shortened R
of 15 min. Even after 5 min at 4 ppm/juice and 50.0 8 C, most of the dextran measured by the haze method was removed (Fig. 7b). ‘‘Concentrated’’ dextranase > 4 ppm/juice at 50.0 8 C had very little further effect (Fig. 7b) as most of the dextran substrate had been depleted and, therefore, 4 ppm/juice was an overdose because of asymptotic behavior under the stated conditions. In addition, the high water (second substrate) content in juice drives the thermodynamic hydrolysis reaction rapidly to further complete the reaction. Consequently, levels as low as 2 ppm/juice of the ‘‘concentrated’’ dextranase after only 5 min will be sufficient in juice at 50.0 8 C. Before addition at such a low level the ‘‘concentrated’’ dextranase should be first diluted with stabilizing sucrose solution to increase enzyme–substrate contact time. These results, therefore, strongly suggest that heating the juice to the maximum dextranase activity temperature of 50.0 8 C markedly improves the efficiency and economics of application and, to some extent, overcomes the limited availabilty of residence time in many factories.
3.5. Addition of dextranases to juice in the presence of biocide
One question that arises about the application of dextranases at the higher temperature of 50.0 8 C is will such a temperature stimulate adverse microbial growth and
possible dextran formation? It may be necessary to simultaneously add biocide and dextranase in the juice. For this reason the effect of dextranase was investigated in the presence of a biocide (dithiocarbamate) commonly used in US factories at approximately 10 ppm/juice levels. Results shown in Fig. 8 show that the ‘‘concentrated’’ dextranase still worked in the presence of the biocide, and 10 ppm levels of biocide may be slightly more favorable than 20 ppm additions. When severe dextran problems occur, an option in the factory could be to add 10 ppm/juice to the mill tandem and 10 ppm to mixed juice.3.6. Storage characteristics of ‘‘non-concentrated’’ and ‘‘concentrated’’ dextranases under simulated factory conditions
Dextranase vendors routinely recommend that barrels of dextranase should be stored in the coolest and shadiest area of the factory in order to prevent loss of activity at higher temperatures. To simulate such factory storage conditions we stored a ‘‘non-concentrated’’ and ‘‘concentrated’’dextranase in a cool and dark corner of a laboratory (ambient temperatures range from 23 to 27 8 C), and analyzed the effect of storage time on the dextranase activity across the approximate length of a grinding season (90 days). By the end of 90 days, the activity of the ‘‘concentrated’’ dextranase had decreased only slightly (9%; Fig. 9). In