You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Characterisation of muscles from Frigate mackerel (Auxis thazard) and catfish (Clarias macrocephalus). Frigate mackerel (Auxis thazard) and catfish (Clarias macrocephalus) can be used as alternative sources for surimi production. However, the functionality of surimi is species-dependent. This study aimed to characterise certain chemical and physical compositions of dark and ordinary muscles from these species. Catfish, particularly ordinary muscle, was composed of higher contents of lipid and carotenoid than Frigate mackerel muscle (p<0.05) but ordinary muscle from Frigate mackerel had the highest phospholipid content (p<0.05). Both dark and ordinary muscles of Frigate mackerel had greater contents of myofibrillar proteins than had catfish muscle (p<0.05). Myosin heavy chain and actin were predominant proteins found in both muscle types of both species. Dark muscle from Frigate mackerel had the highest sarcoplasmic protein content, especially extractable myoglobin (p<0.05). Muscles from Frigate mackerel had greater content of sodium chloride than had catfish (p<0.05). The highest contents of iron, copper and selenium were found in Frigate mackerel dark muscle (p<0.05). The pH of ordinary muscle from both species was higher than that of dark muscle (p<0.05). Frigate mackerel, especilly dark muscle, exhibited the most dark-red colour, as shown by the lowest L(*) and b(*) values with the highest a(*) value and redness index (a(*)/b(*)) (p<0.05).
{"Chemical": ["copper", "selenium", "iron", "sodium chloride"]}