怎么判断刚挤下的牛奶是否掺杂

牛奶是一种营养丰富的饮品，但在市场上，可能存在刚挤下的牛奶被掺杂的情况。那么，我们该如何判断刚挤下的牛奶是否掺杂呢？

首先，可以通过观察牛奶的外观来初步判断。正常的刚挤下的牛奶应该呈现均匀的乳白色，没有杂质和沉淀。如果牛奶中出现了异物、浑浊或者分层的现象，就有可能是掺杂了其他物质。

其次，闻一闻牛奶的气味也是一种方法。新鲜的牛奶应该具有淡淡的奶香，没有异味。如果闻到了刺鼻、发酸或者其他不正常的气味，那么这牛奶可能存在问题。

还可以通过检测牛奶的比重来判断是否掺杂。根据液体的密度原理，牛奶的正常比重在 20℃时为 1.028 - 1.033 之间。实验证明，牛奶中每加入 10%的水时，比重就会降低 0.0029。具体方法是将 200ml 混合均匀的牛奶缓缓倒入容积为 500ml 的量筒内，在 20℃下把比重计放入量筒内（比重计不可与筒壁接触）。静置 2 分钟左右即可读取数值，若数值低于 1.028 时，就可断定牛奶为掺水牛奶。

对于是否掺入淀粉，可以取牛奶 10ml 放入 25ml 试管中，加热煮沸，冷却后加入 2ml 碘液摇晃。若有蓝紫色沉淀产生，说明牛奶中掺入了淀粉。原理是糊化后的淀粉具有遇碘变蓝紫色的特性。

要判断是否掺入碱，可取牛奶 10ml 置于入 25ml 的试管中，然后加入 0.05%的玫瑰红酸乙醇溶液 5ml，摇匀后进行颜色观察。若出现玫瑰红色则说明牛奶中使加了食用碱，而且颜色越深表明碱度越大。若是正常牛奶，颜色应为橙黄色。

总之，判断刚挤下的牛奶是否掺杂需要综合运用多种方法，仔细观察、闻味和进行相关的检测。只有这样，才能确保我们所喝到的牛奶是纯正、安全和健康的。

How to Determine Whether Just-Extracted Milk is Adulterated

Milk is a nutritious beverage, but in the market, there may be cases where just-extracted milk is adulterated. So, how can we determine whether just-extracted milk is adulterated?

Firstly, we can make a preliminary judgment by observing the appearance of the milk. Normal just-extracted milk should present a uniform milky white color without impurities and sediment. If there are foreign substances, turbidity or layering in the milk, it may be adulterated with other substances.

Secondly, smelling the milk is also a method. Fresh milk should have a light milky aroma and no peculiar smell. If there is a pungent, sour or other abnormal smell, then the milk may have problems.

We can also determine whether it is adulterated by detecting the specific gravity of the milk. According to the principle of liquid density, the normal specific gravity of milk is between 1.028 - 1.033 at 20°C. Experiments have proved that when 10% of water is added to the milk, the specific gravity will decrease by 0.0029. The specific method is to slowly pour 200ml of evenly mixed milk into a 500ml graduated cylinder, and put the hydrometer into the graduated cylinder at 20°C (the hydrometer should not touch the cylinder wall). Wait for about 2 minutes and then read the value. If the value is lower than 1.028, it can be determined that the milk is adulterated with water.

For whether starch is added, take 10ml of milk and put it in a 25ml test tube, heat it to boil, cool it, and then add 2ml of iodine solution and shake it. If there is a blue-purple precipitate, it indicates that starch is added to the milk. The principle is that gelatinized starch has the property of turning blue-purple when encountering iodine.

To determine whether alkali is added, take 10ml of milk and put it in a 25ml test tube, then add 5ml of 0.05% rose bengal acid ethanol solution, shake it well and observe the color. If rose red appears, it indicates that edible alkali is added to the milk, and the darker the color, the greater the alkalinity. If it is normal milk, the color should be orange-yellow.

In conclusion, determining whether just-extracted milk is adulterated requires the comprehensive use of multiple methods, careful observation, smell and relevant tests. Only in this way can we ensure that the milk we drink is pure, safe and healthy.