Volatile compounds, bread

To date the aroma of bread consists of a total of 296 volatile compounds (5), which can originate from different stages of bread making (Table I). The... 

For estimating the contribution of volatile compounds to bread aroma Rothe and coworkers (S) defined "aroma value" as the ratio of the concentration of some volatile compounds to the taste threshold value of the aroma. This concept was further developed by Weurman and coworkers (9) by introducing "odor value", in which aroma solutions were replaced by synthetic mixtures of volatile compounds in water. These mixtures showed the complexity of the volatile fractions of wheat bread, because none of them resembled the aroma of bread. Recently two variations of GC-sniffing were presented (10-11), in which the aroma extract is stepwise diluted with a solvent until no odor is perceived for each volatile compound separately in the GC effluent. The dilution factors obtained indicate the potency of a compound as a contributor to the total aroma. 

Sampling and Analysis. A frozen slice of bread was cut in pieces and stacked in an enlarged sample flask of an aroma isolation apparatus according to MacLeod and Ames (74). Volatile compounds were trapped on Tenax TA and afterwards thermally desorbed and cold trap injected in a Carlo Erba GC 6000 vega equipped with a Supelcowax 10 capillary column (60 m x 0.25 mm i.d.) and a flame ionisation detector. Similar GC conditions were used for GC-MS identification of volatile compounds by dr. M.A. Posthumus (Dept. Organic Chemistry, VG MM7070F mass spectrometer at 70 eV El, 75). 

The number of volatile compounds collected from white bread by the dynamic headspace method are presented per class in Table II. The values give only a partial impression, because they depend strongly on the conditions used for isolation and detection of volatile compounds. Overall this method seems to release 1/3 of the total number of compounds published for white bread (5). So, the data obtained can only be used for studying differences compared to control samples. 

Table III shows that l-octen-3-ol and 2-heptenal were only detected in soya containing bread and that the latter has significant larger peak areas of 1-pentanol, 1-hexanol, l-penten-3-ol, hexanal and 2- heptanone compared to the control (p < 0.05). These differences could be caused either by addition of volatile compounds present in soya flour or by its lipoxygenase activity. The main volatile compound found in soya flour was limonene (75), of which the peak areas were similar in the soya flour containing bread and its control sample. Moreover minor volatile compounds of soya flour, like 1-pentanol, 1-hexanol and hexanal, increased steadily in soya samples, as can be seen in Table...

Table L Kind of volatile compounds formed in different stages of bread...

Table II. Number of volatile compounds detected in white bread samples with the dynamic headspace method...

III. These results indicate that the composition of volatile compounds of soya containing white bread is hardly influenced by the volatile compounds of the soya flour itself and that lipoxygenase activity plays a major role. 

As discussed, addition of enzyme active soya flour changes the composition of volatile compounds of white bread. In its practical application as a bread improver component, the soya lipoxygenase isoenzymes are sufficient stable for 5 months to meet the bleaching requirements. 

Table III. Amounts of volatile compounds (ng/kg) detected in white bread by dynamic headspace analysis CX)NTROL is without and SOYA is with 30 g soya flour addition (see recipe 15)...

Table IV. Influence of 16-23 weeks of storage of soya containing bread improver PASTE and POWDER on differences (d) in relative decreases (%) of GC peak areas of selected volatile compounds of white bread (22)...

Bread aroma, volatile compounds, 192-193,195f Bread improvers, 193 Bread making, volatile compounds, 192-194... 

The experiments have been completed by additional reaction of xylose, fructose and some characteristic sugar degradation products like cyclotene, Furaneol and diacetyl and by thermal decomposition of Ama-dori rearrangement products. It is well knwon that sugars can react with suitable amino compounds very easily. In the course of these reactions sugars are mostly decomposed and brown melanoidins are formed. By-products of these melanoidins are many volatile compounds of characteristic aroma properties. They are also responsible for the well known aromas of heated food like meat, coffee and bread. 

It is generally accepted (1 ) that volatile compounds present in the flour are of minor importance to the aroma of bread. Prerequisites for formation of the desired crust flavor compounds are the dough fermentation and, especially, the baking steps (J2, 3). 

W. Grosch P. Schieberle, Bread. In Volatile Compounds in Foods and Beverages, H. Maarse, Ed. Marcel Dekker New York, 1991 pp 41-77. 

During storage of food products, volatile compounds produced by lipid oxidation cause rancidity, especially if the lipids contain linolenic acid. Dark flours become easily rancid on storage. Crackers and other durable bakery products should be stored in an inert gas atmosphere or be protected by antioxidants. Roasted products, such as peanuts, may change their agreeable flavor if stored in air therefore, they are stored either in nitrogen or under reduced pressure. Fried products are most sensitive to oxidative rancidification, especially Med products with weak flavor, such as fried bread, French fries, or potato chips. Dry and deep-frozen products are generally rather sensitive to oxidation because of easier access of air into the inner layers of the food product. 

