■---- 5 mm
121 125 130 135
121 125 130 135
Figure 7. Time taken to reach F0 = 5 in center of particulate.
Temperature (°C) Figure 8. Cooking values versus temperature irrespective of the sterilization temperature. This results in undesirable overcooking of the carrier fluid and the outside surface of the particulates. For example, processing a 19-mm particle to a minimum F0 value of 5 at a sterilization temperature of 135°C will require the carrier fluid to have had an equivalent F0 treatment of 108, as the heat penetration time will have been the order of 4 min. In contrast, ohmic heating can achieve a high F0 level with a low value of cook and nutrient/vitamin destruction. Figure 8 shows the effect of processing temperature on F0 and C0. The C0 value is based on a Z value of 33 and is calculated as wm where y is the [(°C - 100)/Z] and T is the time, min. Sterilization at 140°C to a virtually uniform F0 of 24 equates to a likewise uniform C0 value of less than 5.
This relationship allows such products as diced potato to be subjected to an F0 value greater than 24 and still not be fully cooked; thus, the particle retains its sharp edges throughout the process. Indeed such products processed through the ohmic system therefore require further cooking (as part of the reheat process) by the end user.
The ability to heat particulates uniformly, without mechanical damage, with lower nutrient and vitamin loss, and with no fouling of heat transfer surfaces leaves no doubt that ohmic heating will play a major role in the growing requirement for aseptic food products containing large particulates, as it provides the food processor with an opportunity to produce new high-quality, long-life products previously unseen with alternative sterilization techniques.
Typical of such high-quality added-value products are those already fully tested that contain meat chunks, shrimps, baked beans, mushrooms, diced or sliced vegetables, pasta, whole strawberries, black currants, blackberries and sliced kiwi fruit.
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