Chapter 21
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Quality and Authenticity Control of Fruit-Derived Products Andreas Schieber,*,1 Ralf Fügel,2 Christina Kurz,2 and Reinhold Carle2 1Department
of Agricultural, Food and Nutritional Science, University of Alberta, 410 Agriculture/Forestry Centre, Edmonton, Alberta T6G 2P5, Canada 2Institute of Food Science and Biotechnology, University of Hohenheim, Garbenstrasse 25, D-70599 Stuttgart, Germany *E-mail:
[email protected] The authentication of fruit products such as fruit preparations and jams is most challenging because of their complex composition. In particular the determination of the fruit content, which is an important quality trait of fruit-derived products, is difficult. The situation is further aggravated by the lack of reliable methods. Previous approaches targeted at the determination of low molecular compounds were of limited usefulness because these can easily be manipulated. In contrast, hemicellulose, a high molecular constituent of the plant cell wall, proved to be a promising parameter for the quantification of the fruit content and even for fruit species determination. This contribution provides an overview of our recent investigations on quality and authenticity control of fruit preparations and jams, with a focus on fruit products made from strawberries, cherries, apricots, and peaches.
The adulteration of foods represents a serious economic issue. Fraudulent practices are not an invention of our time but have been observed throughout the history of food production (1), especially since profit margins in the food sector are comparatively low. Adulteration may occur in very different forms and affect virtually all food commodities including functional foods and natural health © 2011 American Chemical Society In Progress in Authentication of Food and Wine; Ebeler, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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products. For example, milk and dairy products have always been a “popular” target, whether through watering or the addition of melamine. The detection of such types of adulteration is relatively straightforward. In contrast, the detection of fraudulent manipulations of fruit products has proved to be challenging in the past, in particular in the case of fruit preparations and jams. Fruit preparations are important intermediates used for the production of dairy products, e.g. fruit yogurts, and in bakery products and ice cream. While there is no food law in place in Germany that governs the composition of fruit preparations, manufacturers are obliged to meet the requirements established by the German Federation of Food Law and Food Science (BLL) for fruit preparations. These guidelines stipulate a fruit content of 35% for fruit preparations in general, whereas preparations based on raspberry, raspberry/blackberry, black and red currant, gooseberry, banana, and pineapple require a fruit content of 25-30%. According to the BLL guideline, the fruit content of yogurt products shall be between 1.5% and 6%, depending on the type of yogurt. The fruit content of ‘fruit yogurt’ needs to be 6%, whereas ‘yogurt with fruit preparation’ contains only 3.5% fruit. The fruit content may even be lower (40% in all strawberry cultivars. The HC fraction of cherries showed 15.1-18.3% arabinose and 20.1-20.8% galactose. The glucose content ranged between 31% and 33% and was lower than in strawberries. The neutral sugar profile of the HC fractions of apples and strawberries was found to be very similar. As a result, fraudulent admixtures of apple purées to strawberry fruit preparations would not be detectable using this method, whereas blends of apple and cherry products would show decreased arabinose contents (5). Astonishingly, the neutral sugar composition of the HC fractions of yellow fruits also revealed significant differences although peaches and apricots belong to the same genus (Prunus). Apricots contained 9.8-12.2% mannose and 4.1-7.2% arabinose, whereas in peaches lower contents (3.9-6.7%) of mannose but higher contents (11.4-17.3%) of arabinose were found. Pumpkins were characterized by somewhat lower levels of galactose (6.5-12.2%) compared to contents of around 16% in apricots and peaches. Mannose was found only in trace amounts (6). The HC content in the AIR proved to be rather constant in the fruits. In strawberries, 12.7-14.6% was found, whereas in cherries the HC content ranged from 15.1% to 16.7% (4). HC levels in apricots were between 9.7% and 12.2%. In peaches we observed HC contents from 11.8% to 15.9%. Only one peach cultivar showed higher contents of 17.9% (6).
