Optimization of pickled herring production - Approaches for process and quality control

Manufacturing of high-quality pickled herring products requires a thorough understanding of the raw material as well as the process. Commonly, the industrial process is based on traditional principles and practical experience. In order to optimize the industrial process, it is essential to understand how the process parameters relate to the raw material characteristics (such as seasonal variation) and affects the product quality and yield. Product yield is one of the main aspects of a profitable production of pickled herring products, and attaining a high product yield without compromising food safety and quality is desired.
The aim of this thesis is to contribute with knowledge to optimize the industrial production of pickled herring products. The experimental work can be divided into three main subjects: i) obtaining a deeper knowledge about the influence of different parameters (biological/process) on the product yield, ii) studying the salt and water diffusion in herring muscle during dry salting and brining to be used in a model describing these phenomena and iii) investigating the use of Near-infrared (NIR) spectroscopy to determine the salt content in herring products.
Regarding the first subject mentioned above, several storage experiments were conducted using single herring fillets, where brining time, marinade composition and marinating time varied. Conducting experiments on single fillets revealed a correlation between fat content and weight change during processing. The greatest weight change occurred within the first 2-3 days and only minor changes were seen during storage up to one year (Paper I).
Different salting conditions were applied to herring fillets in order to investigate the salt and moisture transfer in herring during processing by experiments of global and local concentration profiles. The average salt and moisture changes in fillets during brining were studied under conditions similar to industrial processes using a brine-to-fish ratio of 1:1. An average diffusion coefficient for salt was found, being 2.31 × 10-9 m2s-1 (Paper I). Local concentration profiles were obtained to study the counter diffusion of salt and moisture under the different salting conditions and to determine the diffusion coefficients of salt and water that were used to develop the model. The model can be used for prediction of salt and moisture distribution and equilibrium times (Paper II).
NIR spectroscopy was investigated as a potential method for process control in the herring marinating process. A principal component analysis (PCA) of the spectral data of herring marinade showed that the first principal component was related to the change in salt concentration during processing, and it was possible to calibrate models for prediction of the salt concentration in the marinade and in the herring fillets under the prerequisite that the system was in equilibrium. We concluded that NIR is a good alternative for the time consuming and destructive methods for salt determination often used in the industry (Paper III).
In conclusion, this work has added knowledge about the variation in product yield, developed models that can be used to predict salt concentration and immersion time and we have proposed the use of NIR to achieve fast and non-destructive salt determinations which all-together aid in optimizing the process in the industry.