李偉明 李麗 馬小衛(wèi) 詹儒林 王松標

摘 要 油梨（Persea americana Mill.）是最具有保健作用的水果之一。目前，科學界對油梨的關注與日俱增，特別是在油梨的功能成分、采后處理和加工技術、食用方法、營養(yǎng)價值和保健作用等方面取得了不少顯著研究成果，為油梨功能成分綜合利用的研究積累了豐富的知識和經(jīng)驗，奠定了提高油梨功能成分利用率及其加工產(chǎn)品經(jīng)濟效益的良好理論和技術基礎。本文對過去數(shù)年，尤其是近年來在油梨功能成分測定、產(chǎn)品加工、營養(yǎng)價值、醫(yī)療保健、功能飼料等方面的研究進展進行綜述，以期為后續(xù)油梨綜合利用技術研究和推廣應用提供參考。

關鍵詞 油梨；功能成分；綜合利用；深加工

中圖分類號 S667 文獻標識碼 A

1 Introduction

Avocado （Persea americana Mill.） was considered the most nutrient-dense fruit[1]. It could be eaten raw and used in a range of dishes， desserts and cold drinks. It is used as raw material in the production of cosmetics and industrial feedstock and also used as a staple food. The fat content of avocado varies within 6.50%-30.50%[2-4]， and it was known as the “cream of the forest”. Of its fatty acids， 80% were unsaturated， of which the essential linolenic and linoleic acids account for 27.5%. Avocado fat was easily absorbed by the human body and its digestion rate could be 83.7%， moreover it was cholesterol free. Eating avocado in place of saturated and trans fatty acids in other foods could help reduce blood cholesterol and phospholipid levels[4-7]. The avocados creamy consistency makes it one of the first fresh fruits that a baby can eat[8]. The United States， Japan， Europe and South American countries all treat avocado as a valuable fruit， with features of fruit， grain and oil[9].

Avocado was believed to originate in the United States， Mexico and surrounding areas， it had been used at least 9 000 years[10-12]. It was cherished by the Maya and Aztec civilizations， and semi-domesticated avocado can be traced back to pre-Columbian times[13，14]. Avocado was called “ahuacacuauhitl” by the Aztecs， and was written as “avocado” for the first time in 1 699 by Brit Hans Sloan[15]. Despite the fact that avocado has been paid widespread attention by modern civilization for only more than 100 years， avocado had caused concern around the world due to its unique nutritional values， and its production and consumption have kept growing[16-17]. According to FAO survey data， worldwide total production and yield per hectare of avocado have risen rapidly in the past 10 years（Fig. 1 A and B）[18]. In 2013， world avocado production was close to 500 million tonnes， which is 2.5 times the amount 10 years previously. The top five avocado-producing countries are Mexico， Indonesia， the United States， Colombia and the Dominican Republic （Fig. 1C）[18]. ‘Hass is the worlds most common commercial avocado cultivar and it contains a variety of essential nutrients and important phytochemicals[19]. The 2015 USDA survey data from the‘Hass Avocado Board showed that the United States consumed over 104 million pounds of avocado in 2014， which increased by 1 202.5% compared with the 8 million pounds in 2000. That was enough to fill up a football stadium 40 feet deep[20]. Thus， avocado is an emerging tropical and subtropical fruit attracting attention worldwide with an important position in international trade[3-4]. Given the industry background， theoretical and technological research on functional components of avocado， comprehensive utilization and further processing had been widely conducted with many significant results. There have been studies and reviews on cultivation， storage， function， components， preliminary processing， deep processing and comprehensive utilization of avocado[21-27]. Research advances on the development of avocado functional components and their value were reviewed in this paper to provide reference for further research on comprehensive utilization technology and application of avocado.

2 The functional components of avocado

Many studies had shown that avocado was almost the only low-sugar fruit containing monounsaturated fatty acids， and had the highest content of fat among fruits and vegetables. Avocado flesh had high levels of fat， protein and energy and low levels of sugar. Avocado was included in healthy eating patterns by the worlds leading nutrition organizations and projects， including Dietary Guidelines from United States Department of Agriculture （USDA） for Americans 2010， USDAs My Plate program， the Latin American Diet Pyramid， the Mediterranean Diet Pyramid and the Dietary Approaches to Stop Hypertension Diet Eating Plan[28].

