The polymerization behavior of gluten proteins during durum wheat (Triticum durum Desf.) pasta production and cooking
Pasta is a popular wheat based product. The term pasta refers to extruded and subsequently dried products, but also to fresh extruded or sheet rolled (egg) pasta. To produce dry pasta products, durum wheat semolina (Triticum durum Desf.) and water are transformed into dough particles that are cold extruded or sheet rolled and subsequently dried to less than 12.5% product moisture content. The final pasta cooking quality largely depends on the processing characteristics of durum wheat semolina, on the physico-chemical transformations of semolina constituents as influenced by the processing conditions and the processing conditions themselves. It is assumed that a key for producing high quality pasta is the ability of proteins to form a disulfide (SS) linked protein network. Such network entraps the swollen and gelatinized starch granules during cooking and avoids material leaching. To the best of our knowledge, the protein polymerization behavior or network formation during industrial production is rather unclear at present. Hence, there is a potential improvement of high quality pasta by selecting optimal processing conditions during industrial production.
In this doctoral thesis, the polymerization behavior of gluten proteins during pasta production and cooking was examined. This way, we aimed at providing a scientific basis for optimizing the processing conditions for the production of high quality pasta.
In a first experimental set-up, the different cooking qualities of randomly selected commercial dry spaghetti brands were related to their starch (i.e. swelling) and protein (i.e. network formation) properties after pasta production and cooking. All dry spaghettis had a SS linked protein network which provides shear resistance to particles and restricts starch swelling and subsequent leaching to a different extent during cooking. For obtaining high quality pasta, an optimal degree of protein polymerization during spaghetti production is critical for obtaining resilience in the existing protein network that can then cope with starch swelling during cooking.
The above raised the question which gluten protein types and which polymerization reactions in the network formation are important for obtaining high quality pasta. We therefore studied glutenin linking and gliadin-glutenin cross-linking during laboratory scale fresh pasta production and cooking. During cooking of fresh tagliatelle strands, glutenin polymerized very fast, whereas the incorporation of α- and γ-type gliadins in the protein network occurred much slower. Under the used experimental conditions, the first order kinetics were controlled by using redox agents and N-ethylmaleimide (NEMI) to create cooked pastas with a broad range of polymerized proteins.
Since the bulk of industrially produced pasta is mostly dried to obtain a stable end product, it was important to determine the moment when gluten proteins start to polymerize and form a protein network during pasta drying. Furthermore, the conditions of protein polymerization to obtain superior pasta quality were explored. Under the used high temperature (HT) drying conditions, glutenins started polymerizing at air temperatures below 60 °C [65% relative humidity (RH)], whereas the more heat stable gliadins became incorporated in the protein network at air temperatures exceeding 68 °C (68% RH) through thiol (SH)-SS exchange reactions. Based on the impact of redox agents and NEMI on protein polymerization and quality characteristics, it was postulated that too much gliadin incorporation tightens the protein network which is then not sufficiently resilient to cope with starch swelling during cooking resulting in pasta of low quality.
This hypothesis was further examined on laboratory scale by lowering the pH of pasta dough. Acidification reduced the ionization of protein SH groups and, hence, the extent of gliadin-glutenin cross-linking. Using combinations of acid and redox additions, a sample set with a broad range of (mainly) gliadin-glutenin cross-linking within different cooked pastas was created. We therefore concluded that glutenin must be polymerized to a great extent and that optimal gliadin incorporation in the protein network must be obtained for bringing about high quality pasta.
In the last experimental part, the validity of the above hypothesis was examined during production and cooking of industrial dry spaghetti where different processing conditions govern. Trends were observed similar to those on laboratory scale using the same additives, except that, on industrial scale, they had a greater impact on glutenin linking.
Thus, the procedure to make dry pasta on laboratory scale in this respect is representative for industrial dry pasta production, notwithstanding the differences in processing steps and drying conditions. Furthermore, the hypothesis could be extended for industrial produced pastas that glutenin proteins form the main building blocks of the protein network and must be polymerized to a great extent to provide pasta its ultimate structure. As observed on laboratory scale, optimum gliadin incorporation in the protein network must be obtained to achieve high quality pasta.
In conclusion, this doctoral work increased our understanding of the impact of protein polymerization during (fresh and) dry pasta production and cooking on laboratory and industrial scale. Proteins form a SS linked network during HT drying and further polymerize during cooking. Glutenins start polymerizing at lower drying air temperatures than the more heat stable gliadins. For obtaining high quality pasta, glutenins must be polymerized to a great extent, since they form the core of the protein network. Gliadins incorporate in the glutenin network through SH-SS exchange reactions and increase its flexibility. As a result, the protein network can cope with starch swelling and better retains solids during cooking. However, an optimal incorporation is required since both a high or low level of gliadin incorporation result in low quality pasta. This work provides a scientific basis for optimization of pasta quality.