The other two, were food grade systems with microdroplets of corn or olive oil (m.d. 2.1 and 2.2 μm, respectively) emulsified with peptides and stabilized with xanthan.
Since vitamin C is an important and widely used food supplement, expression of 2,5-DKG reductase with such food grade systems could indeed represent an interesting option.
Moreover, the expression level of RDPE in the newly developed food-grade system was comparable to the level obtained in the conventional kanamycin-based system.
Fig. 3 Expression of RDPE in the food-grade system: a the intra- and extracellular activity analysis of RDPE in 1A751D2R.
Although the reported performance of food grade expression systems is usually low compared to standard expression systems using E. coli (Nguyen et al. 2011a ), an advantage of applying GRAS (i.e., food-grade, Peterbauer et al. 2011 ) expression systems is that the costs to satisfy food safety requirements could be drastically reduced.
A strong argument to employ food grade expression systems however is that such, the costs to satisfy food safety requirements may be significantly reduced (Mierau et al. 2005 ).
In addition, the here applied food grade expression systems do not contain potentially harmful, transferable antibiotic resistence markers (Peterbauer et al., 2011 ).
The aim of the present work was to evaluate the food grade expression systems pSIP and NICE for their capacity to produce the industrially important enzyme 2,5-DKG reductase from C. glutamicum (order Actinomycetales).
To broaden its applications, especially in the food and feed industry, we constructed a novel, stable, food-grade expression system by engineering its type II toxin antitoxin system.
It also introduced a grading system for food — meat, eggs, dairy products, and fresh fruits and vegetables — to flag and discourage substandard quality.
Moreover, the production of RDPE in this food-grade expression system was comparable to that in neo-based system.
