Nutritional Properties of Liquid Oats as Affected by Processing and Storage

Sammanfattning: The cholesterol-lowering effects of oats have led to an increasing interest in them on the food market. Liquid oats has emerged as a new way of consuming oats and has become a commercial product. To produce liquid oats, steam-heated dehulled oat groats are subjected to flaking, wet-milling, enzyme hydrolysis, decanting, formulation, and ultra-high-temperature (UHT) treatment, the resulting beverage being aseptically packaged. The overall aim of the present study was to gain more adequate knowledge of the consequences of processing and storage on the nutritional properties of liquid oats. The retention of vitamins, minerals and phenolic antioxidants (including avenanthramides and hydroxycinnamic acids) was followed in the course of processing. After storage, the fatty acid profile, vitamin retention, dissolved oxygen concentration and sensory attributes of the liquid oats were followed in different prototypes after storage. Its protein quality after processing and storage was investigated in growing rats. The effects of phytase treatment, addition of citric acid and iron fortification either alone or in combination on the non-heme iron absorption from liquid oats in humans were also investigated. A moderate increase on a dry matter basis in the level of native vitamin B6 was observed after the decanting process, whereas much of native phosphorus, zinc, calcium and iron were separated off together with the insoluble fraction. The supplemented vitamins A, B1, B2, niacin, B6 and folate exhibited marked stability during UHT treatment at 140 ?C (holding times 5 s and 20 s), whereas vitamin D3 was lost to about the same extent at both durations, B12 being lost to a moderate extent at the longer duration. The fortified vitamins, except for vitamin A, D3 and B12, remained stable during a year of ambient storage. Phenols were lost to a varying extent after enzyme incubation process, the decanting step causing further losses. During storage, a refrigerating temperature favored the stability of the avenanthramides in the iron-lacking liquid oats. The fatty acid profile changed to only a minor extent after ambient storage for a year, also in an iron-fortified prototype. This could be due to the low dissolved oxygen concentration and also to the presence of antioxidants. Enzyme incubation process together with heating and subsequent decanting reduced the protein content of liquid oats, leading to a slight decrease in the biological value of the proteins in rats, whereas neither UHT treatment nor a year of ambient storage changed the protein quality. The true protein digestibility, biological value and net protein utilization of the various processed and stored forms of liquid oats were comparable to those of casein. The phytase treatment of liquid oats reduced the inositol phosphates to undetectable levels, leading to a significantly improved non-heme iron absorption in humans. A combination of phytase treatment, citric acid addition and iron supplementation increased the iron absorption. Fortification of iron as ferric pyrophosphate or ferric ammonium citrate resulted in similar iron absorption rates but addition of the latter may bring about undesirable sensory attributes. Overall the study showed decanting to change the relative proportions of proteins, vitamins, minerals and phenols, and direct UHT treatment to lead to a good retention of proteins and of most of the vitamins. Ambient storage for a year did not affect the protein quality or the fatty acid profile. The results suggest that the nutritional quality of UHT liquid oats can be improved by phytase treatment, better selection of ingredients, and refrigerated storage.