303 In Progress in Authentication of Food and Wine; Ebeler, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Determination of the Fruit Content of Fruit Products and Dairy Products Because of the striking constancy of the levels of the HC fraction and its exceptional stability during processing we decided to use this cell wall fraction as an analytical tool for the determination of the fruit content of fruit products, in particular fruit preparations. However, we first needed to establish a correlation between the fresh weight of fruits and the HC fraction. For this purpose, we defined the ‘conversion factor’, which allowed us to calculate the fruit content after determining the amount of HC in the fruit products (7). After recovery of the AIR from the fruits and fruit products, respectively, the HC fraction was isolated as described above by sequential extraction and dialysis. Strawberry fruit preparations with fruit contents ranging from 30% to 60% were produced to assess the suitability of the method. An excellent agreement of the specified and determined fruit contents was obtained in most cases, with absolute deviations from the initial fruit contents usually not exceeding 2.4%. Only for the sample with the highest fruit content (60%) we observed an overestimation of approximately 4% (7). Also in the case of cherry fruit preparations excellent results were obtained when single hydrocolloid systems and less complex combinations of hydrocolloids were used (26.8% vs. 30%; 38.6% vs. 40%; 42.5% vs. 40%; 37.6% vs. 40%; 41.2% vs. 40%). However, the deviations were larger (46.2% vs. 40% and 49.6% vs. 40%) when fruit preparations were produced using 3 hydrocolloids for stabilization (8). The latter required the enzymatic digestion of the added matrix compounds. In continuation of the above studies on red fruits, we expanded our investigations to self-made and commercial apricot and peach fruit preparations. Furthermore, jams and spreads made from apricot and strawberry fruits were included. The calculated fruit contents of most self-made fruit products (fruit preparations, jams) were in good agreement with the specified contents, with deviations ranging from 0.3% to 4.1%. Only in one case a considerable overestimation of 6.5% was observed. Remarkably, the determined fruit content of all commercial fruit preparations, jams, and spreads was well in agreement with the specifications obtained from the producers (9). These results are of particular interest because the detailed compositions of the fruit products as well as the cultivars of the fruits used for the manufacture of the jams, spreads and fruit preparations were not known. Since fruit preparations are often used as ingredients in fruit yogurts, the method was applied also to the determination of the fruit content of strawberry yogurt. For this purpose, individually quick frozen strawberries were blended with a commercial yogurt in proportions of 6, 10, 20, and 30%. Subsequently, the blends were homogenized. After digestion of the protein matrix using proteases and lyophilization of the sample, the AIR was prepared and extracted sequentially to isolate the HC fraction. While satisfactory results were obtained for the yogurt sample with high fruit content (30% vs. 31.5%), considerable overestimations were observed in cases of fruit contents, probably caused by the incomplete extraction of high molecular compounds (10). 304 In Progress in Authentication of Food and Wine; Ebeler, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2011.
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Conclusions The results obtained from the above mentioned studies demonstrate that the HC fraction is a promising parameter both for the determination of the fruit content of fruit-derived products and for the authentication of fruit species. The absolute amount of the HC fraction provides information about the fruit content, whereas the neutral sugar profile can be used, although with some limitations, for the differentiation of fruits. Most importantly, this method does not require information about the composition of the fruit products because all ingredients typically added during manufacture are removed through extraction and enzymatic digestion. However, although only simple analytical methods were used, that is, gravimetry, gas chromatography, and capillary electrophoresis, it should be noted that the approach presented is not a rapid method. In particular, sample preparation including lyophilization, sequential extraction, and dialysis take several days. As a result, a high sample throughput cannot be accomplished. Therefore, we recently developed an analytical procedure that employs Fourier transform near infrared spectroscopy and chemometrics for the rapid determination of fruit authenticity and for the quantification of the fruit content (11). Fraudulent practices will continue also in the future and most probably one single method will not be sufficient to detect all possible types of adulterations of fruit products. However, we feel that the availability of as many methods as possible will increase the expenditures of perpetrators to an extent that finally makes adulterations uneconomical. Therefore, efforts targeted at quality and authenticity control of fruit-derived products should be continued also in the future.
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