1. What the Purple Flakes at the Bottom of the Juice Look Like
The objects found at the bottom of the juice were light, thin, and scale-like, measuring about 1–5 mm. Upon investigation, they were identified as aggregates of scale-shaped crystals [2]. The Yokohama City Institute of Public Health concluded that these were potassium bitartrate crystals that had adsorbed grape pigments and settled out of the liquid. The purple color was attributed to the absorption of these pigments. In this specific case, the substance was determined to be harmless to the body [2]. But why do crystals form in juice in the first place?
It is safe to read this as an example where sediment suspected to be a foreign object actually came from the raw ingredients.
2. Where the Crystals Come From: Tartaric Acid Naturally Found in Grapes
According to a food complaint Q&A from Hiroshima Prefecture, the sediment known as "tartrate" in grape juice forms when tartaric acid, naturally present in grapes, binds with ions like potassium and calcium in the juice to create crystals [1]. Since these are components already present in the grape, nothing was added later to cause this. In manufacturing, there is a method to force these crystals out in advance. Juice is cooled to around 0°C for 2–3 weeks to encourage crystal formation, after which the supernatant is removed or the crystals are filtered out [1]. This process utilizes the property that crystals form more easily when cooled. Think of it like hot water with lots of dissolved sugar forming grains at the bottom as it cools. However, the main players in grape crystals are not sugar, but the bond between acid and potassium. Removing crystals is primarily for appearance. What about other sediments?
3. Why Freshly Squeezed Juice Looks Cloudy: Pectin and Tiny Floating Particles
Freshly squeezed juice contains large amounts of pulp and pectin, a large molecule, floating in a colloidal state (tiny particles dispersed in the liquid). This makes the juice viscous and cloudy [1]. To make clear juice, manufacturers use enzymes to break down the pectin into smaller molecules before filtering it. However, if large pectin molecules remain, sediment can form during storage. Even juice that looks clear initially may develop sediment at the bottom over time [1]. So far, we have looked at how juice components naturally gather and settle. Next, we look at how preventing sediment led to another discovery.
4. How Apple Juice Makers Prevent Sediment, and What Polyphenols Are Found in the Waste
According to research reports from the Aomori Prefectural Industrial Technology Research Center, apple juice production in Aomori Prefecture uses centrifugal separation to prevent sediment from forming during distribution [3]. The centrifugal residue produced in this process is estimated at about 2,000 tons per year within the prefecture. This figure is specific to Aomori, not a national total [3]. This residue was found to contain high concentrations of proanthocyanidins, a type of polyphenol [3]. Thus, the material removed to prevent sediment is rich in polyphenols. Research in France on apple juice haze (cloudiness) reported that when apple disease-response proteins are denatured by heat, they can create haze on their own and form complexes with proanthocyanidins, which can then be precipitated by ultracentrifugation [4]. It is also noted that the higher the degree of polymerization of the polyphenol, the stronger its bond with protein. However, this is based on an abstract available via search engines, and this study alone does not explain all aspects of apples.
5. The White Powder on Hijiki Was Mannitol Crystals, but Kombu May Be Different
In a case from the Fukuoka City Institute for Environmental Health and Safety, white substance on dried hijiki was suspected to be mold. Microscopic examination revealed needle-shaped crystals, and instrumental analysis confirmed they were mannitol [5]. Mannitol is a type of sugar alcohol. In seaweeds like hijiki and kombu, it can appear as crystals. The institute’s materials state that it is safe to eat [5]. Note that instrumental analysis was done on hijiki; for kombu, general explanations apply. Regarding kombu umami, it is well known that Kikunae Ikeda identified glutamic acid as the key component of kombu dashi in 1908 and named it "umami" [6]. Mannitol is a different substance from glutamic acid; it is a sugar alcohol. Even on the same kombu, the source of the dashi's umami and the white powder on the surface are different substances. It is generally said that the white powder on kombu, which is said to be mannitol, can be wiped off with a damp cloth, as washing with water washes it away. This is a common suggestion, not a method from public agency documents.
6. Try It at Home: Comparing the Bottom and Surface of Juice in a Cup
Try putting a small amount of juice (like grape or apple) in a transparent cup, cover it, and refrigerate it overnight. The next day, observe the bottom of the cup and the liquid. Nothing may appear, it may become slightly cloudy, or crystals may be visible. Results vary by juice type, and crystals do not always form. Drink the juice the same day and do not leave leftovers. Wash your hands before observing the surface of kombu or hijiki at home. If there is white powder, observe it as is without washing it with water. Please only look at the dried goods before putting them in your mouth. For more detailed reading, the Hiroshima Prefecture food complaint Q&A [1] and the Yokohama City Institute of Public Health foreign object cases [2] contain specific examples of sediment.