2011年5月11日

甜味和鮮味,你能分清嗎?
   嘗和聞是兩種食品檢測的外部感官機制.品嘗食物營養成分的組織是通過舌頭上的味蕾也就是品嘗感覺細胞簇感受食物的營養價值(圖1).就像第一章描述甜味的文章所述,常見碳水化合物刺激豬的甜味味蕾.然而,人類所知的高強度甜味劑使用在豬飼料中(糖精鈉,索馬甜和新橙皮苷二氫查耳酮)時,它們僅僅能觸動少數豬舌頭的味蕾應答,似乎沒有有效地促進飼料的采食,但是它們刺激了胃腸道中的味蕾受體(TR),在碳水化合物的消化/吸收中扮演了關鍵的角色.

圖1 豬的嗅覺和味覺系統:顱神經和味蕾細節

   對于人類,品嘗感覺已經成為了適口性的模式,能夠容易地感覺和鑒定.類似甜味,鮮味也增強了哺乳動物的采食量.一些L-氨基酸(L-AA),如谷氨酸(L-Glu)或谷氨酸鈉（MSG）觸發鮮味。鮮味對于公眾來說仍然是一種保持未知面膜的品嘗感覺，因為它不是很容易同其他味覺（如咸）或者香料（肉香）區分開來。然而，人類有MSG（約1mM）的檢測閾值，大約低于糖(約10mM)的10倍。對于豬來說，MSG的閾值與人相似（分別是約1mM和5-10mM）。而且豬不僅對MSG（和谷氨酸）也對其他氨基酸表現出好感，盡管它們包括谷氨酰胺，丙氨酸和天冬酰胺在內沒有被人類認為是鮮味劑（表1）。這些發現可能表明哺乳動物識別氨基酸比識別糖有更高的敏銳味覺，同時對日糧中蛋白質的喜歡潛勢超過碳水化合物。
表1 豬對L氨基酸的味覺響應能力以及人的主要快感反應

Amino Acid氨基酸	L-isomerL-異構體
	Human taste人類口味	豬的反應(1)
Alanine丙氨酸	Sweet甜	Umami鮮味
Arginine精氨酸	Bitter苦	Umami鮮味
Asparagine天門冬酰胺	Bitter苦	Umami鮮味
Aspartic a.天冬氨酸	Umami, sour鮮味，酸味	Umami鮮味
Cysteine半胱氨酸	Sulphur硫磺味	n/a不適用
Glutamic a.谷氨酸	Umami, salty鮮味，咸味	Umami鮮味
Glutamine谷氨酰胺	Sweet, umami甜，鮮味	Umami鮮味
Glycine 甘氨酸	Sweet甜	Yes一樣
Histidine組氨酸	Bitter苦	No不一樣
Hydroxyproline羥脯氨酸	Sweet甜	Yes
Isoleucine異亮氨酸	Bitter苦	No
Leucine亮氨酸	Bitter苦	No
Lysine賴氨酸	Bitter, salty, sweet苦，咸，甜	Yes
Methionine蛋氨酸	Bitter, Sulphur, umami苦，硫磺，鮮	No
Phenylalanine苯丙氨酸	Bitter苦	No
Proline脯氨酸	Sweet, salty甜，咸	Umami鮮味
Serine絲氨酸	Sweet甜	Yes
Threonine蘇氨酸	Sweet甜	Umami鮮味
Tryptophan色氨酸	Bitter苦	Bitter苦味
Tyrosine酪氨酸	Bitter苦	不適用
Valine纈氨酸	Bitter苦	No

（1）豬的反應：n/a=不適用；YES代表有反應但是味覺不確定;NO代表沒有反應。
(摘自Roura and Tedo. 2009.<豬的飼料食欲：口鼻分別感覺>In: D. Torrallardona 和E. Roura Eds. 豬的采食量. Wageningen 學術出版社, The Netherlands)

這樣的話，豬覓食蛋白多與碳水化合物？
     好像的確如此。在對仔豬偏食性（雙料槽實驗）的研究中，我們觀察與原料的自然特性有顯著效應.測試飼料相對參照組的偏食性表述為測試飼料:總消費(測試組+參照組)的百分比. 50%的百分數相當于中性值也就是兩種供試飼料被采食相同數量. 我們報告稱測試谷物添加比例的1%變化,導致相對中性值的偏食性增加(負增加或正增加) 2%頻率.這項同樣的頻率在纖維原料的測定中也被發現(增加1%即影響2%的偏食性值), 這個結果略低于脂肪性原料(1%添加比例影響大約1%的偏食性值).值得注意的是,在我們的研究中,測試蛋白質每1%的變化導致偏食性超過50%中性值的部分以大約5%的比例增加或減少.而且,已經有文獻報道大多數原料對飼料性能有最大的影響,例如,魚粉,乳清粉濃縮蛋白,噴霧干燥動物血漿以及豬腸膜蛋白粉,它們均具有一些基本特征即含有顯著的咸味和鮮味成分。

