Apple juice from eight different varieties of apples was heated at high-temperature (60-90 degrees C) and short-time (20-100 s) (HTST) combinations. To determine the effect of heating conditions on enzymatic browning and cloud stability in apple juices, the activity of polyphenol oxidase and pectinesterase was analyzed and correlated with the thermal treatment conditions and the quality of the juice. Additional investigations included the measurement of pH value, soluble solid content, titratable acidity, color, and turbidity after 3 and 6 months. The results showed that HTST treatment at 80 degrees C already inactivated polyphenol oxidase, whereas pectinesterase activity was reduced to half and could even at 90 degrees C not be inactivated completely. In fact, highest residual pectinesterase activity was found at 60 degrees C. Heating at 70 degrees C caused stable pectinesterase activity and even a slight increase for 50 and 100 s heating times. Turbidity and lightness increased after HTST treatment. In particular, differences in cloud stability between the varieties were measured. HTST parameters did not correlate with the residual cloud stability after 6 months. The sensory evaluation revealed that only a few combinations were distinguishable. The best stability of cloud and color in relation to heat impact was achieved by HTST treatment between 70 degrees C/100 s and 80 degrees C/20 s.

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In two surveys (600 consumers) (1) the importance of color on a consumer's decision to buy strawberry nectar and jam, (2) the consumer's color acceptance of strawberry nectars from puree and adequate products containing colorants from the market as well as specially prepared samples and (3) the associated attributes to these samples were studied. In addition, a test was carried out in order to (4) characterize strawberry color and its changes during storage sufficiently with one value. Color has a big impact on a consumer's decision to buy strawberry nectars as well as strawberry jam. Neither gender nor age or consumption habits of consumers had a significant impact on color acceptance. For color assessment, an “acceptance factor” (AF) was calculated from colorimetric data that provides processors the prediction of color quality of nectars from puree. Nectars with an AF lower than 0.4 were not accepted, whereas nectars with an AF higher than about 0.7 were described as excellent. PRACTICAL APPLICATIONS The results of this study show the first time that it is possible to characterize the color of strawberry products by means of the calculation of an “acceptance factor” (AF = a*/h), and to predict whether the color will be accepted from the consumers or not (excellent [AF > 0.7], acceptable [AF > 0.4] or not acceptable [AF < 0.4]). This data could be used as basis for the production of new strawberry nectars (containing colorants) and the assessment for the stability and shelf life of strawberry products from puree.

BACKGROUND: Heat treatment during processing of strawberry products has been proposed to negatively affect colour stability. Moreover, the role of enzymes with respect to colour stability is ambiguous when consulting the existing literature. The aim of the present study was to investigate the impact of various processing parameters (pre-freezing, puree content, pasteurisation temperature and heating time) on the colour stability and anthocyanin monomer and L-ascorbic acid contents of strawberry nectars made from puree. In addition, the effect of different enzyme activities on colour stability during storage of strawberry nectars was investigated.

The impact of different amounts of the hydrocolloid xanthan gum (E415) on the typicity and flavour intensity of cloudy apple juice was tested by means of a survey for the first time by consumers. Consumers distinguished juice with even 0.5 g L–1 xanthan gum significantly in its typicity from the original apple juice. Increasing amounts of the stabilizer led to further degradation of typicity. The thickened juices were described as “slimy” with an untypical mouthfeel. Aroma intensity of the apple juice was affected negatively by addition of xanthan gum as well. Even 1.0 g L–1 of the stabilizer caused a significant decline of flavour intensity assessed by the consumers compared to the original juice. Viscosity increased significantly by adding xanthan gum, as well as turbidity. Colour components were affected differently. The addition of xanthan gum can therefore not be recommended for apple juice. Practical applications Presently, a worldwide discussion is going on about the legal permission of hydrocolloids in beverages, especially in fruit and vegetable juices and nectars. In Europe the addition of pectin (E440) is only allowed in pineapple and passion fruit juices. Additives belonging to the food additive group 1 (including xanthan gum (E415), gellan (E418), carboxymethyl cellulose (E446) and others) are allowed in vegetable juices only (regulation (EC) No 1333/2008). Studies show divergent results of the necessity and the impact of hydrocolloids on sensorial properties of beverages. The results of this study show that an average untrained consumer rejects cloudy apple juices thickened with xanthan gum even at low levels. Therefore, neither quality can be improved nor consumption of fruit juice can be increased by the addition of xanthan gum.

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