Evaluation of different cod viscera fractions and their seasonal variation used in a growth medium for lactic acid bacteria

With the long-term goal of developing reliable processes for the conversion of by-products from the fish industry to ingredients for microbial growth media, we have investigated seasonal variation in mixed fish viscera and four subfractions (roe, milt, stomachs, and liver residues), obtained at different time points during two consecutive spawning seasons (2003 + 2004) of north-east Arctic cod. Variation was evaluated by chemical analyses and by a bioassay based on measuring growth of two lactic acid bacteria on media which contained enzymatically prepared hydrolysates of the viscera fractions as complex nitrogen source. The chemical analyses (dry matter, ash, total nitrogen) showed clear differences between the different viscera fractions but did not reveal any systematic seasonal variation. The microbial growth tests showed that Lactobacillus plantarum grew very well, regardless of the hydrolysate used and with little seasonal variation. The more fastidious Lactobacillus sakei generally grew less well than L. plantarum and growth depended both on the type of hydrolysate and on the season. These results show that seasonal variation must be considered when preparing microbial peptones from fish viscera which are to be used for the most fastidious of bacteria. However, seasonal variation seems not to be a problem when developing fish viscera-based peptones for less fastidious bacteria, such as L. plantarum and many other common microorganisms which are less demanding than lactobacilli.