08-06-2013, 04:56 PM
Specific yeasts developed for modern ethanol production
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INTRODUCTION
Yeasts for ethanol production are subjected to new and different
conditions and expectations with innovative process alterations in
ethanol plant design attributed to microbiological and engineering
advances. Driven by the explosive growth of the North American
dry grind corn (maize) fuel ethanol market, modern yeast selection,
production and formulation technologies have led to valuable
specialty yeast products suited for today’s ethanol production facilities.
As a result, the ethanol producer has a number of viable
commercial yeast product options to choose from, including a variety
of yeast strains and formulations. Additionally, in-plant yeast
management options abound. The optimal yeast choices – strain,
formulation and management – for each facility depend on a
number of interrelated factors. Decisions are getting more complicated
– and more critical – as ethanol production technologies and
markets mature.
Yeast selection, step one:
Review ethanol production plant objectives
Prior to selecting a yeast strain, formulation type, or yeast management
protocol, a review of plant objectives and priorities should
be conducted. To facilitate selection of appropriate yeast-related
options for a specific ethanol plant, choose one of these three main
operating objectives:
– efficiency (or yield);
– throughput;
– consistency.
If each ethanol production plant was designed to achieve one of
these main objectives, the remaining two objectives will also be addressed
by good plant designs. However, when analyzed in depth,
the overall theme of each plant design can be associated with one
main objective.
Yeast strains
For much of human history, alcohol (ethanol) production has been
accomplished by ‘wild’ yeast strains – the yeast population naturally
resident on the fermentation feedstock or other ingredients. By
the early 1900’s, yeast strains were routinely selected from ‘good’
fermentations and used over and over again. This practice, despite
the crude yeast propagation techniques employed, led to improved
consistency and quality of final products.
Yeast management
A number of yeast management practices are available to today’s
ethanol producer. The most complex aspect of yeast management
is commercial-scale yeast production. It is possible for yeast to be
produced at the ethanol plant from seed vials by end-users, but
because of the need for appropriate amounts of air (O2) and the
programmed addition of low but constant amounts of utilizable
sugar – all with sterile medium and equipment – this task is best
undertaken by companies with the proper technology, equipment
and downstream processing. Yeast companies provide large volumes
of yeast in a number of product formats, each with positive
and potentially negative attributes.
After yeast products are received into the ethanol plant there are
several in-plant yeast management practices to be considered.
Depending
on the physical and physiological states of the yeast when
it enters the plant, it may take some time to rehydrate yeast cells
and initiate the biochemical pathways for cell metabolism and ethanol
production. Rehydration in clean, warm water can take several
hours. Nurturing yeast cells through the lag phase and into the log
growth phase, during which maximum ethanol production occurs,
can take 6 or even up to 12 hours.
Commercial yeast production
Yeast reproduce by a process called budding; a single yeast cell develops
a ‘bud’ on its surface which progresses to a fully functional
daughter cell. The daughter cell consists entirely of cellular matter
from the mother cell. In order to produce daughter cells, yeast cells
must be in good general condition and have access to all of the
nutrients they need to grow before budding. The main objective of
commercial yeast production is to propagate a single yeast cell to
a very large number of genetically identical yeast cells. Propagation
is certainly necessary for economic ethanol production, but
an equally important objective of modern yeast production is to
ensure consistently excellent performance of the yeast in end-user
applications. Both of these objectives can be accomplished under
meticulously controlled conditions.
Batch yeast propagation
Propagating yeast in a singe-batch process has many of the drawbacks
and advantages of continuous propagation. Batch propagation
is the process used for most of today’s commercial yeast
production, but in ethanol plants it is typically characterized by
limited investment in support of achieving appropriate levels of
cleanliness, sterility of mash, process control and quality control.
As practiced today, in-plant yeast propagation is never completely
achieved. True propagation is only achieved by utilizing pure yeast
culture, sterile conditions in early stages, and low carbohydrate and
high oxygen levels in the final fermentation. The term propagation
should therefore not be used to describe lower levels of yeast cell
growth under what are in reality anaerobic conditions. Sterility can
not be maintained in a commercial ethanol plant setting and appropriate
aeration and substrate control are not achieved. Risk of
contamination is therefore high.