Industrially Pre-Treated Potatoes (Solanum tuberosum L.) Served in Large-Scale Food Service Systems

In Swedish schools, a warm lunch is served every day. The lunch is prepared in a largescale food service system and includes steam-cooked potatoes several times per week. The potatoes are often industrially pre-treated to facilitate the handling and logistics required to cook hundreds of meals in a couple of hours. During the industrial pretreatment, the potato tubers are usually peeled by abrasion, knife peeling, or a combination of both, followed by preservative actions and packaging. When arriving at the large-scale food service systems, the tubers are usually cooked by steam-cooking (SC) in a combi-steamer (an oven with saturated steam as the medium for heat transfer). To manage logistical issues, the potato tubers might have to be held warm until serving. These processing steps are often very rough, causing stress and mechanical damage to the tubers, followed by a poorly controlled and understood cooking process. Unfortunately, this contributes to an unpleasant product being served with reduced
eating quality. Preservative actions are required to prolong the shelf-life and include reduction of enzymatic browning and reduction of microbiological growth. These actions can be performed by chemical treatment with organic acids (OA) and/or sodium metabisulfite (SMS). Textural analyses by puncture of the samples show that tubers treated with OA, SMS, and OA+SMS might develop a tough surface compared to untreated samples, referred to as Ref. Preservation with both OA and SMS prolongs the shelf-life but has shown to reduce the eating quality by contributing to the creation of an unpleasant, tough surface. Textural analysis revealed that the hardness of the surface depends on the chemical pre-treatment, with hardness order of Ref<OA<SMS<OA+SMS. Visual inspection showed that SMS and OA+SMS contributed to the most pronounced tough surface, while analysis by light microscopy revealed that OA developed brick-like cells at the surface, indicating that the different preservative treatments contribute to a tough
surface by two different mechanisms.
SC is used to cook the samples due to decreased cooking time and easier handling compared to conventional boiling (CB) in a cooking vessel. The cooking degree has shown to mainly depend on the core temperature of the tuber at the end of the cooking process, but differences were also found depending on the cooking method (CB compared to SC) and potato variety.
Warm-holding (WH) is often conducted in the combi-steamer with a relative humidity (RH) of 100% at a minimum temperature of 72°C for a maximum of 2 h since this used to be the recommendation in Sweden. During WH, the cooking process continues, where temperature has a larger impact than time. WH can easily cause overcooking, with an unpleasant watery core as a result. However, if tubers that will be warm-held are slightly undercooked, the continuation of the cooking process can easily be adapted to achieve a good eating quality. Control of relative humidity (RH) has shown to be crucial, where too low RH contributes to a tough surface due to evaporation of water, which is a third mechanism causing a tough surface.