猪的好口感-鲜味（第二章）

2011年5月11日

甜味和鲜味,你能分清吗?
   尝和闻是两种食品检测的外部感官机制.品尝食物营养成分的组织是通过舌头上的味蕾也就是品尝感觉细胞簇感受食物的营养价值(图1).就像第一章描述甜味的文章所述,常见碳水化合物刺激猪的甜味味蕾.然而,人类所知的高强度甜味剂使用在猪饲料中(糖精钠,索马甜和新橙皮苷二氢查耳酮)时,它们仅仅能触动少数猪舌头的味蕾应答,似乎没有有效地促进饲料的采食,但是它们刺激了胃肠道中的味蕾受体(TR),在碳水化合物的消化/吸收中扮演了关键的角色.

图1 猪的嗅觉和味觉系统:颅神经和味蕾细节

   对于人类,品尝感觉已经成为了适口性的模式,能够容易地感觉和鉴定.类似甜味,鲜味也增强了哺乳动物的采食量.一些L-氨基酸(L-AA),如谷氨酸(L-Glu)或谷氨酸钠（MSG）触发鲜味。鲜味对于公众来说仍然是一种保持未知面膜的品尝感觉，因为它不是很容易同其他味觉（如咸）或者香料（肉香）区分开来。然而，人类有MSG（约1mM）的检测阈值，大约低于糖(约10mM)的10倍。对于猪来说，MSG的阈值与人相似（分别是约1mM和5-10mM）。而且猪不仅对MSG（和谷氨酸）也对其他氨基酸表现出好感，尽管它们包括谷氨酰胺，丙氨酸和天冬酰胺在内没有被人类认为是鲜味剂（表1）。这些发现可能表明哺乳动物识别氨基酸比识别糖有更高的敏锐味觉，同时对日粮中蛋白质的喜欢潜势超过碳水化合物。
表1 猪对L氨基酸的味觉响应能力以及人的主要快感反应

Amino Acid氨基酸	L-isomerL-异构体
	Human taste人类口味	猪的反应(1)
Alanine丙氨酸	Sweet甜	Umami鲜味
Arginine精氨酸	Bitter苦	Umami鲜味
Asparagine天门冬酰胺	Bitter苦	Umami鲜味
Aspartic a.天冬氨酸	Umami, sour鲜味，酸味	Umami鲜味
Cysteine半胱氨酸	Sulphur硫磺味	n/a不适用
Glutamic a.谷氨酸	Umami, salty鲜味，咸味	Umami鲜味
Glutamine谷氨酰胺	Sweet, umami甜，鲜味	Umami鲜味
Glycine 甘氨酸	Sweet甜	Yes一样
Histidine组氨酸	Bitter苦	No不一样
Hydroxyproline羟脯氨酸	Sweet甜	Yes
Isoleucine异亮氨酸	Bitter苦	No
Leucine亮氨酸	Bitter苦	No
Lysine赖氨酸	Bitter, salty, sweet苦，咸，甜	Yes
Methionine蛋氨酸	Bitter, Sulphur, umami苦，硫磺，鲜	No
Phenylalanine苯丙氨酸	Bitter苦	No
Proline脯氨酸	Sweet, salty甜，咸	Umami鲜味
Serine丝氨酸	Sweet甜	Yes
Threonine苏氨酸	Sweet甜	Umami鲜味
Tryptophan色氨酸	Bitter苦	Bitter苦味
Tyrosine酪氨酸	Bitter苦	不适用
Valine缬氨酸	Bitter苦	No

（1）猪的反应：n/a=不适用；YES代表有反应但是味觉不确定;NO代表没有反应。
(摘自Roura and Tedo. 2009.<猪的饲料食欲：口鼻分别感觉>In: D. Torrallardona 和E. Roura Eds. 猪的采食量. Wageningen 学术出版社, The Netherlands)

这样的话，猪觅食蛋白多与碳水化合物？
     好像的确如此。在对仔猪偏食性（双料槽实验）的研究中，我们观察与原料的自然特性有显著效应.测试饲料相对参照组的偏食性表述为测试饲料:总消费(测试组+参照组)的百分比. 50%的百分数相当于中性值也就是两种供试饲料被采食相同数量. 我们报告称测试谷物添加比例的1%变化,导致相对中性值的偏食性增加(负增加或正增加) 2%频率.这项同样的频率在纤维原料的测定中也被发现(增加1%即影响2%的偏食性值), 这个结果略低于脂肪性原料(1%添加比例影响大约1%的偏食性值).值得注意的是,在我们的研究中,测试蛋白质每1%的变化导致偏食性超过50%中性值的部分以大约5%的比例增加或减少.而且,已经有文献报道大多数原料对饲料性能有最大的影响,例如,鱼粉,乳清粉浓缩蛋白,喷雾干燥动物血浆以及猪肠膜蛋白粉,它们均具有一些基本特征即含有显著的咸味和鲜味成分。

图2 猪的偏食性（用总采食量的%表示），日粮中含有5,10或20%的不同大白纸来源，采用双料槽试验，并包含常规参考日粮。

使用不同长度的柱子表示显著差异(P<0.05).
*表示偏食性与50%中性值显著差异(P<0.05)
(来源: Torrallardona and Solà-Oriol. 2009. In: D. Torrallardona and E. Roura Eds. Voluntary Feed intake in Pigs. Wageningen Academic Publishers, The Netherlands).

猪舌头中的鲜味受体对某些蛋白质原料改善饲料性能和吸收表现出积极的推动作用,尤其是仔猪.在蛋白质缺乏或者营养严重不良的情况(比如断奶时),这些受体超常表达,增加了部分对丰富蛋白质日粮采食欲望的应答反应.因此,在这些关键时期,刺激鲜味感官能够促进饲料采食量增加,另一方面,在早期阶段,高适口性蛋白质来源的使用能够增加断奶后饲料采食和生长速率.

   在胃肠道黏膜中也能发现鲜味感官受体, 能感受到管腔中的氨基酸.据称,它们参与了氨基酸的吸收和消化代谢,使用同样的方式,甜味受体影响了一般碳水化合物的代谢. 反过来,这些小肠营养营养感觉机制与饥饿-饱腹周期控制有关.因此,高度偏食蛋白质不仅能确保巩固短期的饲料采食,也能产生长期效应,这种效应也能通过小肠机制解释.最后,越来越清楚的是小肠中碳水化合物和氨基酸感官(味蕾细胞)被归为串扰细胞机制,旨在促进这两种日粮营养来源的模糊使用,以此来满足能量需求.确定这些关于猪和人的研究还需要更多的研究。

猪对鲜味有高度敏感性，高于甜味的10倍。关于日粮中原料偏食性测定的数据表明仔猪对蛋白质营养的偏好。营养专家应当关注蛋白质的偏好级别，以便在关键时期尤其是断奶后早期或者饲料转换期（特别是在饲料配方大幅变化后），预期增加猪的饲料采食量。另外，在这样的关键时期过后，鲜味（蛋白质）和甜味（碳水化合物）口味的平衡是满足长期最大采食量的充分战略。

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.