圖2 豬的偏食性（用總采食量的%表示），日糧中含有5,10或20%的不同大白紙來源，采用雙料槽試驗，并包含常規參考日糧。

使用不同長度的柱子表示顯著差異(P<0.05).
*表示偏食性與50%中性值顯著差異(P<0.05)
(來源: Torrallardona and Solà-Oriol. 2009. In: D. Torrallardona and E. Roura Eds. Voluntary Feed intake in Pigs. Wageningen Academic Publishers, The Netherlands).

豬舌頭中的鮮味受體對某些蛋白質原料改善飼料性能和吸收表現出積極的推動作用,尤其是仔豬.在蛋白質缺乏或者營養嚴重不良的情況(比如斷奶時),這些受體超常表達,增加了部分對豐富蛋白質日糧采食欲望的應答反應.因此,在這些關鍵時期,刺激鮮味感官能夠促進飼料采食量增加,另一方面,在早期階段,高適口性蛋白質來源的使用能夠增加斷奶后飼料采食和生長速率.

   在胃腸道黏膜中也能發現鮮味感官受體, 能感受到管腔中的氨基酸.據稱,它們參與了氨基酸的吸收和消化代謝,使用同樣的方式,甜味受體影響了一般碳水化合物的代謝. 反過來,這些小腸營養營養感覺機制與饑餓-飽腹周期控制有關.因此,高度偏食蛋白質不僅能確保鞏固短期的飼料采食,也能產生長期效應,這種效應也能通過小腸機制解釋.最后,越來越清楚的是小腸中碳水化合物和氨基酸感官(味蕾細胞)被歸為串擾細胞機制,旨在促進這兩種日糧營養來源的模糊使用,以此來滿足能量需求.確定這些關于豬和人的研究還需要更多的研究。

豬對鮮味有高度敏感性，高于甜味的10倍。關于日糧中原料偏食性測定的數據表明仔豬對蛋白質營養的偏好。營養專家應當關注蛋白質的偏好級別，以便在關鍵時期尤其是斷奶后早期或者飼料轉換期（特別是在飼料配方大幅變化后），預期增加豬的飼料采食量。另外，在這樣的關鍵時期過后，鮮味（蛋白質）和甜味（碳水化合物）口味的平衡是滿足長期最大采食量的充分戰略。

18-May-2011·                                
Eugeni Roura

Sweet and umami tastes: who is what?
Taste and smell are two of the peripheral sensing mechanisms devoted to food identification. The taste system senses the nutritional value of foods through the taste buds defined as clusters of taste sensory cells present in the tongue (figure 1). As discussed in Part I (The good taste of pigs (part I): let it be sweet) simple carbohydrates stimulate sweet taste in pigs. However, high intensity sweeteners known to humans and that are commonly used in piglet diets (Sodium Saccharin, Thaumatin and Neohespiridine dehydrocalcone) they only trigger minor sweet taste responses in pigs tongue and do not seem to be effective in increasing feed intake but they may play a key role in carbohydrate digestion/absorption by stimulating the sweet taste receptors (TR) present in the gastrointestinal tract (GIT).

Figure 1. The smell and taste system in the pig: cranial nerves and a detail of the taste buds.

In humans sweet perception has become a paradigm of “good taste” easy to sense and identify. Similar to sweet, the umami taste also enhances voluntary intake in mammals. Several L-amino acids (L-AA), such as Glutamic acid (L-Glu) or monosodium glutamate (MSG), trigger umami taste. Umami remains largely as an unknown taste modality to the general public likely because it may not be easy to distinguish from other tastes (i.e. salty) or flavours (meaty). However, humans have a detection threshold for MSG that is around 10 fold lower than for sugar (around 1mM and 10 mM respectively). The detection thresholds for MSG and sugar in pigs are very similar to humans (around 1mM and between 5 to 10 mM respectively). Furthermore pigs show a positive preference not only to MSG (and L-Glu) but also to other amino acids not perceived as umami by humans such as glutamine, alanine and asparagine among others (table 1). These findings may indicate that mammals have a higher acuity of taste in detecting amino acids than sugars and a potential higher craving for dietary protein compared to carbohydrates.

Table 1. Gustatory responsiveness of pigs to L-amino acids and predominant hedonic response in humans.

