# Upgrading of Mixed Food Industry Side-Streams by Solid-State Fermentation with _P. ostreatus_
## 1 Introduction  
Agro-industrial wastes and side-streams (AISS), which are produced in large amounts worldwide, are usually rich in fermentable components. Specifically, food industry side-streams, such as brewer's spent grains (BSG) and malt spent rootlets (MSR) from breweries, wasted fruits and vegetables form agricultural production, landfills, restaurant wastes, etc., whey from the dairy industries, and molasses from the sugar industries, have the high potential for biotechnological utilization. However, apart from molasses, they are used in limited amounts as livestock feed or are discarded in the environment since their high organic load makes biological treatment impossible [1; 2; 3; 4; 5; 6; 7; 8; 9; 10]. Bio-utilization of these materials and the conversion to products of added-value is, therefore, of great importance from both economical and environmental points of view. Single cell protein (SCP) production has been the subject of extensive research mainly for the production of animal feeds that are enriched in protein by microbial treatment of various AISS using both submerged (SmF) and solid-state fermentation (SSF) processes. The most common examples of food grade SCP are baker's, brewer's, and distiller's yeasts, as well as food and feed supplements (as protein and vitamin sources and flavor enhancers). Molasses is an ideal carbon source that is commonly added to other substrates that contain low amounts of fermentable sugar, in order to facilitate their bioconversion [7; 9; 10]. Whey also has a hugepotential for bioconversion to added-value products, such as food additives, protein, SCP, organic acids, enzymes, bioethanol, etc. [6; 9; 10]. The development of efficient and cost effective processes for its exploitation is a great scientific and industrial challenge, since it is generated in large amounts and has a high organic load. BSG and MSR can both be treated with cellulolytic, amylolytic, and/or ligninolytic species, to produce protein enriched livestock feeds, or other products of added-value. The use of BSG for such applications is limited but with promising results, while MSR have not been very widely exploited [3; 4; 5; 9; 10]. The exploitation of the above AISS for SCP production of various species, such as _Kluyveromyces marxianus_, _kefir_, and _Saccharomyces cerevisiae_, was recently reported [9; 10]. A promotional effect of the addition of orange pulp was shown during a SmF process using mixed substrates of orange pulp, cheese whey, molasses, and potato pulp. Specifically, a 2- to 3-fold increase of cell mass was observed in the presence of orange pulp. Likewise, the promotional effect of BSG on cell growth during SSF of mixtures containing BSG, whey, molasses, potato pulp, MSR, and orange pulp, was also shown. The _K. marxianus_ and _S. cerevisiae_ growth was enhanced by three-fold and two-fold, respectively, in these substrates, thus proving their suitability for SCP production without need for synthetic nutrients [9; 10]. Therefore, the mixed AISS can be used for low cost SCP production, achieving at the same time waste vaporization and minimization.  
The most studied food flavor enhancer is monosodium glutamate (MSG), although it has been accused for undesirable health effects, including the so-called _Chinese restaurant syndrome_[11; 12; 13]. Substitution of MSG with a safer natural flavor enhancer has been the subject of many research efforts based on the use of low cost biotechnological products, such as autolyzed microbial extracts [14; 15]. Autolysis is a degradation process that produces soluble peptides, amino acids, nucleotides, and amino acid derivatives. Autolyzed SCPs can also be used as low cost extracts to replace the expensive commercial ones that are used in synthetic growth media. In this study, the enrichment of six AISS (BSG, MSR, molasses, whey, potato, and orange pulp) by SSF with _Pleurotus ostreatus_ is described, as well as the autolysis and the proximate analysis of the produced SCP for potential use as multipurpose nutritious extract.