MacConkey agar contains bile salts and crystal violet to inhibit most gram + organisms.Lactose is a carbohydrate that may be used as a nutrient. The utilization of lactose is important in identifying gram negative rods. Lactose + means that the organism can use lactose as an energy source whilst lactose – means they cannot. Differential media is used to distinguish organisms from each other based on a reaction that occurs as they grow. Lactose + organisms ferment the lactose and produce acid whilst lactose – organisms do not produce any acid as they grow. MacConkey agar contains a pH indicator called neutral red that detects the acid production in lactose + organisms. As lactose + bacteria produce acid, it causes the pH to drop and where neutral red is absorbed and the colony turns red.
Lactose – bacteria remains colorless and translucent as they do not produce any acid.
When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products. O-quinones then produce the well documented brown color by reacting to form compounds with amino acids or proteins, or they self-assemble to make polymers. One question that often accompanies yours is, “Why do some apples seem to brown faster than others?” Well, nearly all plant tissues contain PPO, however, the level of PPO activity and concentration of substrate can vary between varieties of fruits. In addition, a tissue’s PPO level can vary depending on growing conditions and fruit maturity. One approach the food industry employs to prevent enzymatic browning is to select fruit varieties that are less susceptible to discoloration; either due to lower PPO activity or lower substrate concentration. This approach, however, may not be practical for the home “culinary scientist.” In the home kitchen enzymatic browning can be prevented by either reducing PPO oxidation activity or lowering the amount of substrate to which the enzyme can bind. Coating freshly cut apples in sugar or syrup can reduce oxygen diffusion and thus slow the browning reaction. Lemon or pineapple juices, both of which naturally contain antioxidants, can be used to coat apple slices and slow enzymatic browning. In addition, both fruit juices are acidic and the lower pH that they bring about causes PPO to become less active. Heating can also be used to inactivate PPO enzymes; apples can be blanched in boiling water for four to five minutes to nearly eliminate PPO activity. Enzymatic browning is not unique to apples. PPO,a mixture of monophenol oxidase and catechol oxidase enzymes is present in nearly all plant tissues and can also be found in bacteria, animals and fungi. In fact, browning by PPO is not always an undesirable reaction; the familiar brown color of tea, coffee and cocoa is developed by PPO enzymatic browning during product processing.
Various aspects were left out in the video so i added the missing substantial information. However the video answered some interesting questions about the browning of apples, a case where many people do not really understand. The video could improve vastly b including structural formula and equations to validate the information being given. I have finally learned why my fruits turn brown when i leave it exposed for a while. The video was quite nice and provided some relevant information but as mentioned before, as a biochemistry student, i would have liked to see the reactions,structural formula and equations of the various components.
BiochemAholic 
