No abstract

Although Haematococcus pluvialis is one of the most important natural sources of the carotenoid astaxanthin as a pigmentor for the aquaculture industry, the thick sporopollenin cell wall in the cysts hinders astaxanthin extraction and its subsequent bio-availability to fish. A range of physical and chemical processes were tested to promote the disruption of the encysted cells. The efficacy of these processes was evaluated in terms of astaxanthin recovery, which was assessed by determining the extent of leaching of astaxanthin into an organic solvent. The processes tested were: autoclave 30 min, 121 • C, 1 atm; HCl 0.1 M, 15 min and 30 min; NaOH 0.1 M, 15 min and 30 min; enzymatic treatment with a mixture of 0.1% protease K and 0.5% driselase in a phosphate buffer, pH 5.8, 30 • C, for one hour; spray drying, inlet 180 • C, outlet 115 • C; and mechanical disruption, with a cell homogeniser developed for this purpose. The mechanical (homogenisation) and autoclave treatments were the most effective in terms of extraction and availability.

The deposition of natural, optically active, astaxanthin fatty acid esters in rainbow trout (Oncorhynchus mykiss) was studied. Mono‐esterified and di‐esterified (3S,3′S) astaxanthin were purified from the green microalga Haematococcus pluvialis and incorporated into extruded diets and compared with diets containing synthetic racemic astaxanthin (Carophyll Pink) and a total carotenoid extract from the alga. All sources of astaxanthin achieved >4 mg kg−1 in the white muscle after 6 weeks feeding. No significant difference (P > 0.05) between the deposition of astaxanthin or total carotenoid for the different diets was observed. Other xanthophylls, namely lutein, zeaxanthin and idoxanthin were found in the white muscle of rainbow trout fed all diets and together accounted for 10–14% of total carotenoid. Astaxanthin was deposited in the white muscle in the stereochemical form administered in the diet, i.e. racemic astaxanthin for Carophyll Pink and ˜100% (3S,3′S)‐astaxanthin for the algal sources. In contrast, epimerization of (3S,3′S) astaxanthin from the alga was observed for the astaxanthin esters deposited in the skin of rainbow trout, with a ratio close to 1.0:2.0:1.0 (3S,3′S:3R,3′S:3R,3′R).

The absorption of astaxanthin from diets (30 mg kg−1 inclusion) supplemented with either unesterified astaxanthin; isolated astaxanthin monoesters, diesters or a cell‐free carotenoid extract from Haematococcus pluvialis were studied in rainbow trout (>200 g). No significant differences (P > 0.05) were recorded in the apparent digestibility coefficients (ADC) (≈60–65%) between astaxanthin sources. However, following consumption of a single meal, peak serum astaxanthin levels at 32 h (≈1.0–1.6 μg mL−1) were significantly higher (P < 0.05) in fish fed unesterified astaxanthin and astaxanthin monoester, compared to fish fed astaxanthin diester and the cell free extract. However, no significant differences (P > 0.05) were recorded in serum astaxanthin uptake rates between sources of astaxanthin. Results suggest that the extent of carotenoid esterification negatively influences the peak serum levels of astaxanthin in rainbow trout.

A modified pectin agar medium was evaluated for the rapid isolation and presumptive identification of Yersinia enterocolitica. Of 118 isolates of Enterobacteriaceae tested, only the 13Y. enterocolitica and the three Klebsiella oxytoca strains produced colonies that depressed and sank into the agar. Yersinia enterocolitica was also easily identified in mixed cultures, even from inocula containing three times as many other Enterobacteriaceae organisms as Y. enterocolitica. The recovery of Y. enterocolitica was evaluated on Mueller-Hinton, pectin, Hektoen enteric, xylose lysine desoxycholate, Salmonella-Shigella, and MacConkey agars. Compared with Mueller-Hinton agar, the pectin agar showed a 100% recovery of Y. enterocolitica, with all strains having depressed colonies, while the other media showed lesser recoveries of only 5 to 25%, with no other discriminating colonial characteristic.