The Kashrut Authority – in line with similar policies adopted by the London and Manchester Batei Din as well as others - takes a view in accordance with the following:
Almost all poskim are of the view, that if an ingredient which is not kosher is nullified in a final product manufactured by gentiles for gentiles, the final product is kosher. ( Hagohos Shaarei Dura Shar 1, Shu”t Maharam Lublin 104; Chatam Sofer YD 62; Darkei Teshuva YD Siman 108 s”k 20 and more… see also Noda B’Yehuda Mahadura Tinyono Y”D 56 ). The two ingredients most likely to cause problems in soft drinks are either grape juice or cochineal (120). (The Manchester Beth Din does not consider grape derived anthocyanin as a problem but others do and as such we give the option of being mehader if 163 is contained in a product). If either of these ingredients were to be in a fruit juice or soft drink they would have to be mentioned in the ingredient listing and if they are the KA warns in the directory that they should not be used.
Any add-back ingredients not listed in the ingredients are precisely that – add back. That means that if it is an orange juice then orange juice derived ingredients may be added back. They invariably would pose no kashrus problem – but even if they did the amount would be so small as to pose no problem as above.
Similarly, any flavours that might be used in a soft drink or juice would invariably not pose a kashrut isssue. Either because the flavour is kosher or even if it contained a sub-ingredient that was not kosher it would be nullified.
In relation to equipment used:
The standard CIP (Cleaning In Place) used in cleaning pasteurisation and bottling equipment would certainly kasher from grape juice or from any other non-kosher ingredient and this would be normal procedure at a plant. Similarly for those who do not keep Chalav Yisrael the normal CIP between milk products and juice would be sufficient for the juice to be considered Pareve. However for those who strictly keep Chalav Yisrael relying on a CIP is more problematic – even though there are strong reasons to argue that the since milk will cause a bad taste (be pogem) in the juice, CIP would be sufficient, never-the-less were it to be common practice to use the same dairy equipment there may be grounds to be stricter. However our experience with the major juice and (certainly soft drink companies) is that they generally do not use the same equipment for milk and juice or soft drinks.
Specialised UHT equipment poses even less of a problem as it is always sterilised at very high temperatures and under circumstances that are considered a full kashering.
Therefore, it is the view of the Kashrut Authority that provided the guidelines are followed; any juice and soft drink is kosher and does not need to be further investigated. Any uncertainty as to the possibility of a non kosher ingredient or sub ingredient is covered by the above Halachic ruling of nullification. (It should be noted that the above ruling applies even if we know with certainty that a non kosher ingredient is nullified - the KA however only employs it in cases such as the above to obviate any residual doubt- see the article entitled "Halachic Standards today" ) .Products that have been verified to actually contain non kosher ingredients – even in nullifiable amounts – are generally not allowed by the KA.