How to comprehensively utilize fish processing by-products

The fish heads, fish tails, minced meat, scales, skin, viscera, bones, gallbladders, etc. left over from fish processing contain large amounts of protein, amino acids, trace elements and vitamins. Using these protein resources, various deeply processed products can be made, including soluble edible fish protein powder, liquid fish protein feed, fish scale glue, fish skin glue, bone paste, fish protein calcium candy, bile pigment calcium salt and bile acid salt, etc. Comprehensive utilization of fish processing by-products can reduce the cost of main products and increase the added value of aquatic products; it can also protect the environment and reduce wastewater and waste emissions, achieving relatively high economic, ecological and social benefits. The above several products are described separately below.

I. Soluble edible fish protein powder. Soluble edible fish protein powder is rich in nutrients, has good water solubility, and is easy to digest and absorb. It can be used as a base material for amino acid-fortified foods and as a substitute for milk powder; it can also be developed into products such as nutritional soup, seasonings and fish protein beverages. It is an effective medicine for treating diarrhea in children caused by indigestion, and can also serve as a protein source in liquid foods for cancer patients or postoperative patients who have difficulty eating. There are many methods for preparing this product, such as enzymatic hydrolysis, acid hydrolysis, alkali hydrolysis, and alcohol extraction degreasing. Enzymatic preparation is especially preferable, as it not only maintains the inherent nutritional components of fish, but also has good taste and is easy to dissolve. It is a new technology developed abroad in recent years.

1. Process flow: raw material treatment -> enzymatic hydrolysis -> centrifugal filtration -> fish protein hydrolysate -> decolorization -> vacuum drying -> crushing -> finished product.

2. Process. Use fish minced meat, fish heads and fish tails from processing by-products as raw materials, wash them clean, add water and mash them. Take the mashed fish paste into a reaction pot, add water to adjust the pH to 6.5-7.5, make the solid-liquid ratio 5:8, add 1%-2% protease, react at 45°C-60°C for 2 hours-4 hours, heat and boil for 10 minutes to inactivate the enzyme, then centrifuge and filter. Discard the filter residue (use as fertilizer or feed) to obtain fish protein hydrolysate. Decolorize with 10 grams per liter of activated carbon at 65°C for 30 minutes, vacuum dry at 50°C-55°C, crush with a ball mill, and pass through a 200-mesh copper sieve to obtain soluble edible fish protein powder.

II. Liquid fish protein feed. Liquid fish protein feed is not only rich in nutrients, but its protein exists in the form of polypeptides and amino acids, making it easier for animals to digest and absorb. Methods for preparing this feed include the molasses fermentation method (using by-products from sugarcane sugar production), acidification method, alkalization method, etc. The acidification production method is as follows.

1. Process flow: raw material treatment -> acidification -> liquefaction -> finished product -> storage.

2. Process. Liquid fish protein feed uses spoiled fish that have lost edible value, low-quality small miscellaneous fish, fish heads, viscera, etc. as raw materials. Mash them with a hammer mill, place the mashed raw materials in an acid-resistant container, add formic acid with a volume concentration of 85%, using an amount equal to 3.5% of the raw material weight. The pH of the acidified raw material must be lower than 4. Place the acidified raw material in an acid-resistant liquefaction tank for liquefaction. Liquefaction time varies depending on temperature and raw materials; generally at room temperature of 30°C-35°C, one to two days is sufficient. Store the liquefied fish protein feed in an acid-resistant covered container; at room temperature, the storage period is 1 year-2 years.

III. Fish scale glue and fish skin glue. Use the skin, scales, etc. discarded during fish processing to extract animal glue, replacing glue made from pig skin, cowhide, mule skin, and turtle and soft-shelled turtle shells. Fish skin accounts for about 5% of the fish body, and fish scales about 2.5%-4%. Therefore, not utilizing fish scales and fish skin is a great waste. Practice has proved that making glue from fish scales and fish skin is one of the best raw materials among animals. Fish scale and fish skin glue can have the same effects as turtle shell glue and donkey-hide gelatin in nourishing yin and stopping bleeding, and has unique effects in moistening and tonifying the lungs. The glue-making technology is introduced below.

(I) Preparation of fish scale glue

1. Process flow: fish scales -> washing -> alkali soaking -> acid soaking -> lime soaking -> rinsing -> glue boiling -> filtration -> solidification -> glue scraping -> drying -> finished product.

