The aim of the investigations was to determine the natural NPN-contents of fresh hams before, during and after industrial cooked ham production in order to show how the NPN-content is influenced by varying technological and raw material parameters. It was found that the NPN-content of fresh meat increases during refrigerated storage as a result of native proteases (calpains) to a level as yet insufficiently determined. The same applies as a function of the temperature in the case of high bacterial counts for bacterial proteases. The effect of the increase in NPN in fresh meat on the end product is not entirely clear. When storing the end product within the minimum shelf life period no rise in the NPN content is to be expected if current hygiene measures, refrigeration temperatures and packaging methods are observed. The breed of the fattening animals influences the NPN-content of the fresh meat indirectly via the protein and moisture content and the feed given to fatteners may possibly directly alter the native NPN-content. When using animals of the same breed and from units with standardized feeding there would be no noticeable differences in these influences. The NPN and protein content of the cooked ham depend, amongst other things, on different production patterns in the units. In the case of the moisture content however the different influences of the various types of muscle groups and the pH of the fresh meat predominate. The statistically certain highest NPN-content in the fresh meat and in the end product was found in the M. semimembranosus as compared with the M. biceps femoris and the underpart of the leg. The protein content of the M. semimembranosus was only higher than in the other muscle groups in the case of fresh meat. The moisture content of cooked ham from the M. biceps femoris had the statistically significant lowest value as compared with the other cuts after heat treatment. There was a statistically certain negative correlation between the moisture and the protein content. The NPN-content of fresh meat and sometimes of the end product correlated significantly positively with the protein content. A high protein content in the sample thus pointed to a high NPN-content. In view of this correlation it is suggested that the NPN-content should be related to the crude protein content. In this way differences between units could be relativized. In the present case they were sometimes even reversed. An analytical assessment of the NPN-content that is independent of the protein content and thus more realistic could thus be achieved with the result that there would be no disadvantages due to high NPN content to a meat products manufacturer whose initial material had a high protein content. A comparison of the NPN-contents of 191 cooked hams from the trade and a consideration of the present results suggest a limit for the natural NPN-content in cooked hams of 2.4 % (% fresh matter).
