substructures can be achievedspontaneously by the natural phenomena of
adhesion and electrostaticforces.However, strongerstructures are best achieved
by forcingparticles to bindtogether usingmoisture or especially addedbinders
whilethe particles are being fluidisedor mixed.
A definition of a dry product in the senseof microbial stabilityis not so easily
given,since it can changeslightly fromproductto product. As a rule of thumb,
one can say that whenthe wateractivity is below60%,littleto no microbial
growthwilloccur.Drymaterials can be characterisedby boththeir single
particle(suchas shapeand size) and theirbulk(such as bulkdensityand
flowability)characteristics.It should be emphasisedthat the bulkcharacteristics
of industrial dry materialsare at leastas important as their singleparticle
characteristics, and for each material, the most important characteristics
influencingmaterials handlingwill vary.
Flowability is an importantcharacteristic for dry material retention in
equipment, and generallyimproveswith:
∑ increase in particle size and particlesphericity;
∑ decreaseof moisturecontent;
∑ decreasein fines content;
∑ decreasein surface stickiness;
∑ decreasein neutralisationof surfaceenergy/charge.
Hygienic processing also influencesthe dry material quality propertiesof:
∑ aroma;
∑ chemical, biologicalor physical activity;
∑ colour;
∑ flavour.
In general it can be statedthat,basedon theircharacteristics,powders have
the tendencyto stickto productcontactsurfaces and are morelikelyto remainin
the process line as compared with liquids. Also, lump formation and
hygroscopicpropertiesare importantparameters in enhancing this effect.As
mentioned earlier,an increasedmoisturecontentin the powder (andremaining
powder residues!!) can causeserious proliferation of microorganisms. The
above-mentioned effects have to be takeninto serious considerationwhen
designing equipmentprocessingpowders.
13.3 Cleaning regimes
Thecriteria for hygienicdesignof equipment and plants for dry materials
handlingdependuponthe moisture contentof the dry material and the method
of cleaning.
Whetherthe equipmentis cleanedwet or dry has a significant effecton the
designcriteria. If wet cleaning procedures are appliedthe designhas to fulfil the
generalrequirementsfor equipmentin the liquidareaas described in several
Improving the hygienic designof equipmentin handling dry materials 221