2.1 Basic nutritional composition of avocado flesh

The basic nutritional composition of avocado flesh had water at the highest （70.0%-75.6%）， followed by crude fat （6.5-30.0%） and the protein content was low， usually less than 2.5% （Table 1）. It is noteworthy that the fat content of different avocado varieties varies greatly， and this directly affects the oil content. Therefore， varieties with high fatty acid content should be selected as raw materials for avocado oil extraction[29-31].

One-half an avocado may have a wide range of potential health benefits due to being nutrient and phytochemical dense： 4.6 g of dietary fiber， 0.2 g of total sugar， 345 mg of potassium， 5.5 mg of sodium， 19.5 mg of magnesium， 5.0 μg of RAE vitamin A， 6.0 mg of vitamin C， 1.3 mg of vitamin E， 14 μg of vitamin K1， 60 mg of folate， 0.2 mg of vitamin B6， 1.3 mg of niacin， 1.0 mg of pantothenic acid， 0.1 mg of riboflavin， 10 mg of choline， 185 μg of lutein/zeaxanthin， 18.5 μg of cryptoxanthin， 57 mg of phytosterols and 6.7 g of high-monounsaturated fatty acids and with energy of 114 kcal or 1.7 kcal/g[19， 32-33]. The nutritional and phytochemical composition of ‘Hassavocado had obvious advantages compared with three nuts， as summarized in Table 2 [19].

2.2 Extraction and composition of avocado oil

Werman et al.[34] conducted single factor experiments on avocado oil extraction using three factors： extraction temperature， pH and sodium chloride （NaCl） concentration. Then they further optimized conditions of oil extraction using orthogonal design. The combined optimized conditions were extraction temperature of 75 ℃， pH5.5 and 5% NaCl. Ortiz et al. [35] compared different methods of avocado oil extraction including microwave-extrusion and hexane and acetone extraction through microstructure and ultra-microstructure analysis. The oil extraction rate was highest（67%） using microwave-extrusion， and the original cell shape was basically maintained with slight changes. The oil extraction rate by hexane extraction was 59%， and irregular cell shapes and surface roughness occurred. The extraction rate by acetone extraction was the lowest， only 12%， and oil quality was the worst. Xu et al.[36] extracted avocado oil by aqueous enzymatic method. The oil extraction process was optimized by single factor and orthogonal experiments， and reached 75.38%； furthermore， the pineapple aroma gave the extracted avocado oil a better flavor. The optimal conditions reported by Cui et al.[31] using aqueous extraction were solid： liquid ratio 1：6（g/mL）， extraction time was 3 h， extraction temperature was 25 ℃ and centrifugation speed was 4 400 r/min-this resulted that the extraction rate of avocado oil was 72.86%. The quality of extracted avocado oil was good-among the tested physical and chemical properties， acid value was 0.62 mg/g， iodine was 47.20 g/100g， saponification was 197.80 mg/g and peroxide was 6.40 mmol/kg.

Cui et al.[31] determined fatty acids in avocado oil using gas chromatography. They found 19 mainly unsaturated fatty acids in avocado oil by comparing with the standard mixture of fatty acid methyl esters. The main fatty acids were 9c18：1， 11c18：1， 9c12c18：2n-6， C16：0 and 9c16：1. The main unsaturated fatty acids in avocado oil were oleic， linoleic and palmitoleic acids， and palmitic acid was the main saturated fatty acid （Table 3）.

2.3 Extraction of protein from avocado pulp and amino acid composition of avocado protein

Cui et al.[31] studied the effect of alkali extraction and acid precipitation on avocado protein extraction rate using de-oiled avocado meal as raw material and determined the optimum process conditions. The protein extraction rate reached its optimum of 49.13% when the process conditions were solid：liquid ratio of 1：15， extraction pH10， extraction time 2 h and extraction temperature 40 ℃. A solution pH4.6 resulted in the maximum protein precipitation， indicating that the isoelectric point of avocado meal protein was 4.6.

