Biomedical Engineering Reference
In-Depth Information
6.5.2.1
Fresh Baker's Yeast
Fresh baker's yeast is commercialised as block or compressed yeast, granulated
yeast and liquid (cream) yeast. Compressed yeast is available in blocks with variable
consistency. The consistency varies from high plasticity (kneadable) to friable
(crumbly texture). Granulated yeast is in the form of small granules. Liquid yeast is
a suspension of yeast cells in water with a cream-like viscosity. The dry matter content
of fresh baker's yeast varies depending on the formula, the fermentation performance,
and the consistency/friability. Liquid and compressed yeast with high plasticity and
friability have a dry matter content of 15-21, 27-31 and 30-35%, respectively. The
dry matter of granulated yeast is 31-37%. The nitrogen content of the dry matter has
a value of ca. 8.0%. The ash content of the dry matter is ca. 6%, and the value of pH
is usually ca. 5.0. Fresh baker's yeast is suitable to be used directly in dough. As it
is a perishable product, it should be stored under refrigerated conditions.
6.5.2.2
Dry Baker's Yeast
Dry baker's yeast is sold as active dry yeast (ADY) and instant dry yeast (IDY).
ADY is characterised by spherical particles with a diameter of 0.2-3 mm. The yeast
is reactivated under warm water at ca. 38°C (not exceeding 45°C) and then used as
fresh yeast. IDY consists of porous cylindrical particles with a diameter of 0.5 mm
and length up to a few millimetres. Rehydration in water is not necessary and it may
be added directly to the flour. Depending on the activity and recipe, IDY is added to
the flour at a level that corresponds to one-third of that of fresh yeast. Direct contact
of the yeast with salt, fat or sugar has to be avoided to prevent osmotic stress and
dispersion during rehydration of the flour. Careful rehydration of the flour with
water at room temperature is needed. The dry matter ranges from 92-96 to 93-97%
for ADI and IDY, respectively. The density is 75-95 and 55-80% for ADI and IDY,
respectively. The nitrogen content of the dry matter has a value of ca. 8.0% and the
ash content of the dry matter is ca. 6%,with a pH of usually ca. 6.0.
References
1. Gobbetti M (1998) The sourdough microflora: interaction of lactic acid bacteria and yeasts.
Trends Food Sci Technol 9:267-74
2. Mäntynen VH, Korhola M, Gudmundsson H, Turakainen H, Alfredsson GA, Salovaara H,
Lindström K (1999) A polyphasic study on the taxonomic position of industrial sour dough
yeasts. Syst Appl Microbiol 22:87-96
3. Pulvirenti A, Solieri L, Gullo M, De Vero L, Giudici P (2004) Occurrence and dominance of
yeast species in sourdough. Lett Appl Microbiol 38:113-117
4. Rocha JM, Malcata FX (1999) On the microbiological profile of traditional Portuguese sour-
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