Carrots blanching

Extensive carrot blanching (100°C for 60 min) cansed losses of 26 to 29% in total p-carotene content, along with increased IS-CM-P-carotene contents up to 10% after pasteurization 95°C, P = 3) and to 14% after sterilization 121°C, F = 5). ° However, unheated juices produced from carrots blanched at 80°C for 10 min were devoid of cis isomers, and further pasteurization or sterilization processes formed only IS-di -P-carotene, at 2 and 5%, respectively. ... 

The structural features of ceU wall polysaccharides of carrots have been studied by Stevens and Selvendran (1984) and Massiot et al.(1988). Plat et al.(1991), Ben Shalom et al.(1992) and Massiot et al.(1992) investigated the changes in pectic substances of carrots after blanching, dehydration and extended heat treatment. Data on the changes in ceU waU polysaccharides of canned carrots are lacking. This study aims to investigate the effect of preheating time at low temperature and the addition of CaCL on texture and on the composition of various pectin fractions of carrots canned by conventional and by a new process. 

Effect of preheating time on firmness and pectic polysaccharides Firmness of canned carrots preheated at 65 C for 15, 30 and 60 min is illustrated in Fig. 1. Preheating for 15 min showed a firmness of 261.6N. When preheating time was increased higher tissue firmness was observed. Similar observations have been reported by Lee et al., (1979) and Quintero-Ramos et al., (1992) who found that firmness of carrot tissues was increased with increasing the time of blanching at 65 C. 

Degree of methylation (DM%) of total pectin of carrot alcohol insoluble sohds (AIS) was decreased from 60.73% for fresh carrots to 48.70, 44.62 and 43.83% for canned carrots preheated at 65°C for 15. 30 and 60 min, respectively (Fig. 2). Similar levels of demethylation were also reported in potato (Bartolome and Hoff, 1972), in carrots (Lee et al., 1979) and in snap beans (Adams and Robertson, 1987) when they were blanched at low temperature between 65°C and 70°C. 

Figure 1. Effect of blanching time on texture of canned carrots...

Prakash, S., Jha, S.K. and Datta, N., Performance evaluation of blanched carrots dried by three different driers, ]. Food Eng., 62 (2004) 305-313. 

Fuchigami, M., Miyazaki, K., and Hyacumoto, N. (1995). Frozen carrots texture and pectic components as affected by low-temperature-blanching and quick freezing. ]. Food Sci. 60, 132-136. 

Kidmose, U. and Martens, H. (1999). Changes in texture, microstructure and nutritional quality of carrot slices during blanching and freezing. J. Food Sci. Agric. 79,1747-1753. 

Bourne, M. C. (1987). Effect of blanch temperature on kinetics of thermal softening of carrots and green beans. 

Canet, W., Espinosa, J. (1984). The effect of blanching and freezing rate on the texture of potatoes, carrots and peas, measured by mechanical tests. In P. Zeuthen, J. C. Cheftel, C. Eriksson, M. Jul, H. Leniger, P. Linko, G. Varela (Eds.), Thermal Processing and Quality of Foods (pp. 678-683). Elsevier Applied Science, London. 

Common unit operations of food processing are reported to have only minor effects on the carotenoids (Borenstein and Bunnell 1967). The carotenoid-protein complexes are generally more stable than the free carotenoids. Because carotenoids are highly unsaturated, oxygen and light are major factors in their breakdown. Blanching destroys enzymes that cause carotenoid destruction. Carotenoids in frozen or heat-sterilized foods are quite stable. The stability of carotenoids in dehydrated foods is poor, unless the food is packaged in inert gas. A notable exception is dried apricots, which keep their color well. Dehydrated carrots fade rapidly. 

An example of this approach is shown with carrots (19). Seventeen cultivars of carrots were sliced transversely and blanched 4 min at 212F or 165F, then canned and processed 23, 30,45,60,75 min at 250F. The firmness of the phloem tissue and xylem tissue (core) were separately measured by a puncture test. Figure 5 shows log firmness versus process time for the cultivar Dominator Sunseed together with the equations for the lines of best fit and the correlation coefficients R. Similar plots were obtained for the other sixteen cultivars. Sufficient data was obtained to measure substrate "b" and the thermal firmness value. No data were collected to measure substrate "a" because it is not needed. However, the firmness at zero process time was measured and these points are plotted on the ordinate. The correlation coefficients range from r = 0.98 to r = 1.00 which shows the excellent fit of the empirical data to the kinetic model. 

A three level four factor three response design of RSM was employed to optimize the carrot dehydration process. The four factors were drying temperature in HTST fluidized bed dryer, exposure time in HTST fluidized bed dryer, concentration of biopolymers and blanching time. The three responses were rehydration ratio, bulk density and carotenoid loss during the process. Second degree polynomial equations and Statistical Analysis System were used to fit the data for RSM. 

For dehydration of diced potatoes, a three level four factor four responses design was used. The four factors stayed the same as in the carrot study. The four responses were rehydration ratio, bulk density, non-enzymatic browning and water holding capacity. Optimal conditions were determined to be 1450C arid 10 min in HTST fluidized bed dryer with blanching time of 4.5 min and biopolymer... 

Park, 1987) also reported no significant differences in the /3-carotene content or colour between conventional and microwave cooked carrots, showing that the methods and the processing conditions had similar effects. However, different coimnodities or different processing conditions may result in inconsistent results. Drake et al. (1981) found that water-blanched sweetcom was superior in yellow colour to microwave blanched com. 

Prestamo, G., Fuster, C., and Risueno, M.C. 1998. Effect of blanching and freezing on the structure of carrots cells and their implications for food processing. Journal of the Science of Food and Agriculture 77 223-229. 

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