for 4-6 h. Under these anaerobic conditions, the maximum amount of ethanol found in the headspace increased steadily with anaerobic storage time while the amount of hexanal decreased steadily (Figures 5-6).

Figure 5. Maximum amount of ethanol released after different times of storage under nitrogen in tomato fruits allowed to recoverfor 4-6 h. Data are the mean of 5 replicates and error bars are ±SD. Because the flavor composition of tomato fruits changes rapidly, control fruit were included for treatments up to 12h (control 1) and for treatments over 12h (control 2). These experiments show that anaerobic conditions have significant effects on the amount of ethanol present in the fruit (this might be a marker for poor storage conditions) but prolonged anaerobic storage (24h) is necessary to halve the hexanal content. Similar trends were seen for the other volatile flavor compounds in tomato.



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Figure 6. Maximum amount of hexanal released after different times of storage under nitrogen in tomato fruits allowed to recover for 4-6 h. Data are the mean of 5 replicates and error bars are ±SD.

Figure 7. Effect of refrigerated storage (6°C for 3 days) on tomato fruits allowed to recover 4-6 h (Day 1) and 72 h (Day 3). The release of volatiles is expressed as the ratio of the maximum concentration for test fruits/control fruits.


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Effect of storage temperature Storage at 6°C for 3 days was followed by a period of 1 or 3 days at ambient temperature to study whether recovery took place. Again, because the experiments were carried out over 6 days, control fruit from the same batch were stored at ambient temperature and analyzed each day. The changes in volatile flavor compounds were expressed as a ratio of the test fruit content to the control fruit (a value of 1 indicates no change, 1 indicates an increase in the test fruit). Figure 7 summarizes the changes for the nine compounds monitored. A l l the ratios were

Freshness and Shelf Life of Foods - American Chemical Society

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