Amino Acid	L-isomer
	Human taste	Pig response(1)
Alanine	Sweet	Umami
Arginine	Bitter	Umami
Asparagine	Bitter	Umami
Aspartic a.	Umami, sour	Umami
Cysteine	Sulphur	n/a
Glutamic a.	Umami, salty	Umami
Glutamine	Sweet, umami	Umami
Glycine	Sweet	Yes
Histidine	Bitter	No
Hydroxyproline	Sweet	Yes
Isoleucine	Bitter	No
Leucine	Bitter	No
Lysine	Bitter, salty, sweet	Yes
Methionine	Bitter, Sulphur, umami	No
Phenylalanine	Bitter	No
Proline	Sweet, salty	Umami
Serine	Sweet	Yes
Threonine	Sweet	Umami
Tryptophan	Bitter	Bitter
Tyrosine	Bitter	n/a
Valine	Bitter	No

(1)Pig response: n/a=not available; YES means that there is a response but the type of taste has not been identified; NO means no response.
(Adapted from Roura and Tedo. 2009. Feed appetence in pigs: an oronasal sensing perspective. In: D. Torrallardona and E. Roura Eds. Voluntary feed intake in pigs. Wageningen Academic Publishers, The Netherlands)

So, do pigs find proteins tastier than carbohydrates?
It is likely to be so. In our studies on piglet feed preferences (double choice tests), we have observed marked effects related to the nature of the ingredients. Test feed preference over a reference feed is expressed as a per cent consumption of the test feed over the total consumption (test + reference diets). A preference of 50% corresponds to a neutral value, that is when equal amounts of the two feeds presented were eaten. We have reported that changes in 1% of the inclusion rate of a test cereal result in increments (positive or negative) of preference compared to the neutral value of around 2%. A similar ratio is found for fibre sources (1% increase for 2% impact on preference value) and slightly lower value for fat sources (1% inclusion for around 1% changes in preference). Remarkably, in our studies, every 1% change in the inclusion of a test protein resulted in ca 5% increase or decrease in preference over the 50% neutral value. This data indicates that protein sources have the highest relative impact on feed preference among the families of feed ingredients that we tested (table 2). Furthermore it has been widely reported in the literature that most of the ingredients with the highest impact on feed preferences such as fish meal, whey protein concentrate, spray-dried animal plasma and dried porcine solubles, have the common feature of containing significant salty and umami taste compounds.

Table 2. Pig’s preference (% of total feed intake) for diets containing 5, 10 or 20% of different protein sources offered in a two-way choice with a common reference diet.

Columns with different letters indicate significantly different preference values (P<0.05).
* Indicate a preference value significantly different from 50% (P<0.05).
(Source: Torrallardona and Solà-Oriol. 2009. In: D. Torrallardona and E. Roura Eds. Voluntary Feed intake in Pigs. Wageningen Academic Publishers, The Netherlands).

Effects of protein on feed intake
The presence of umami receptors in the tongue contribute to the positive effect of some protein sources on feed preference and intake particularly in piglets. Under protein or general malnutrition (such as at weaning) these taste receptors are over-expressed as part of the response that increases the craving for protein rich diets. During these critical periods, therefore, stimulating the umami sensors will result in significant increases in feed intake. In other words, the use of highly palatable protein sources in early phases will result in increases of feed intake and growth rate post-weaning.

Umami taste receptors have also been found in the GIT mucosa sensing amino acids in the lumen gut. It is believed that they are involved in amino acid absorption and metabolism much to a similar fashion that sweet receptors affect simple carbohydrate metabolism. These intestinal nutrient sensing mechanisms are, in turn, linked to the controls of the hunger-satiety cycle. High preference proteins, therefore, are not only securing a short term feed intake but a long-term effect might be also explained through intestinal mechanisms. Finally, it is becoming clear that at the intestinal level carbohydrate and amino acid sensors (taste sensory cells) are integrated into a crosstalk cellular mechanism aimed at facilitating the indistinct use of the two dietary nutrient sources to meet the energy requirements. Additional research is required to verify the relevance of such findings in pigs and humans.

In summary
Pigs have high sensitivity for umami taste that is about 10 fold than of sweet taste. Data on dietary ingredient preferences confirm the craving of piglets for proteins. Nutritionists should pay attention to the preference rankings of proteins to anticipate increased feed acceptance by pigs at critical stages such as immediately after weaning or in feed transitions (particularly after drastic formulation changes). In addition, after such critical periods, the balance between umami (protein) and sweet (carbohydrate) sensing is the adequate strategy to target maximal long term feed intake.