Carrot Juice Processing - Effects on Various Quality Aspects
Abstract: Carrots are a cold-weather crop well suited for cultivation in the south of Sweden. Carrots are generally rather inexpensive, with a market price below € 1 /kg (Sweden, 2003), whereas fresh carrot juice has a price of around € 5 /kg. The fresh carrot juice produced today has a short shelf-life of approximately three days, which makes centralised industrial production and distribution difficult, and there is also a high degree of spoilage in the retail outlet. It is therefore of interest to produce a carrot juice with a sustainable high quality and thus a longer shelf-life. The aim of the present work was to investigate the effect of different unit operations on the quality and shelf-life of carrot juice. High-quality carrot juice was defined as a cloud-stable juice with fresh taste, appealing intense colour, high carotene content and a healthy dietary fibre balance. The effects on the pomace were also considered in order to gain knowledge on by-product applications of the waste material and thus decrease the environmental load. Three different two-stage expression lines were investigated. It was concluded that the composition of the juice and pomace, as well as the physical properties of the pomace, were affected by both juice yield and the type of equipment used. The primary juice expressed was found to have higher quality than secondary juice in terms of higher values for colour, optical density, carotene and dietary fibre content. During juice expression the density of the press-cake seemed to influence primarily the amount of carotene expressed, but also that of dietary fibre. The water-holding capacity of the pomace decreased after hard pressing, although the total amount of dietary fibre per gram dry matter increased. Cloud formation was investigated and a combined effect of temperature and acidification was found. Cloud formation occurred at lower temperatures for lower pH. Carrot juice has a pH of 6.3, and at a pH of 4.4 cloud formation was found to occur immediately at room temperature. The shelf-life of fresh carrot juice is primarily determined by microbial growth in the juice. The method of isothermal calorimetry in determining the microbial status of the juice was compared with total colony forming units found by plate counts. The results showed that isothermal calorimetry could be used to determine the shelf-life of carrot juice. When stored at 17°C the shelf-life was less than 20% of that under refrigerated storage at 8°C, and the shelf-life was found to be limited by the mesophilic flora in both cases. The shelf-life of carrot juice was prolonged by mixing with other juices and acidification. The effect on shelf-life varied according to the acid used, and was related to the undissociated acid concentration. Using combinations of acids had greater effects at the same pH and thus a positive effect on shelf-life could be achieved without cloud formation. However, storage of the carrot juice under a modified anaerobic atmosphere had no effect on shelf-life.
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