Optimization of Agaricus bisporus blanching. Modeling of shrinkage, heat transfer and quality loss

ALEJANDRO LESPINARD; SANDRO GOÑI; RODOLFO MASCHERONI

Lisboa, Portugal

Conferencia; EFFoST / EHEDG Conference 2007 – Practical application of research results; 2007

Blanching is an important operation during the production of mushroom preserves. One of its main objectives is to induce volumetric contraction, that can reach up to 50-70% of the volume of fresh product, avoiding that this shrinkage take place during sterilization. It is important to optimize this process, not only for controlling contraction but also deteriorative processes during heating. In this sense, modelling volume contraction and temperature histories is a valuable tool. This work aims at developing a kinetics for size change and a model to describe heat transfer to the mushroom, to use them to optimize the blanching process. Experiences were performed at different temperatures (60, 70, 80ºC) where thermal histories and volume reduction were determined. The time necessary to reach 18% of contraction was determined as adequate for the lenght of this process (80% of maximum volume reduction). Volume variation was determined with a computational video system and further image processing by photogrammetry (Image Processing Tolboox, Matlab®), which enabled to characterize the proposed contraction kinetics (Eq. (1)), being k a constant dependent of water bath temperature (Eq. (2)). The developed heat transfer model was solved by finite elements (COMSOL 3.2) with constant thermophysical properties and irregular 3D domain. Simulated temperatures for the thermal center showed a somewhat higher time lag than experimental ones owed to unavoidable heat conduction through thermocouples (Sastry et al, 1985; Akterian, 1995). Asumming that measured temperatures match average mushroom temperatures, absolute relative differences lower than 8% arose between experimental and predicted data. As a quality index the average Cooking Value for each process, (C100 25), was computed. Their values were 0.84, 1.21 and 0.76 minutes, respectively, for the three temperatures. These results show that blanching at 80 ºC is the best due to lower quality loss and process time.                        (1)            (2)