A new method for HTST sterilization of particulate foods
Abstract
A batch High-Temperature-Short-Time (HTST) sterilization system was designed and used to study heat transfer aspects and quality changes in food particulates subjected to microwaves and/or steam heating. Cylindrically cut potato of equal diameter and length was used as an illustrative food material. Particle sizes studied were 13 mm, 28 mm, and 35 mm. Microwave output power of 336 W and steam temperatures ranging from 110$\sp\circ$C to 132.2$\sp\circ$C were used. A time-dependent finite element model for a two-dimensional axisymmetric body was developed and added to analyze the experimental results. For the direct steam heating, lack of lethality in the particulate center became evident when the particulate size increased. Computer simulations indicated that rate of heat penetration into the particulate center was sensitive to thermal conductivity of the particulate, but was insensitive to the steam-particulate interfacial heat transfer coefficient. Microwaved particulates showed pronounced center heating with decreasing temperature gradient from inside out. Initially, center heating rate of the particulate under atmospheric pressure was linear. This was followed by a declining heating rate up to a maximum temperature, which then quickly dropped to 100$\sp\circ$C. Simultaneously, the quick temperature drop caused an evident tissue failure in the particulate center. Sequential heating of a particulate with microwave energy followed by steam showed a remarkable center temperature drop followed by a delayed temperature rise. Steam was found to be inert to the microwave heating. A combination process of simultaneous microwaves and steam resulted in rapid and uniform heating of particulates under HTST conditions. No tissue failure in the particulate center was also found for particulates heated by the simultaneous combination process. A lower z value found for microwaved spores of Bacillus stearothermophilus suggested possible reduction in process time by the use of microwave energy. A transient axisymmetric finite element model related to heat conduction without internal heat generation was used to determine the steam-particulate interfacial heat transfer coefficient (h) semi-empirically. The estimated h for 35 mm, 28 mm, and 13 mm potato samples were 948, 1605, and 2600 W/$m\sp{2\circ}$C, respectively. A heat generation term related to the microwave irradiation was derived and incorporated into the preceding model for the analysis of temperature distribution in the microwaved particulate. Using dielectric properties of the potato, particulate temperatures below 80$\sp\circ$C were predicted well. However, particulate temperatures beyond 80$\sp\circ$C were corrected using dielectric properties of water.
Degree
Ph.D.
Advisors
Singh, Purdue University.
Subject Area
Food science
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