Various natural amino acids play different roles in the human body and could promote metabolism. Avocado contains 17 amino acids， including seven that are essential-threonine， valine， methionine， leucine， isoleucine， lysine and phenylalanine-making it a nutrient-rich protein source （Table 4）[30-31].

3 Value of avocado functional components and utilization of avocado

3.1 High-energy， high-fat and low-sugar healthy fruit

Avocados was a medium energy dense fruit because about 80% of the edible fruit consists mainly of water （72%） and dietary fiber （6.8%）， and had been shown similar effects on weight control to low-fat fruits and vegetables[19， 32， 37]. The energy in avocado was over 221 kcal per 100 g of flesh， which was 2-4 times higher than that in most tropical and subtropical fruits and twice that in lean meat. However， the sugar level in avocado was the lowest in fruits and the sodium content was also very low， making avocado a high-fat， high-energy and low-sugar healthy fruit [4]. One-fifth of a medium avocado （about 1 ounce） contains 50 calories and provides 1/8 of the daily value for fiber[28].

3.2 Avocado oil

Avocado oil had long been recognized as food[38]， and had been widely applied in cosmetics and daily necessities[39]. Avocado was rich in monounsaturated fatty acids. An average edible portion of a ‘Hass avocado was 136 g， of which 71% was monounsaturated fatty acids， 13% was polyunsaturated fatty acids and 16% was saturated fatty acids[40]. Eating avocado can significantly reduce levels of total cholesterol， low-density lipoprotein cholestero （LDL） and triglycerides， thus benefiting the blood lipids[41]. Avocado reduced the LDL cholesterol circulation by enhancing decomposition of triglyceride protein and simultaneously suppressing liver very low-density lipoprotein（VLDL）， thereby reducing blood lipid levels[42]. Avocado consumption enhances human postprandial provitamin A absorption and conversion， especially in populations in which vitamin A deficiency was prevalent[43]. However， most such studies have been at a small scale， and whether the conclusions apply on a large scale needs further validation[44-45].

Among a variety of commercially available edible oils， only refined avocado oil and cold-pressed avocado oil contain a relatively large amount of palmitic acid[46-48]. With respect to unsaturated fatty acids， the palmitoleic acid content of avocado oil is much higher than that of other edible oils-its ω9 oleic acid content ranks second only after camellia oil， its linolenic acid content was higher than other edible oils except soybean oil， and its ω6 linolenic acid content lies in the middle range. Among saturated fatty acids， palmitic acid had the highest content， and stearic acid was the lowest. Avocado had high content of fat， with relatively high levels of saturated and unsaturated fatty acids. There are small differences between oil composition and proportions of refined and cold-pressed avocado oils. Refined avocado oil has slightly lower levels of palmitic， palmitoleic， stearic and ω6 linolenic acids than cold-pressed avocado oil， and higher ω9 oleic acid content （Table 5）[46].

Although levels of oleic and linoleic acids of avocado oil were comparable to tea seed oil， the relatively high content of saturated fatty acids in avocado oil was undesirable for a healthy diet[46] （Table 6）. However， clinical studies have found that eating avocado in place of saturated fat is more beneficial to health compared with directly adding avocado to a baseline diet[41]. Therefore， increasing numbers of studies suggest that eating more avocado as advocated by health policy and public health recommendations should focus on emphasizing avocado as a substitute for saturated fat and not just as a food added to a free diet[41， 49-50].

3.3 Antioxidant activity

Leong et al.[51] examined the total antioxidant capacity of 27 types of fruits in the Singapore market including avocado， cherimoya and oranges by different methods. They found that the total antioxidant capacity of avocado ranked ninth， before oranges. Polyphenols and ascorbic acid were the main hydrophilic antioxidants in vegetables and fruits[52-53]. However lipophilic antioxidants could penetrate cell membranes and had higher bioavailability than hydrophilic antioxidants， and they were finally excreted through the urine[54]. Therefore， investigating specific types of lipophilic antioxidants and their contribution to human health was a promising research field[55]. Avocado was considered an important source of healthy lipophilic antioxidants， such as unsaturated fatty acids， polyphenols， carotenoids， sterols and Annonaceous acetogenin[56].

