Figure 1. Overview on
alkyl pyrazines. The most processed alkyl pyrazines1 -7 . Annual volumes of use of pyrazine derivatives
used as flavoring agents in Europe 2004 is stated in kg/year, whereas
the * represents the quantitative natural occurrence of alkyl pyrazines
in foods in Europe in tonnes per year [6]. Special attention belongs
to compounds 1 , 3 , 4, and 8 which
are commercially available as pure substance declared as natural whereas
others are at least partially chemical synthesized. Short notice: To the
best of our knowledge these are the latest official volumes of the WHO
and EFSA [7]. Non-commercial Synthetic alkyl-pyrazines11 -16 .
The consequences are high extraction costs and a product line that
suffers from high risk of change in taste and smell. Therefore,
industrial research is turning towards bio-based and/or biocatalytic
strategies. Nowadays, the use of certain microorganisms [8] is well
known and declared as natural and safe by food authorities [9]
(EFSA, FDA etc. ), and offers the chance for a better economic
feasibility with mild reaction conditions, high specificity and
simplified downstream processes due to higher purities, better
reproducibility and constant sourcing in contrast e.g. to the natural
feedstock extraction.
Pyrazines are volatile, nitrogen-containing heterocyclic compounds
widely distributed in plants [10], insects [11], fungi and
bacteria [12]. They act as an odor signal to repel predators and
effectively prevent vegetative tissue or immature fruit from being eaten
[13]. For this reason, pyrazines find various applications as
ingredients in pesticides, insecticides, dyes, and pharmaceutical
compounds [14]. Pyrazines gained attention from the food industry as
important ingredients in raw and roasted foods. Especially alkylated
pyrazines are in the focus, as they have strong olfactory properties
(Figure 1). They can be extracted from natural sources such as potatoes,
nuts and coffee with applications in numerous areas, e.g.
2,3-dimethypyrazine 6 (coffee, nutty, roasty) is used in
various beverages and sauces, while 2,5-dimethylpyrazine 3(nutty,
musty,
earthy,
roasted
cocoa) is
used in breakfast cereals. 2,3,5-Trimethylpyrazine 1 (chocolate
enhancer) and 2,6-dimethylpyrazine 4 (roasty, nutty, beef) are
used in meals with coffee, cocoa, meat or potatoes as a roasted flavour.
2-Ethyl-3-methylpyrazine 2 (nutty, musty, corn, raw, earthy,
oily) is used in peanut products, popcorn, and in bread. The perceived
odor depends on the dilution (recommended 0.1% or less in dipropylene
glycol) of the substance. Methoxylated pyrazines are mainly used in the
perfume industry to improve the odor of cosmetics and toiletries
[15] (fresh, green, woody, ambery, oriental, musky, minty and
herbaceous). Pyrazine flavors are often added to microwave foods because
roasting odors are not formed in microwaves at temperatures below 200°C
[16].
The growing demand for the ‘natural‘ alkyl pyrazines described above
cannot be satisfied by extraction and concentration techniques from
natural sources alone, since the natural content of e.g. 0.01 wt. %
pyrazines in sugar beet molasses is low [14c, 17]. Modern
biotechnology, recognized as a sustainable economic process, could cope
with the growing demand for ‘natural‘ compounds in the future and may
further increase the amount may fulfil the market demand of selectively
substituted pyrazines.