And given that all mammals lactate,

所有哺乳動物都會泌乳

why do we favor certain types of milk over others?

但人們為何喜歡某些牛奶 勝過其他種類？

Milk is the first thing we drink,

奶水是我們最先喝到的東西

and thanks to developments in the production and variety of dairy products,

多虧了製造業的發展和奶製品的多樣

it can take on countless forms for our dietary and sensory well-being.

它可以呈現許多 有益於我們飲食及感官的形式

Milk's primary function is as a complete source of nutrition for newborns.

奶水最主要功能 是為新生兒提供全面營養

In fact, since it has all of the vital nutrients for development and growth,

實際上，它含有 所有成長所需的重要營養

proteins,

蛋白質

carbohydrates,

碳水化合物

fats,

脂肪

vitamins and minerals,

維他命和礦物質

and water,

以及水分

milk is the only thing a baby even needs to ingest

奶水是新生兒 6 個月大之前

for the first six months of life.

唯一需要攝入的

The unique makeup of milk can vary depending on factors like species,

它的獨特成分取決於多樣的因素

diet,

比如物種

and location.

膳食習慣

Reindeer of the Arctic Circle, for example,

和地理位置

make energy-dense milk that's about 20% fat,

比如，北極圈馴鹿

roughly five times more than human or cow's milk,

其奶水是能量密集型 含有 20% 的脂肪

to help their young survive the harsh, freezing climate.

大概比人奶或牛奶多 5 倍

So how is milk made?

以保證小馴鹿在嚴寒之地可以生存

In the uniquely mammalian process of lactation,

奶水是怎樣產生的呢？

a special class of milk-secreting cells known as mammocytes

在哺乳動物獨特的泌乳過程中

line up in a single layer around pear-shaped alveoli.

有一種泌乳細胞稱為「乳腺細胞」

Those cells absorb all of the building blocks of milk,

它們沿著梨狀濾泡排成單層的一排

then synthesize tiny droplets of fat

這些細胞汲取所有製奶的基本成分

on structures called smooth endoplasmic reticula.

然後在「平滑內質網」組織 製造微小的脂肪滴

The droplets combine with each other and other molecules

脂肪滴與脂肪滴 及其他分子互相結合

and are then expelled and stored in spaces between cells.

然後排出並儲存在細胞之間的空間中

Mammary glands eventually secrete the milk through the breasts, udders,

乳腺最終經由乳房、乳頭排出乳汁

or, in the rare case of the platypus, through ducts in the abdomen.

或在鴨嘴獸的罕見案例中 經由腹部的管道

Although this process is typically reserved for females,

雖然這個過程通常發生在雌性

in some species, like dayak fruit bats,

但在一些物種，如棕櫚果蝠

goats,

山羊

and even cats,

甚至貓

males can also lactate.

雄性也能泌乳

Milk drinkers worldwide consume dairy from buffalo,

全世界的牛奶飲用者 飲用的牛奶是取自水牛

goats,

山羊

sheeps,

綿羊

camels,

駱駝

yaks,

犛牛

horses,

馬匹

and cows.

和乳牛

Almost all of these species are ruminants,

幾乎所有這些物種都是反芻動物

a type of mammal with four-chambered stomachs

一種四室胃的哺乳動物

that yield large quantities of milk.

能產生大量的奶水

Of these, cows were the most easily domesticated

其中，乳牛是最容易馴養的

and produce a milk that is both easily separated into cream and liquid

可生產易於分離成奶油和液體的奶

and has a similar fat content to human milk.

並具有與人乳相似的脂肪含量

In their natural environment,

在自然環境中

mammals secrete milk on call for immediate consumption by their young.

哺乳動物分泌乳汁 隨時供牠們的幼仔使用

But with the demands of thirsty consumers,

但隨著消費者的大量需求

the dairy industry has enlisted methods to step up production,

乳製品產業 已經採取了提升生產的方法

enhance shelf life,

增長保質期

and provide a variety of milk products.

