Digestion, General | September 15, 2015 | Author: The Super Pharmacist
Raw fruits and vegetables are an important natural source of many essential vitamins, minerals, and polyphenols. At present, there is a wide availability of fruit and vegetable juice products that market themselves as containing the same nutritional value of their solid vegetable counterparts, although the processing and juicing of different fruits and vegetables has been evidenced to destroy some of the nutrients that are naturally found in them (1). The vitamins, minerals, polyphenols and enzymes that are present in uncooked fruits and vegetables are generally the same as those found in juices (2), but it is the process of juicing that reduces the amount of each naturally occurring ingredient. Although fruit juices are widely marketed as providing health benefits such as weight loss, flushing toxins from the body, and increased energy, there remains very little scientific evidence to support this assertion.
Presses have traditionally been the most usual and traditional method of removing juice from fruit and vegetables. However, in light of increasing demand and greater commercial interest, a number of other methods such as diffusion extraction, centrifugation, and specialized ultrafiltration techniques have been explored to determine which gives the greatest yield, flavour and retention of nutritional value.
Normally, a yield efficiency diagrams that relate juice yields to mash feed rates provide a mechanism for comparing presses and other processes such as enzyme treatments or decanter centrifuges for efficiency under a stated set of circumstances (3). Other studies, largely driven by commercial interest, compare techniques for managing some of the well known side effects of juicing such as increased bitterness and an astringent taste when compared to fruit consumed in its solid form.
Research has shown that diffusion extraction is capable of removing 90 to 94% of soluble solids from properly prepared fruit slices, but the resulting juice is often heavily diluted with extraction water and is high in extracted tannins (a naturally occurring polyphenol responsible for making the juice bitter and astringent) (4). Concentration is necessary to obtain juice solids equivalency, and the resulting juice must be treated with tannin absorbants to provide an acceptable flavour for consumers. At present, decanter centrifuges are widely used commercially: when naturally coloured and flavoured juices are desired, the decanter provides a useful alternative to presses because it is easily inert gas blanketed (a process of cooling that is also used in the commercial production of wine). The use of metallic ultrafilters as a press has been patented, but has not been used commercially and there is no available research regarding its use when compared to other more established methods.
There is no formal evidence to suggest that juicing carries any additional benefits when compared to eating fruit in its normal form. Much of the material that suggests that juicing is beneficial is biased due to strong commercial interests – the actual scientific evidence that exists suggest that the edible skin of many fruits, which are often excluded from fruits in juicing filtration processes, contains the large majority of the fruit’s nutritional value (such as carotenoids and flavenoids). There is also some evidence to suggest that the juicing process results in the loss of some of the fruit’s fibre content (5), and many juiced fruits available commercially only contain a small percentage of real fruit juice that is often supplemented by added sweeteners such as sucrose or high fructose corn syrup. Centrifugal juicers can also speed up the oxidisation process of fruits, with natural enzymes and nutrients beginning to deteriorate when exposed to oxygen.
A medical case study, published in the The American Journal of Medicine in 2013, reported a case of oxalate nephropathy (an acute kidney injury that often results in kidney failure) as a result of a patient spending six weeks drinking nothing but fruit that had been prepared with a number of different juicing methods (6). The patient had kept their own diary of their juicing habits, noting that for over 6 weeks he had taken a daily average of 1260mg of oxalate from beets, collard greens, kiwi, parsley, spinach, and soy products. When high-oxalate juice is consumed, the oxalate gradient is increased. In addition, because of increased water flux, the absorption of oxalate is enhanced more so than if the fruit was consumed without being juiced. This is obviously a very rare case, and individuals with normal renal function who do not consume high oxalate juice will run a very low risk of juicing related nephropathy. However, because juice consumption enhances oxalate absorption, there is perhaps an argument for commercial juicing products to label the oxalate content and provide an evidence-based limit of daily consumption for consumer protection. For those who have consumed a high quantity of oxalate who may have predisposing factors that place them at increased risk of kidney harm, an evaluation including kidney function and urinalysis with microscopic examination to look for oxalate crystals is warranted (albeit the amount of patients with underlying kidney problems who choose to juice excessively is probably very small). In conclusion, there is no strong scientific evidence to suggest that juicing, or drinking juiced fruit, is nutritionally any more beneficial than eating fruit in its original solid form. The process of juicing has been evidenced to reduce the nutritional content of solid fruit.