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Previous studies have indicated that 15N enrichment of solid-associated bacteria (SAB) may be predicted from the same value in liquid-associated bacteria (LAB). The aims of this study were to confirm this and to measure the error in the nutrient supply from SAB, when LAB are used as the reference sample. For this purpose, the chemical and amino acid (AA) compositions of both the bacterial populations were studied in four experiments carried out on different groups of three rumen cannulated wethers. Diets (one in Experiments 1 and 4 and three in Experiments 2 and 3) had forage-to-concentrate ratios (dry matter (DM) basis) between 2 : 1 and 40 : 60, and were consumed at intake levels between 40 and 75 g DM/kg (BW)0.75. The bacteria samples were isolated after continuous infusion of (15NH4)2SO4 (40, 18, 30 and 25 mg 15N/day, in Experiments 1 to 4, respectively) for at least 14 days. In all experiments, SAB had consistently higher concentrations of organic matter (826 v. 716 g/kg DM, as average) and total lipids (192 v. 95 g/kg DM, as average) than LAB. Similar CP concentrations of both populations were observed, except a higher concentration in SAB than in LAB in Experiment 3. A consistent (in Experiment 4 only as tendency) higher AA-N/total N ratio (on average 17.5%) was observed in SAB than in LAB. The 15N enrichment in SAB was systematically lower than in LAB. On the basis of the results of all studies a close relationship was found between the 15N enrichment in SAB and LAB, which was shown irrespective of experiments. This relationship was established from Experiments 1 and 2 and the above cited previous results (n = 20; P < 0.001; R2 = 0.996), and then confirmed from the results of Experiments 3 and 4. These relationships between SAB and LAB demonstrate that CP supply from SAB is underevaluated by, on average, 21.2% when LAB are used as the reference. This underevaluation was higher for true protein and even higher for the lipid supply (32.5% and 59.6%, respectively, as an average of the four experiments). Large differences in AA profile were observed between SAB and LAB. The prediction equation obtained using 15N as the marker may be used to correct the errors associated with the traditional use of LAB as the reference sample, and therefore to obtain more accurate estimates of the microbial nutrient supply to the ruminants.

Aflatoxin M1 (AFM1) distribution in curd, whey, Manchego cheese, the traditional Spanish whey cheese Requesón and Requesón whey, and its stability during two different cold treatments, have been studied. Raw ewe’s milk was artificially contaminated with AFM1 in a final concentration of 50 and 100 ng kg-1, and was used to produce Manchego cheese. AFM1 determinations were carried out by HPLC with fluorimetric detection after immunoaffinity clean-up. The mean AFM1 concentrations in the produced curd and Manchego cheese were approximately 2- and 3-fold higher than the initial milk they were made from, and the levels of this toxin remaining in whey were 42.3 % and 51.3 % of the initial concentrations. In the Requesón samples, the mean AFM1 values were 1.7 times higher than those in the corresponding whey, while 33.7 % and 44.4 % of the AFM1 concentration detected in milk also appeared in the Requesón whey. Short refrigeration and freezing periods did not affect the toxin levels in either curd or Requesón samples. When ewe’s milk destined for Manchego cheese-making is AFM1-contaminated at the EU limit level (50 ng kg-1) or double, a concentration of this toxin will appear in the manufactured products, but values will be considerably below the toxic doses (Tolerable Daily Intake = 2 ng kg-1 body weight per day), which poses a human health problem.