Downloaded by CALIFORNIA INST OF TECHNOLOGY on June 14, 2017 | http://pubs.acs.org Publication Date (Web): November 17, 2011 | doi: 10.1021/bk-2011-1081.ch014
Chapter 14
Phenolic Compounds as Markers for the Authentication of Sherry Vinegars: A Foresight for High Quality Vinegars Characterization M. C. García-Parrilla,* A. B. Cerezo, W. Tesfaye, and A. M. Troncoso Área de Nutrición y Bromatología, Facultad de Farmacia, c/P García Glez nº2, Sevilla 41012, Spain *E-mail:
[email protected] Sherry vinegars are appreciated products reaching higher prices in the market than Sherry wine. Actually, the Protection of Designations of Origin is recognized since 1995. Their elaboration encompasses the acetification process to reach the required acetic degree and aging in wood. Phenolic compounds proved to be valid markers to differentiate vinegars according to their origin and elaboration process. Multivariate analysis of phenolic composition data including Linear Discriminant Analysis (LDA) and Artificial Neural Networks trained by Backpropagation (BPANN) classifies correctly vinegar according to the acetification process (LDA=92.5, BPANN =99.6) and origin (LDA=88, BPANN= 96.5). Good recalling classification rates were also obtained for aging periods. Indeed, phenolic aldehydes as syringaldehyde, vanillin, coniferaldehyde increase their concentration during aging. New trends in Sherry vinegars elaboration intends to shorten the production time. This is achieved by obtaining the acetic degree with submerged culture or by accelerating aging with chips. This communication presents the impact of both strategies on phenolic profile. Nowadays certain innovations to produce high quality vinegars include the type of wood used. Compounds released from it as (+)-taxifolin in the case of cherry wood or (+)-dihydrorobinetin for acacia wood are suitable chemical markers to characterize vinegars aged in these woods.
© 2011 American Chemical Society Ebeler et al.; Progress in Authentication of Food and Wine ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on June 14, 2017 | http://pubs.acs.org Publication Date (Web): November 17, 2011 | doi: 10.1021/bk-2011-1081.ch014
Introduction FAO/WHO (1) defines vinegar as any liquid fit for human consumption, produced exclusively from suitable agricultural products containing starch or sugars by a double fermentation process. The first step is the transformation of a fermentable carbohydrate source into ethanol by yeasts; the second step is the use of ethanol by acetic acid bacteria to produce acetic acid. Vinegars´ quality is determined by the raw materials used and the acetification process involved in their elaboration. Raw materials used for vinegar making are very different: wine, cereals, rice, apples, cider are the most common but whey and honey can also be used to make vinegars. Acetic acid bacteria (AAB) require oxygen to ferment alcohol. The availability of oxygen determines the growth of the AAB. If oxygen is not supplied, the bacteria grow just on the surface and the process is slow. Traditional vinegars are obtained by this surface culture method which involves large periods of time to obtain the acetic degree and meanwhile an aging process occurs simultaneously. Normally, these vinegars are produced in barrels where the contact with wood enhances the products organoleptic properties but a reduced production volume is obtained. On the other hand, oxygen can be supplied into the liquid to be acetified by different systems (agitation, diffusions…) increasing its availability in a large volume of liquid. This fact allows the growth of a submerged culture AAB and a quick acetification process. By this method a volume of 30000 litres can be fermented in a short period of time as 36 hours. The installations need less space and the overall process is less expensive. Most wine vinegars consumed nowadays all over the world are produced in this way. However, volatile compounds are lost mainly due to the strong aeration in the system and sensorial properties are less appreciated than those of traditional vinegars.
