Analysis of Apple Juice

Because of the simplicity of UV spectra of procyanidins, it is reasonable to only form one absorbance ratio. For the characterization of procyanidins the ratios from absorbances at 220 nm / 275 nm [179] and 260 nm / 290 nm [263] (only monomers) are reported. The diastereoisomers (-)-epicatechin and (-i-)-catechin could not be distinguished properly [263], The value of absorbance ratios is dramatically diminished in the presence of matrix, which has been demonstrated in the analysis of apple juices [179]. We calculated... 

Figure 5 Verification of a fruit juice sample (rediluted apple juice from Poland). Left the 400 MHz H spectrum in the region near 2 ppm (black trace) is plotted over a quantiles plot (gray-scale) of the model spectra set (univariate analysis of apple juice at 2 ppm). Right influence plot of a multivariate analysis (circles reference samples star test sample green region representative of group red region not representative).

However, sometimes simple methods such as that developed by Low in the early 1990s using capillary gas chromatography, which is readily available in well-equipped laboratories around the world, can expose major problems of adulteration (Low Hammond, 1996). Here a well-established technique, analysis of sugars by capillary gas chromatography, was able to be applied to detect two routes that were known to be used to extend juices, the addition of HFCS and invert syrups. The method also discovered the adulteration of apple juices with another type of sugar syrup that had only been proposed as a possible adulterant. 

Bayindirli, L. 1992. Mathematical analysis of variation of density and viscosity of apple juice with temperature and concentration. J. Food Proc. Preser. 16 23-28. 

Table 8-8. Comparison of the results obtained with IC and enzymatic techniques for carbohydrate analysis in apple juice (taken from [57]).

Table IX shows the monomer analysis of freeze dried dialysis retentate of apple juices made in the laboratory with the three technologies. It provides information on the degree of...

Procyanidin samples are commonly chromatographed on C-18 supports. In the analysis of wine and grape tissue some authors prefer C-8 columns, because retention times are shorter [204,238]. C-6 columns have been used in the analysis of apple tissue, juices and ciders [66,197,239]. The eluent should include an acid to suppress ionization of phenolic acids. Gradients are applied starting at a high percentage of aqueous phase... 

Monakhova et al presented validation studies for multieomponent quantitative NMR analysis of apple fruit juice that includes, apart from other compounds, such sugars as galacturonic acid, arbutin, fructose, glucose, sucrose, quinic acid, maltotriose, maltose and xylose. The NMR analysis can be used for quality control of different fruit juices. 

Phenolic compounds are of interest due to their potential contribution to the taste (astrin-gency, bitterness, and sourness) and formation of off-flavor in foods, including tea, coffee, and various fruit juices, during storage. Their influence on the appearance of food products, such as haze formation and discoloration associated with browning in apple and grape products, is also significant. Furthermore, analysis of these phenolic compounds can permit taxonomic classification of the source of foods. The importance of each phenolic compound and its association with the quality of various foods is described further in Sec. IV, on food applications. 

Most HPLC applications used for phenolic analysis simply allow the room temperature to determine the operating temperature of the column, but elevated temperatures of between 30°C and 40°C are often applied for phenolics and derivatives in apples (14), carrots (15), apple juice (6,13), bilberry juice (16), and for cis-trans isomers of caffeic and p-coumaric acids in wines (17). Generally, a change in temperature has only a minor effect on band spacing in reversed-phase HPLC and has essentially no effect in normal-phase separations. Thermostatic control of the column temperature is generally recommended to provide reproducible retention. 

Since phenolic compounds occur in many fruits and most of them contribute to color and taste, phenolic analysis of fruits has been an active research area, especially in apple, grape, and citrus fruits and their products, such as juice, cider, and wine. 

High-performance LC analysis of phenolics in apples and pear juices are often performed by gradient procedures (49-54), due to the presence of several phenolic classes. Most of the HPLC methods applied to apple phenolics are very similar to that of Oleszek et al. (55). 

Figure 10.3 HPLC trace of a suspect apple juice run on Dionex HPLC using PA-10 column and PAD detection. Conditions column Dionex PA-10 250 X 4 mm solvent = 100 mM sodium hydroxide in HPLC water PAD detection sample diluted 1 100 in water prior to filtration and analysis.

Separation of the organic acids is also possible using an ion-exclusion column, such as a Bio-Rad HPX-87H. However, in this case it is essential that the acids are separated from the sugars otherwise a number of peaks co-elute, thus distorting the quantification. This method is particularly good for the analysis of fumaric acid in apple juice because no pre-treatment is required and the acid elutes late in the chromatogram, well separated from other components (Figure 10.7). 

Owing to the varied structures of various food dyes, they can often be differentiated from one another by their characteristic ultraviolet/visible absorbance spectra. Using HPLC coupled with a diode array detector (HPLC-DAD) it is possible to collect a compound s absorbance spectrum as it elutes from the HPLC column, which greatly assists in identification. At Reading Scientific Services Ltd (RSSL) this type of detector is routinely used in a range of analyses of such substances as patulin, a mycotoxin found in apple juice, and in the analysis of colours and vitamins, which allows a more certain assignment of a particular peak to a specific compound to be made. 

In recent years, the use of pasteurized apple juice as a growth medium for the initial inoculum ofL. oenos, ML 34, has become common. This is readily available, requires no preparation, and allows the malo-lactic organism to grow rapidly. Where possible, the inoculation of a lot of wine is accomplished at a level in excess of 10 percent inoculum. Once the initial lot of wine shows completion by paper chromatographic analysis, as described by Kunkee (22), it is used to inoculate a larger lot of wine, again at a 10 percent level or higher. This procedure is followed in subsequent inoculations. 

---