Plaza[57] found that permeable stearic acid from monounsaturated or polyunsaturated fatty acids had a high total antioxidant capacity even at low concentrations. Studies by Villa-Rodríguez[58] and Jacobo-Velázquez[59] also reached the same conclusion. In contrast， Wu et al.[60] found that antioxidant capacity of hydrophilic antioxidants was 2.5 times that of lipophilic antioxidants. There was no significant correlation between the concentration of total lipophilic substances in avocado and oxygen radical absorption capacity[55]， and these fatty acids blocked oxygen radicals by providing hydrogen ions[58]. The antioxidant capacity of the lipophilic substances separated from a crude acetone extract of avocado pulp by low-temperature partition chromatography was higher than that of the hydrophilic substances， and primary separation showed that acetyl derivatives（Annonaceous acetogenin） were likely to contribute to the antioxidant capacity. The biological activity of these substances was confirmed by performing lipid peroxidation in separated mitochondria[55]. Recent studies showed a significant positive correlation between the value of total antioxidant capacity and concentrations of oleic， linoleic and linolenic acids in avocado[58].

Rodríguez-Sánchez et al.[55] found seven lipophilic antioxidants-1-acetoxy-2，4-dihydroxy-n-heptadeca-16-ene， persediene， persenone-C， persenone-A， persenone-B， persin and 1-acetoxy-2，4-dihydroxy-heneicosa-12，15-diene-in the stone of avocado by optimizing the extraction and purification process， and of these the second and third compounds were reported in avocado pulp for the first time.

Fatty alcohols were commonly found in a variety of fruits， and the oxidation product of one of its members， hexanol， is hexanal. Studies showed that the hexanal content of avocado oil was usually low and oleic acid content was high-hexanal and other oxidation products were detected in mesocarp， indicating that all volatile components of avocado diminished during processing， therefore the quality and antioxidant capacity of avocado oil depends on its processing[46-47， 61]. Avocado phenols are receiving increasing attention[46]. Zhong et al.[46] measured phenols in several edible oils and found that the total phenolic content of cold-pressed avocado oil was 11.6 μg/g as caffeic acid， which was equivalent to that of cold-pressed camellia oil， slightly lower than that of cold-pressed pumpkin oil， but only half the amount in cold-pressed sesame oil. However， total phenolic content of refined avocado oil was 12.8 μg/g as caffeic acid， which was three times than that of refined camellia， pumpkin and soybean oils（Table 7）. The total phenolic content of fresh avocado fruit was 40 times than that of avocado oil， and total phenolic content of avocado seed was six times that of fresh fruit and 200 times that of avocado oil（Table 7）， indicating that total phenolic content of avocado seed was much higher than that of flesh， phenols were little transferred to the oil， and the content of phenolic compounds was greatly reduced during the extraction of avocado pulp oil. Moreover， the total phenolic content of cold-pressed avocado oil was slightly lower than that of refined avocado oil， indicating that with more and complicated extraction steps， the greater the loss of easily oxidized phenols. The extraction rate of phenols from fruits was higher using anhydrous methanol than aqueous methanol[46]. The extraction efficiency of phenols from avocado flesh using anhydrous methanol was 1.3 times than that using 70% aqueous methanol solution[46]. Vitamin C in the sample could affect the determination of phenolic compounds and lead to high values[46]. García-Alonso et al.[62] examined 28 types of fruit samples including plums， apricots and blueberries and found that flavanol content of avocado was very low， only 0.24 mg/100g dry weight， much lower than that of the plum which had the highest value of 366 mg/100g dry weight； however， no positive correlation was found between flavanol content and total antioxidant capacity. There were also no significant correlations between carotenoid[63] and sterol[57] concentrations of avocado with total antioxidant capacity.