Jerez Vinegars, Sherry Vinegars Jerez vinegars are one of the most famous in the world. The production is around 4 millions of litres per year. Almost 200.000 litres are exported to the USA. In order to protect these vinegars, an official label of the Denomination of Origin Council is mandatory after bottling to ensure its origin. There are a few DO for vinegars in the world; Jerez vinegar, Condado de Huelva and Montilla Moriles in Spain and Aceto Balsamico Tradizionale di Modena and Aceto Balsamico Tradizionale di Reggio Emilia in Italy. The conditions required to be included in the Jerez vinegars DO are directly linked to Sherry wine (2). Indeed, the same Council protects both Sherry wine and vinegar. The raw material must be wine from the white grapes Palomino, Pedro Ximenez or Muscat; the producing area is restricted to a geographical area in the Southwest of Spain; the aging process in oak woods by a period of time no less than 6 months restricted to Jerez, Sanlúcar and El Puerto area. Nowadays this area includes up to 48 wineries. 202 Ebeler et al.; Progress in Authentication of Food and Wine ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on June 14, 2017 | http://pubs.acs.org Publication Date (Web): November 17, 2011 | doi: 10.1021/bk-2011-1081.ch014
Traditionally, Jerez vinegar is elaborated by surface culture; however, it is accepted to obtain the required acetic degree by a quick acetification process and submit the obtained vinegars to aging in barrels afterwards. The aging process defines the type of Jerez vinegar as follows: 6 months (Jerez vinegar), 2 years (Vinagre de Jerez Reserva) and Vinagre de Jerez Gran Reserva (10 years). Almost the 70% of the production is aged 6 months and a 30% aged for 2 years, being the most aged production very limited. The regulation fixes a minimum acidity of 70g/L (7ºAcetic acid) and 1.3g/L dry extract per degree of acetic acidity. The analyses of chemical composition of Jerez vinegars demonstrate their special characteristics. Criteria to determine authenticity and traceability of the products are established on the basis of these scientific data. This paper presents the state of the art in the characterization of Jerez vinegars based on phenolic compounds as well as a discussion of recent advances in this field.
Authentication of Vinegars The price of common vinegars is based on acetic degree. Thus, the earliest falsification was to sell synthetic acetic acid dilution as natural vinegar obtained by a natural fermentation process. 13C isotope ratio mass spectrometry allows differentiating acetic acid obtained from fermentation from organic origin. However, adulteration is performed by mixing dilutions of natural vinegar and synthetic acetic acid. Recent advances to detect these additions are based on SNIF-RMN (Site Specific Natural Isotopic Fractionation Nuclear Magnetic Resonance Spectrometry) (3), an addition of just a 5% of synthetic acid can be detected. Below this limit, the practice becomes a non profitable trade. Moreover, isotopic methods allow to trace back the origin of chemically identical molecules. The 18C/16C ratio of water permits to differentiate vinegars from wine than those of raisins (4). Vinegars produced from different raw materials can be differentiated by their content in polyalcohols as they are related to the substrate and persist after both alcoholic and acetic fermentations. Indeed, apple vinegar presents a particularly high content in sorbitol (3296 mg/L) and honey vinegar in manitol (958 mg/L) whilst vinegar obtained from alcohol lacks polyalcohol (5). However, polyalcohol content is not useful to characterize wine vinegars. L-proline is a grape characteristic aminoacid and therefore it is present in their derivate products. Wine vinegars can present values up to 355-2187mg/L in Sherry vinegars (6), values 149-360 mg/L were reported in wine and lesser concentrations are suspicious of adulteration. The efforts in the characterization of vinegars have been addressed to most expensive products: Jerez Vinegar and Aceto Balsamico Traditionale. Different aspects of their chemical composition specially related to aroma compounds have been revised recently (7, 8). The present paper will focus on phenolic compounds as markers of quality vinegars. Phenolic compounds are widespread in the plant kingdom and have been proposed as taxonomic marker in wines and different foods (9–12). 203 Ebeler et al.; Progress in Authentication of Food and Wine ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on June 14, 2017 | http://pubs.acs.org Publication Date (Web): November 17, 2011 | doi: 10.1021/bk-2011-1081.ch014
