Marine peptones are defined as those sources of organic nitrogen, protein material obtained from fish wastes, which are part of the culture media designed for the growth of microorganisms.

In economic terms, peptones are the most expensive ingredient in microbial media.
There are two main procedures for obtaining these ingredients: through the enzymatic hydrolysis of the by-products or through the solid-liquid thermal extraction of the aqueous protein-rich fraction.

In the first case, the production process is quite similar to that previously described for the protein hydrolysates, that is, crushing of substrates, enzyme digestion of these substrates under controlled conditions, separation of non-digested materials, and recovery of oils from liquid hydrolysates by centrifugation-decantation.

The difference between peptone and FPH is that the enzyme deactivation process is carried out in an autoclave (at 121ºC for 15 min) to:

1) deactivate the enzyme 

2) kill the possible presence of microorganisms in the hydrolysates and 

3) help in the flocculation and removal of insoluble proteins.

In the second alternative for the production of peptones, the crushed material is mixed in a solid-liquid ratio (1:1) and thermally processed by autoclaving to improve the extraction of protein-rich liquid (peptone) together with the sterilisation of this material.

Two types of bacterial strains are especially interesting to evaluate the validity of marine peptones: lactic acid bacteria and bacteria of marine origin with probiotic properties.
Focusing on the case of lactic acid bacteria, the low-cost media that can be formulated with these marine peptones are a reproduction of the Man, Rogosa, Sharpe (MRS) complex medium.

Thus, the residual medium presents a carbon source, commonly glucose, a set of mineral salts, a surfactant, and the corresponding marine peptone incorporated at the same concentration of total soluble protein of the commercial peptones replaced.

The fermentation process begins when a small amount of an active culture of the selected lactic acid bacteria is inoculated into the culture flasks, and they are placed in an orbital incubator with shaking conditions and an adequate temperature for its growth.

Samples are taken from each flask at pre-established times to analyze the different variables that allow us an exhaustive knowledge of each fermentation.

The determination of bacterial growth is carried out using two types of methodologies. On the one hand, the measurement of the optical density at 700 nanometers leads us to the knowledge of biomass production in dry weight.

On the other hand, the seeding of agarized media allows us to quantify the number of viable bacteria. Cell-free samples are stored frozen for the analysis of nutrients and metabolite content.

The determination of the concentration of total soluble protein in the peptones recovered from marine by-products is essential both for the formulation of the culture media and to know their consumption throughout the time of fermentation.

The method of Lowry is the most classical protocol, which combines the reaction of copper ions with peptide bonds in an alkaline medium together with the oxidation of aromatic residues of proteins.

The intensity of the blue-purple color generated is dependent on the protein concentration present and directly quantifiable by visible spectrophotometry at 750 nanometers.

Lactic acid bacteria are producers of significant amounts of a primary metabolite such as lactic acid. In heterofermentative species, other organic acids such as acetic acid are also produced.

The production of both metabolites, as well as the glucose consumption at each of the sampled culture times, are routinely determined by high-performance liquid chromatography (HPLC) equipped with a cationic exchange column and a refractive index detection.

Finally, all the kinetic data of bacterial growth, production of metabolites, and nutrient consumption (mainly soluble protein and glucose) are mathematically modeled to estimate the yields and the main productive parameters.

These calculations are essential to establish and predict the nutritional capacity of marine peptones to replace commercial peptones.