並提供各種奶製品

In the dairy, centrifugation machines spin milk at high speeds,

在乳製廠，離心機高速旋轉牛奶

forcing less dense fats to separate from the liquid and float up.

迫使較低密度的脂肪 與液體分離並浮起

After being skimmed off,

脫脂後

this fat, known as butterfat, can be used in dairy products

這種脂肪，稱為乳脂 可以用於乳製品

like butter,

像黃油

cream,

奶油

and cheese.

和奶酪

Or it can be later added back to the liquid in varying proportions

或稍後以不同比例加回到液體中

to yield different fat content milks.

以產生不同的脂肪含量乳

Full fat milk, sometimes referred to as whole milk, has 3.25% butterfat added

全脂牛奶──有時稱為全脂奶 添加了 3.25％ 的乳脂

compared to 1-2% for low and reduced fat milk,

相比之下，低脂和減脂牛奶為 1-2％

and less than half a percent for skim milk.

脫脂乳則是少於 0.5%

To stop reseparation of the fat from the water, or creaming,

為了防止脂肪與水的再分離 ── 即「乳狀液分層」

the mixture undergoes the high-energy pressurized process of homogenization.

混合物經過高能量加壓的 均質化處理 (homogenization)

Before milk hits the shelves, it's also typically heat treated

在牛奶送上貨架之前 它通常也經過加熱處理

to reduce its level of microbes,

以減少其微生物含量

a government-sanctioned process

這是政府批准的製程

that raw milk enthusiasts argue may reduce milk's nutritional worth.

但生奶愛用者認為 這會降低牛奶的營養價值

Milk spoilage is started by microbes,

牛奶腐敗始自微生物

which consume and break down the nutrients in milk.

它會消耗和分解牛奶中的營養物質

That process causes butterfat to clump together,

這會導致乳脂聚集成塊

leading to a visually unpleasant product.

形成看起來令人不快的物質

And the byproducts of the microbes' consumption

而且微生物消耗後的副產品

are compounds that taste and smell nasty.

是味道和氣味令人作嘔的化合物

But there's a bigger problem.

但是有一個更大的問題

Raw milk can carry microbes that are the sources of deadly diseases,

生奶含有致命疾病來源的微生物

so in order to kill as many of those microbes as possible,

因此為了盡可能殺死微生物

and keep milk fresh longer,

並使牛奶保鮮更久

we use a technique called pasteurization.

我們使用一種稱為 「巴氏殺菌」的技術

One version of this process involves exposing milk

其中一個方法是讓牛奶 處於高溫下約 30 秒

to about 30 seconds of high heat.

另一個方法稱為 「超高溫消毒法」（UHT）

Another version, called ultra-high temperature processing,

或「超巴氏殺菌」

or ultra pasteurization,

以相當高溫加熱牛奶數秒鐘 （用 135~150°C 加熱 2~3 秒）

blasts the milk with considerably higher temperatures over just a few seconds.

UHT 牛奶有更長的保質期

UHT milk boasts a much longer shelf life,

不放冰箱也可保質 12 個月

up to twelve months unrefrigerated,

而巴氏殺菌的牛奶 放冰箱則可保質兩週

compared to pasteurized milk's two weeks in the fridge.

這是因為溫度較高的 UHT 處理

That's because the higher temperatures of UHT processing

消滅更多的微生物

inactivate far more microbes.

然而，較高的加工溫度

Yet the higher processing temperatures

可能不利於牛奶的營養和質感

may adversely affect the nutritional and sensory properties of the milk.

最終，這種選擇在於消費者的口味

Ultimately, that choice lies in the consumer's taste

與便利性的需求

and need for convenience.

幸好，這個每年超過 8.4 億噸產品的商業

Fortunately, there are many choices available

提供了很多的選擇

in an industry that produces in excess of 840 million tons of products each year.

翻譯：Yue Chen