Volatiles, fruit/vegetables

Historically, most studies on plant volatiles were undertaken with the aim of identifying the substances responsible for the characteristic aroma and flavor of plant materials. Since the 1970 s studies on the pathways and control mechanisms of flavor formation have been carried out ( 1), showing that fruits, vegetables and spices contain volatiles originating predominantly from secondary plant metabolism (2j. 

Model System Theory. In contrast to static headspace, which looks only at the concentration of the volatiles in the headspace, the aim of dynamic headspace is to transfer as much as possible of the volatiles in the food to the trap. Some understanding of the transfer can obtained by considering model systems. As fruits, vegetables and plant materials have high concentrations (70-90%) of water, it is usefiil to consider model systems of solutions of volatile components in water. With such systems it is possible to calculate the total volume of sweep gas (Vg) needed to transfer a certain pCTcentage (P) of the dissolved volatile component to the trap (22,28). This can be done with the following relation ... 

Lyophilization is mainly used for the sample treatment of plants (and biological materials) having undefined quantity of water (fruits, vegetables) as well as when volatile elements are to be determined. Water is removed under low temperature and pressure. The freezing temperatures do not influence the mass of the final product. The enrichment factor depends on the quantity of water and is in the range 4-20 for fruits and vegetables. 

The enzymic formation of aldehydes, ketones, alcohols, and oxoacids (from linoleic and linolenic acids) on disruption of plant tissues is an important biosynthetic pathway by which fruit and vegetable volatiles are formed. Some examples are (E)-2-hexenal ("leaf aldehyde") and ( )-3-hexenol ("leaf alcohol") in tea (E)-2-hexenal in apples (E,Z)-2,6-nonadienal ("violet Teaf aldehyde") and (E)-2-nonenal in cucumber ( Z)-5-nonenal in musk melon (Z,Z) -3,6-nonadienol in water melon, and 1-octen-3-ol ("mushroom alcohol") in certain edible mushrooms and Fungi. The enzyme system is highly substrate specific to a (Z,Z)-1,4-pentadiene system (like lipoxygenase) splitting the >C = C< double bond at the W - 6 and/or W - 9 position. Therefore linoleic-, linolenic-, and arachidonic acids are natural substrates. It seems to be a common principle in leaves, fruits, vegetables, and basidiomycetes. 

Szafranek, B. Szafranek, J. (2008). Volatiles of Solanum spp. analysis, composition and ecological significance. Fruit, Vegetable and Cereal Science and Biotechnology, Vol.2, No.l, pp. 145-155, ISSN 1752-3419... 

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). 

Separation of individual bioactives from a complex matrix is important in discovery of new potential volatile and nonvolatile components from herbal medicines, fruits, vegetables, and spices. Based on... 

Hydroxycarboxyhc acids are mostly non-volatile and polar compounds that are not important as odour-active food components. Certain hydroxy acids are, however, the major substances influencing the sour taste of fruits, vegetables and other foods. Some of their reaction products are odour-active substances, especially lactones. 

The aroma substances that comprise flavors are found in nature as complex mixtures of volatile compounds. A vast majority of volatile chemicals that have been isolated from natural flavor extracts do not provide aroma contributions that are reminiscent of the flavor substance. For instance, n-hexanal is a component of natural apple flavor (1) however, when smelled in isolation, its odor is reminiscent of green, painty, rancid oil. Similarly, ethyl butyrate has a nondescript fruity aroma although it is found in strawberries, raspberries, and pears, it does not uniquely describe the aroma quality of any of these individual fruits. It has long been the goal of flavor chemists to elucidate the identity of pure aroma chemicals that have the distinct character impact of the natural fruit, vegetable, meat, cheese, or spice that they were derived from. Often, these are referred to as character impact compounds (2). 

Similar information for other foods would be of great value. With dehydrated vegetables and fruits, large errors are not to be expected, because these foods do not normally contain much nonaqueous volatile matter and some of it will have been lost in the process of dehydration. 

Although SPME was applied initially for the analysis of relatively volatile environmental pollutants in waters, rapid developments have enabled SPME to be successfully applied for the analysis of pesticides in water, wine and more complex food samples such as honey, fruit juice and pears, vegetables and strawberries. With food samples, most analysts recognize the need for some sample pretreatment in order to minimize matrix effects. The matrix can affect the SPME efficiency, resulting in a reduced recovery of pesticides. The most common method is simply to dilute the sample or sample extract with water. Simpltcio and Boas comminuted pears in water prior to the determination of pesticides. Volante et al. extracted over 100 pesticides... 

Determination of parathion deposits on fruits and vegetables indicates such small amounts present that there should not be a residue problem with this insecticide. Parathion apparently volatilizes or decomposes so rapidly that less than 0.10 p.p.m. of residue... 

The types of spoilage caused by bacteria in fruits and vegetables are diverse they include sensory changes, degradation of compounds, and formation of new substances such as acids, volatile compounds, and polymers. For example, the bacteria produce a set of enzymes such as pectinases, cellulases, proteases, and others that causes maceration and softening of tissue. Off-flavor development is common in contaminated tissues, caused by volatile compounds produced by microflora (Jay 1992). 

Flavor is one of the major characteristics that restricts the use of legume flours and proteins in foods. Processing of soybeans, peas and other legumes often results in a wide variety of volatile compounds that contribute flavor notes, such as grassy, beany and rancid flavors. Many of the objectionable flavors come from oxidative deterioration of the unsaturated lipids. The lipoxygenase-catalyzed conversion of unsaturated fatty acids to hydroperoxides, followed by their degradation to volatile and non-volatile compounds, has been identified as one of the important sources of flavor and aroma components of fruits and vegetables. An enzyme-active system, such as raw pea flour, may have most of the necessary enzymes to produce short chain carbonyl compounds. 

The central question that I want to approach here is the possible relationship between flavor preferences and nutritional value. There are a lot of data to work with. More than 70(X) volatile flavor substances have been identilied in foods and beverages. The situation may not be quite as complex as this would suggest. While it is true that any single fruit or vegetable may synthesize a few hundred volatile compounds, only a modest subset of these will contribute to its flavor profile. So the task is to sort out what these are, identify their sources, and link, where possible, these sources to nutritional value. Studies with the tomato provide a great example. The bottom line is Virtually all of the major tomato volatiles can be linked to compounds providing health benefits to humans. ... 

Many factors affect the volatile composition of fruit and vegetables, e.g. genetics, maturity, growing conditions and postharvest handling. Furthermore, preparation of the fruits and vegetables for consumption and the method for isolation of volatile compounds may change the volatile profile and key aroma compounds compared to non-processed fruits and vegetables. 

A large number of volatile phenols and related compounds occur in vegetables and fruits, and some of them are potent aroma compounds. The majority of volatile phenols and related compounds in plants are formed mainly through the shikimic acid pathway, and are present in intact plant tissue either as free... 

Fig. 7.2 Examples of volatile lactones important for the flavour of fruits and/or vegetables...

The flavour of fruits and vegetables is a very important aspect of quality. This review has focused on the most important aroma compounds in fruits and vegetables of moderate climate and demonstrated that a wide variety of volatile compounds are formed naturally in the products or after processing that influence the aroma and flavour of fresh and processed fruits and vegetables. 

A complete understanding of the flavour chemistry and biochemistry of volatile components of fruits and vegetables is important in order to improve the flavour quality of fresh and processed produce that complies with the consumer needs for better quality vegetable and fruit products. 

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