First papers in the field described phenolic profile of Jerez vinegars by means of liquid chromatography coupled to diode array detectors (13). Suitability of this technique has been previously proved in many foods. Recently, these values have been compiled in the Phenol Explorer, a database on food polyphenols (14). Briefly, Jerez vinegars contain phenolic acids (gallic, protocatechuic, p-hydroxybenzoic, vanillic, caffeic, p-coumaric and ferulic acids and the tartaric esters of the hydroxycinnamic acids), aldehydes (vanillin, protocatechualdehyde, syringaldehyde, coniferaldehyde, p-hydroxybenzaldehyde), the flavanols (+)-catechin, (-)-epicatechin and caffeic ethyl ester and coumaric ethyl ester. Multivariate statistical analyses proved useful to differentiate vinegars using their phenolic profile. A total of 92 wine vinegars representing most of the brands in the market at that moment were sampled and vinegars from two geographical close regions (Condado de Huelva and Montilla Moriles) were included. Linear discriminant analyses and also artificial neural network trained by backpropagation permits to differentiate vinegars obtained by surface culture from those obtained from submerged culture with following recalling rates: LDA (mean=92.5) and BPANN (mean =99.6) (15). Many variables in our data matrix did not fit normality requirements to be included in a linear model. The cause was that not detected compound means a zero. When several zeros are present in the data matrix it causes a lack of normality in the variable limiting its inclusion in further statistical analysis. However, the presence of a certain compound in a group of vinegar and its absence in another group must be a tool for differentiating these vinegars and should be taken into account for discriminating purposes. The solution to this problem was the use of artificial neural networks which do not require a normal distribution of data. A short description of the statistical analysis of data is as follows: Phenolic compounds were used as variables to build up the data matrix. Cluster analysis explored data trends and natural groupings of samples. Clusters obtained by Wards’ method showed some data trends including most vinegars obtained by submerged culture or those samples from Condado de Huelva. Principal Components Analysis revealed which variables contribute most to the variance of data; therefore, it was advisable to include them in the discriminate functions. Indeed, the first PC corresponded to those phenolic compounds presenting a hydroxycinnamic acid structure (caffeic, caffeoyltartaric, coumaric acid…) and the second PC to flavonoid structure compounds. Then, supervised pattern recognition was applied. Samples were divided into two sets (training set (75% of samples) and test set (25% of samples)). To validate both the goodness of the classification and the goodness of the prediction both training and prediction sets were repeated 10 times for different constitutions. This procedure was applied to differentiate between vinegars from very close geographical origins: Jerez, Condado and Montilla. Indeed, they are no more than 200 Km one from another. The recalling rates were 88.8 for LDA, and 96.5 for BPANN (15). Table 1 displays classification functions obtained. On the other hand, aging in oak wood is an important aspect to develop organoleptic properties thus influencing quality. Indeed a Jerez category is determined by the aging period (less or more than 2 years). In order to determine whether or not phenolic profile could be useful to discriminate aged vinegars from 204 Ebeler et al.; Progress in Authentication of Food and Wine ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
Downloaded by CALIFORNIA INST OF TECHNOLOGY on June 14, 2017 | http://pubs.acs.org Publication Date (Web): November 17, 2011 | doi: 10.1021/bk-2011-1081.ch014
not aged vinegars, vinegar were sampled from wineries and divided according to their period of aging: less or more than 2 years as declared by producers. Some trends were observed in the concentrations of phenolic compounds however results obtained from BPANN do not allow to extract further conclusions (just a 90.4 for classification test and a 60% for the test set) (16). Nevertheless if aging period is strictly controlled in the samples used to construct the classification test, the assessed phenolic profile is useful to differentiate among vinegars with the following reference periods of aging (90, 180 and 225 days) as proved in a controlled experience. After 90 days of aging in oak wood barrels, the concentration in 5-(hydroxymethyl)-2-furaldehyde, 2-furaldehyde, vanillin, syringaldehyde and coniferaldehyde increased significantly (p
