In steam blanchers, a product is transported by a chain or
belt conveyor through a chamber where ‘‘food-grade’’
steam at approximately 100C is directly injected. Usually
temperature in the headspace is measured and the flow
rate of steam is controlled. Steam blanching is usually
used for cut and small products, and requires less time
than water blanching because the heat transfer coefficient
of condensing steam is greater than that of hot water (refer
to the articles ‘‘Convection Heat Transfer in Foods’’ and
‘‘Convective Heat Transfer Coefficients’’). However,
because of the high-temperature gradients between the
surface and the center of the product, larger products or
pieces of product can be ‘‘overblanched’’ near the surface
and ‘‘underblanched’’ at the center. To increase heat
transfer efficiency, forced convection blanchers have been
designed. These blanchers are made of nested chambers
that allow recirculating steam with a fan that interconnects
both chambers. The fan forces the flow of steam through
a packed bed of product conveyed by a mesh belt. This
technology allows higher product bed depths and higher
product throughput. Figure 1 shows a picture and a
schematic of the cross section of a forced convection
blancher. Another technology, individual quick blanching
(IQB), was developed to minimize product treatment
nonuniformities. In IQB, a single layer of product is
conveyed through the steam chamber and each ‘‘individual’’ piece of product immediately enters in contact with
the steam.
Steam blanching is more energy-efficient and produces
lower BOD and hydraulic loads than water blanching.
In addition, nutrient leaching is reduced compared to
water blanching.