Since commercial juices are pasteurized in the USA, it destroys many vitamins including C. Then they add synthetic ones to it.
Flash pasteurization, also called "High Temperature Short Time" processing (HTST), is a method of heat pasteurization of perishable beverages like fruit and vegetable juices, beer, and some dairy products. Compared to other pasteurization processes, it maintains color and flavor better, but some cheeses were found to have varying responses to the process.
It is done prior to filling into containers in order to kill spoilage microorganisms, to make the products safer and extend their shelf life. Flash pasteurization must be used in conjunction with sterile fill technology (similar to aseptic processing) and therefore has the risk of post-pasteurization contamination if hygiene standards are not rigorously enforced. Flash pasteurization is often used for the pasteurization of bulk products such as keg beer, milk, and kosher wines.
The liquid moves in a controlled, continuous flow while subjected to temperatures of 71.5 °C (160 °F) to 74 °C (165 °F), for about 15 to 30 seconds.
Flash pasteurization is widely used for fruit juices. Flash pasteurization was first developed by Tropicana Products, which has used the method since the 1950s. The juice company Odwalla switched from non-pasteurized to flash-pasteurized juices in 1996 after tainted unpasteurized apple juice containing E. coli O157:H7 sickened many children and killed one.
Int J Food Sci Nutr. 1995 May;46(2):107-15.
A model study on rate of degradation of L-ascorbic acid during processing using home-produced juice concentrates.
Roig MG, Rivera ZS, Kennedy JF.
Departmento de Química Física, Facultad de Farmacia, Universidad de Salamanca, Spain.
The rate of degradation of L-ascorbic acid in solution has been investigated under varying conditions, such as temperature, level of dissolved oxygen, pH, amino acids, sugars and processing conditions. Changes in pH between pH 1.5 and 7.0 accelerate L-ascorbic acid degradation. The most important factor that determines its stability is storage temperature, on which the rate of degradation of L-ascorbic acid is directly dependent. Similarly, the deleterious effect of variables such as oxygen and pH are influenced by temperature. Therefore, low temperature storage is imperative in order to regard L-ascorbic acid delay. A definite role of amino acids in L-ascorbic acid degradation has not been identified.
Br J Nutr. 1950;4(2-3):95-100.
The losses of ascorbic acid during the large-scale cooking of green vegetables by different methods.
ALLEN MA, BURGESS SG.
The processing of orange juice - looks pretty disturbing:
Cien Saude Colet. 2010 Jan;15(1):51-62.
[Vitamin C loss in vegetables during storage, preparation and distribution in restaurants].
[Article in Portuguese]
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Moraes FA, Cota AM, Campos FM, Pinheiro-Sant'Ana HM.
Departamento de Nutrição e Saúde, Universidade Federal de Viçosa, Viçosa, MG. firstname.lastname@example.org
The main objective of this study was to evaluate vitamin c loss following vegetable manipulation stages, reception, storage, preparation, and distribution, in commercial and institutional restaurants. Seven vegetables were used: lettuce, carrots, chicory, collard greens, cauliflower, cabbage and tomatoes. Vitamin c analysis was made by high-performance liquid chromatography (hplc). After all manipulation stages, cumulative vitamin c loss was high in vegetables prepared in institutional restaurants and commercial restaurants corresponding to 44.38%-67.03% and 32.90%-71.32%, respectively. In both restaurants, storage accounted for the most expressive loss of vitamin c in the majority of the vegetables. The adoption of measurements to control vitamin c loss in vegetables is suggested, such as purchase and reception according to demand, storage at refrigerated temperatures, monitoring of hygiene and sanitation times, slicing near serving time, cooking preferably steamed, by pressure or stewed, monitoring of cooking time and temperature, and control of the exposure time between preparation and distribution. The results obtained for vitamin c content contribute to the nutritional characterization of vegetables, since studies related to this area are scarce in brazil and in the world.