Reviewed studies of avocado intake were unable to show a statistically significant decrease in serum total cholesterol（TC）， LDL and triglyceride levels. However， avocado intake significantly increased serum high-density lipoprotein cholesterol（HDL） levels and consequently decreased TC：HDL ratio and LDL：HDL ratio， indicating a potential association between avocado intake and reduced cardiovascular diseases risk[64].

3.4 Vitamins and minerals

Avocado was rich in nearly 20 types of vitamins， including vitamins A （20-60 mg/100g）， C （8.5 mg/100g）， D（10 mg/100g）， E （3 mg/100g）， K （8 mg/100g） and H （10 mg/100g） and B vitamins [thiamin （100 mg/100g）， riboflavin（120 mg/100g）， niacin（1 mg/100g）， biotin （10 mg/100g）， pantothenic acid， vitamin B6， vitamin B12 and folic acid][7，65]. Minerals in avocado pulp include calcium（9 mg/100g）， iron（0.17 mg/100g）， sodium（7 mg/100g）， phosphorus（55 mg/100g） and potassium （720 mg/100g）[29]. Avocado could effectively promote absorption of fat-soluble nutrients such as vitamins A， D， K and E， and so it was known as “nutrient booster” [4，8].

3.5 Other functions and applications

Other functional components in avocado include phytosterol， ergosterol， folate， inositol， phosphoric acid， lecithin and sesquiterpene. Avocado leaves and bark contain volatile oil， methyl chavicol， d-pinene， flavonoids， tannins and other compounds[7]. These components also have different functions， such as stimulating menstrual flow， regulating menstruation， relieving pain， deworming， disinfection and skin care[4].

In the 1990s， a number of avocado clinical trials consistently showed positive effects on blood lipids in a wide variety of diets[66-71]. In recent years， some studies had reported that avocado meals improved postprandial glucose， insulin and lipid profiles with concomitant improvements in vascular reactivity[72]. Therefore， avocado had been called high-energy， low-sugar “super food”[28]， in full compliance with the requirements of nutrition- and health-oriented food development-notably it meets consumption requirements of a high-energy low-sugar food for diabetics[9]， and is also a high-quality raw material for production of advanced cosmetics[4].

A few results illustrated the benefits of dietary avocado on pig breeding and on the oxidative stability of processed pork loins. Dietary avocado significantly affected the content and composition of intramuscular fat， reducing the lipid content in LTL muscles and increasing the degree of unsaturation， when providing a mixed diet enriched with avocado waste for finishing industrial genotype pigs[73]. Dietary avocado did not protect against oxidation of tryptophan and thiols but cooked and chilled loins from pigs fed an avocado-supplemented diet had significantly lower concentrations of lipid and protein carbonyls than controls. Likewise， dietary avocado alleviated the formation of Schiff bases during cooking[74]. Maximum antioxidant and antimicrobial responses against some food-borne bacteria were obtained with a mixture of 61% avocado peel extract and 39% of nisin， with concentrations of 1 mg/mL； there was radical scavenging capacity and antioxidant activity of avocado peel and seed extract， due to their contents of polyphenols， including kaempferide， epicatechin， chlorogenic acid and epicatechin gallate[75].

Additionally， avocado seed powder could efficiently treat dye-contaminated wastewater[76]. It had been suggested that the lipid-soluble bioactive-rich fraction of avocado fruit had antioxidant and antihemolytic activities. These avocado-derived substances could be served as a starting point for further research and applications in the preparation of natural pharmaceuticals and herbal formulations[77].

4 Prospects

a） The major producers and consumers of avocados were the United States， Mexico， South Africa and Israel， have experience that will be valuable for China.

b） Peoples consumption consciousness should be enhanced by new discovery of avocado functional components.

c） Avocado production should not be ignored and advanced production technology should be introduced and popularized by government.

Acknowledgements

This research was supported by the Agricultural Wild Plant Resources Protection Project of Science and Education Department of the Ministry of Agriculture and Basic Scientific Research Projects for State Level（No. 1251416305010）， National Crop Germplasm Resources Platform（No. NICGR2017-95） and Species Conservation （Crops） Project （No. 2017NWB053）.

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