Besides energy efficiency, product quality is gaining importance in the design of drying processes for sensitive biological foodstuffs. The influence of drying parameters on the drying kinetics of apples has been extensively investigated; the information about effects on product quality available in literature however, is often contradictory. Furthermore quality changes obtained applying different drying parameters are usually hard to compare. As most quality changes can be expressed as zero, first or second order reactions and mainly depend on drying air temperature and drying time, it would be desirable to cross-check the results in function thereof. This paper introduces a method of quality determination using a new reference value, the cumulated thermal load. It is defined as the time integral of the product surface temperature and improves the comparability of quality changes obtained by different experimental settings in drying of apples and tomatoes. It could be shown that quality parameters like color changes and shrinkage during apple drying and the content of temperature sensitive acids in tomatoes vary linearly with the integral of product temperature over time.
Post harvest technology
(2015)
When designing drying processes for sensitive biological foodstuffs like fruit or vegetables, energy and time efficiency as well as product quality are gaining more and more importance. These all are greatly influenced by the different drying parameters (e.g. air temperature, air velocity and dew point temperature) in the process. In sterilization of food products the cooking value is widely used as a cross-link between these parameters. In a similar way, the so-called cumulated thermal load (CTL) was introduced for drying processes. This was possible because most quality changes mainly depend on drying air temperature and drying time. In a first approach, the CTL was therefore defined as the time integral of the surface temperature of agricultural products. When conducting experiments with mangoes and pineapples, however, it was found that the CTL as it was used had to be adjusted to a more practical form. So the definition of the CTL was improved and the behaviour of the adjusted CTL (CTLad) was investigated in the drying of pineapples and mangoes. On the basis of these experiments and the work that had been done on the cooking value, it was found, that more optimization on the CTLad had to be done to be able to compare a great variety of different products as well as different quality parameters.
In modern fruit processing technology, non-destructive quality measuring techniques aresought for determining and controlling changes in the optical, structural, and chemical properties of theproducts. In this context, changes inside the product can be measured during processing. Especiallyfor industrial use, fast, precise, but robust methods are particularly important to obtain high-qualityproducts. In this work, a newly developed multi-spectral imaging system was implemented andadapted for drying processes. Further it was investigated if the system could be used to link changesin the surface spectral reflectance during mango drying with changes in moisture content andcontents of chemical components. This was achieved by recovering the spectral reflectance frommulti-spectral image data and comparing the spectral changes with changes of the total soluble solids(TSS), pH-value and the relative moisture contentxwbof the products. In a first step, the camera wasmodified to be used in drying, then the changes in the spectra and quality criteria during mangodrying were measured. For this, mango slices were dried at air temperatures of 40–80◦C and relativeair humidities of 5%–30%. Samples were analyzed and pictures were taken with the multi-spectralimaging system. The quality criteria were then predicted from spectral data. It could be shown thatthe newly developed multi-spectral imaging system can be used for quality control in fruit drying.There are strong indications as well, that it can be employed for the prediction of chemical qualitycriteria of mangoes during drying. This way, quality changes can be monitored inline during theprocess using only one single measuring device.