2. Process. After washing, soak fish scales in 5% caustic soda solution for about 5 minutes to remove oil and fatty skin and cause the scales to swell. Immediately rinse the alkali-soaked fish scales with water, then soak them in clean water to remove alkalinity until no red color appears with phenolphthalein. Next, soak the cleaned fish scales in hydrochloric acid; the amount used is initially 12% (hydrochloric acid volume concentration 31%), then gradually decreases: 8%, 3%, and 1% in 4 acid soaking steps, with a scale-to-water ratio of 2:3. After acid soaking, rinse the fish scales and place them in limewater of 2 grams-4 grams per liter for 7 days-14 days to remove other impurities, then rinse with clean water. Add a small amount of hydrochloric acid to the washed fish scales to adjust the pH to 5-6. Heat the fish scales treated as above in a jacketed steam pot at 60°C-70°C for 2 hours-3 hours until extraction is complete. Filter the boiled glue to remove impurities, place the transparent and clear glue liquid in a fixed tray to let it naturally condense, scrape it into 5 cm thick sheets with a scraper, and dry the scraped sheets to obtain fish scale glue. Every 100 kg of wet scales can produce 10 kg-12 kg of dry glue. Its uses fall into four major categories: photographic glue, biological preparation glue, medicinal glue, and industrial glue, each with different quality standards.

(II) Preparation of fish skin glue. The glue-making process for fish skin is basically the same as for fish scale glue, but the first few steps differ, described separately below: alkali soaking: the alkali water concentration is 0.04%, replace the alkali water once daily until the fish meat and most of the oil are removed.

Lime soaking: after alkali soaking, soak the fish skin in limewater (concentration 0.4%), replacing the lime once every 2 days-3 days, requiring about 30 days (including the alkali soaking period).

Lime washing: wash off the lime. First wash with water; after most of the lime is removed, wash with dilute acid water (pH between 3-4), then wash with water again.

The following steps, such as glue boiling, are the same as for fish scale glue. The glue yield from fish skin is about 15%.

IV. Bone paste and fish protein calcium candy. During fish processing, the middle bones (backbones) of fish are often discarded as waste. In fact, fish bones contain natural high-quality active calcium and various trace elements that can promote human growth and development, such as phosphorus, iron, selenium, etc. The middle bones of fish can be processed through steps such as steaming, drying, crushing, and adding auxiliary materials to make a nutritional health food - bone paste. It is the best health food for pregnant women, infants and young children, and the elderly to supplement calcium and trace elements.

Another product, "fish protein calcium candy," also uses fish protein bone powder as the main raw material, and is made by deodorizing, removing fishy odor, and adding vitamin D, etc. This product is rich in nutrients, easy to digest and absorb, can improve appetite and promote growth and development. It is also suitable for pregnant women, infants and young children, and the elderly, and has certain health-care and therapeutic functions.

V. Bile pigment calcium salt and bile acid salt. Fish gallbladders are often discarded first as waste during fish processing, which is truly a pity. Bile can be processed into bile pigment calcium salt, bile acid salt and taurine, etc., whose value is more precious than gold. These bile products can be used as raw materials for the pharmaceutical industry, and can be used to make artificial cow bezoar and antibiotic preparations; medically, they can promote the digestion and absorption of fats by pancreatic juice; they can also be used as one of the components of bacterial culture media for scientific research. The production technology is introduced below.

1. Bile pigment calcium salt. Add 20% saturated calcium hydroxide solution to bile, then pass in steam and heat for 4 hours-5 hours. At this time, a yellow-green solid floats on the liquid surface or precipitates on the inner wall of the container. Let stand for 2 hours-3 hours to separate this solid, which is bile pigment calcium salt. After drying, it is the finished product and can be bottled for storage.

If dilute hydrochloric acid is added to the above calcium salt mixture to generate calcium chloride, it can dissolve in water, while the reduced bilirubin and biliverdin do not dissolve in water. After filtration and separation, add chloroform to the filter residue to separate bilirubin from biliverdin (the former is soluble in chloroform while the latter is not), then respectively purify by recrystallization to obtain pure bilirubin and biliverdin.

2. Bile acid salt. Use bile from which bile pigment calcium salt has already been extracted, adjust its pH to the level of the original fresh bile (pH 7.8), and filter to remove impurities. Place in a concentration pot and heat to concentrate, removing 5/6 of the water. After it becomes paste-like, add 3 times the amount of 95% alcohol and 5% activated carbon, transfer to a distillation flask, and distill according to a fractionating column (the distilled alcohol can be recovered and reused) until the residue in the flask is 1/3 of the raw material volume. Take it out and filter; extract the filter residue twice more with alcohol and then discard it (it can be used as fertilizer). Combine the three filtrates and place them in a cold storage room at 0°C. Add ether while stirring until a stable emulsified state appears. The amount of ether used is about 2 times that of the concentrated liquid, and the time is about 10 minutes. Then let stand for 48 hours, and layering can be seen. Separate the lower liquid (the upper layer is ether, which can be distilled, purified and reused), then pour it into an enamel tray, spread it into a thin layer, blow away residual ether under ventilation, and place in a vacuum drying oven to dry. After drying, take it out and grind it fine with a ball mill, screen through a 150-mesh sieve, and immediately bottle and package it as the finished product. The finished product absorbs moisture very easily, so the grinding and bottling work must be carried out in a dry environment. This bile acid salt is a mixture of sodium salts of taurocholic acid and glycocholic acid, and can be separated and purified.