For years researchers have investigated the sulfur compounds present in various foods. Cooked foods typically contain numerous sulfur compounds, especially heterocyclic compounds like thiazoles, thiophenes, thiazolines, etc. In 1986, Sha-hidi et al. (7) reported that 144 sulfur compounds had been identified in beef. Other heated food systems like bread, potato products, nuts, popcorn, and coffee also contain many sulfur compounds. Aliphatic thiols have been found in fruits, vegetables, dairy products etc., as well as in heated foods. No discussion of the occurrence of sulfur compounds in foods would be complete without mention of their major role in the various allium species. Indeed, more than half of the volatile compounds reported in garlic, onion, leek, and chive contain sulfur (2). Comprehensive reviews of the literature concerning the role of thiazoles, thiophenes, and thiols in food flavor through 1975 can be found in Maga s series of review articles (3-5). 

HS-SPME analysis of volatile compounds for food samples was performed according to a previously reported method applied to bread samples [29] with slight modifications. Similar conditions to those proposed by Blanda et al. [4] were used in the study with strawberries. The SPME fiber used was divinylbenzene/carboxen/polydimethylsiloxane (DVB/CAR/ PDMS) 50/30 pm, StableFlex, 1 cm long mounted to an SPME manual holder assembly from Supelco (Bellefonte, PA) (Fig. 1). Prior to use, the fiber was conditioned by following the manufacturer s recommendations. The needle of the SPME device was inserted into the container through the septum and the fiber was exposed to the food sample headspace for 30 min at room temperature. The fiber was then retracted into the needle assembly, removed from the container, transferred to the injection port of the GC unit and immediately desorbed. 

Identification of volatile compounds in strawberries and sliced bread headspace was performed in full scan mode (m/z 30-550). Carvacrol and thymol were identified by a combination of the NIST mass spectral library and gas chromatographic retention times of standard compounds. The rest of volatiles were tentatively identified by their GC/MS spectra. In this sense, the compounds having 90% similarity with spectra in the NIST library were not taken into consideration. Chromatographic responses of detected volatile compoimds (peak area cormts) were monitored for comparative measurements of each compotmd in the studied samples. 

The FD chromatograms of the volatile compounds of white bread and French fries are presented in Fig. 5.4 and 5.8, respectively. 

There are a number of similarities in volatile compounds found in cereal-based foods. Bread, crackers, cooked basmati rice, and pearl millet often elicit a response similar to a roasty or popcornlike aroma due to presence of the same type or most often the same compound. There are not many reports on volatile compounds from freshly popped popcorn the report by... 

The composition of the volatile fraction of bread depends on the bread ingredients, the conditions of dough fermentation and the baking process. This fraction contributes significantly to the desirable flavors of the crust and the crumb. For this reason, the volatile fraction of different bread types has been studied by several authors. Within the more than 280 compounds that have been identified in the volatile fraction of wheat bread, only a relative small number are responsible for the different notes in the aroma profiles of the crust and the crumb. These compounds can be considered as character impact compounds. Approaches to find out the relevant aroma compounds in bread flavors using model systems and the odor unit concept are emphasized in this review. A new technique denominated "aroma extract dilution analysis" was developed based on the odor unit concept and GC-effluent sniffing. It allows the assessment of the relative importance of the aroma compounds of an extract. The application of this technique to extracts of the crust of both wheat and rye breads and to the crumb of wheat bread is discussed. 

The first comprehensive investigation of the volatiles of wheat bread was carried out by Mulders et al. (11. 16-18). After a qualitative analysis (16-18) which led to the identification of 90 compounds, the authors attempted to get an insight into the sensory relevance of the major components which were found in the headspace extract of white bread. A mixture of the main compounds identified was prepared in water to match the gas chromatogram obtained from the bread sample (11,). The odor of the synthetic mixture resembled that of the fermented dough but not that of wheat bread. 

Sizer at al. (20) observed that the compounds causing the pleasant odor which resembled bread crust occur in the basic volatile fraction of white bread. Identification experiments yielded the five pyrazines listed in Table II. A comparison of the odor threshold of each pyrazine in water to its concentration found in bread indicated that 2-ethyl-3-methylpyrazine and 2-methyl-6-propylpyrazine were present at concentrations above their odor thresholds. The authors described the odors of these two pyrazines as "butterscotch, nutty" and "burnt, butterscotch" notes (Table II). 

GC-effluent sniffing of wheat bread aroma concentrates has shown the presence of low level volatiles that smell like the fresh bread crust. As discussed in the preceeding sections, these compounds (3-7 in Figure 1) were proposed to be responsible for this odor note. 

The review shows that the improvement of the methodology of flavor analysis has led to a systematic evaluation of the volatile neutral and basic key compounds of the flavors of wheat and rye breads. The... 

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