Tag Archive for: acid

Jammin’ With Fruit

Now that summer is in full swing, what better way is there to use all the berries, apricots, plums, peaches, and nectarines in season than to make jam? Jam has a great texture that makes it the perfect spread for brioche toast and a sweet complement for porridge. The base ingredients of fruit, sugar, pectin, and acid are cooked until the jam reaches a spreadable consistency [1]. Each ingredient plays an important role in the texture of the final product.

Strawberry jam. Photocredit: Julia Khusainova (Jullclous/Flickr)

Strawberry jam. Photocredit: Julia Khusainova (Jullclous/Flickr)

Sugar Skills

Sugar has many roles besides adding sweetness to the jam. When sugar is mixed with mashed fruit, it begins to dissolve and draw water out of the fruits through osmosis [1]. This occurs because fruit has a lower concentration of sugars than the amount of sugar that is typically added. The hydrophilic groups on sugar make it miscible with other polar molecules like water. Sugar also acts as a preservative by forming bonds with water molecules, making fewer water molecules available to support the growth of various microorganisms that might cause spoilage, such as Aspergillus glaucus and Saccharomyces rouxii [2,3].


The making of blackberry jam. Photocredit: (mrskupe/Flickr)


Pectin is soluble dietary fiber that naturally occurs in certain fruits such as apples, plums, and quinces. When heated and mixed with acid, this carbohydrate creates a thick gel that contributes to the consistency of jam. Acid from citrus increases the hydrogen ion concentration in the solution, which results in the pectin molecules losing some charge. With less electrostatic repulsion, the molecules can now aggregate to form a physical gel at a higher temperature of around 220ºF [4], resulting in watery fruit liquid dispersing itself within a web of pectin molecules [5].

Typically, under-ripe fruits have more pectin because fruit enzymes convert it to pectic acid during the ripening process [5]. This means that high pectin content can often be a trade-off for lower flavor, so it is recommended to use two parts of ripe fruit for every part of under-ripe fruit for the best consistency and taste [1]. Alternatively you can buy powdered pectin from the store to use with fruits that naturally have low levels of this carbohydrate, such as apricots, peaches, and raspberries.

Sugar Inversion

Acid not only contributes to the texture of jam, but it also catalyzes the conversion of sucrose (from added sugar) into its constituent fructose and glucose molecules with the help of heat. This process is called sugar inversion, and it is necessary to prevent recrystallization during jam storage [6]. However, this rarely occurs because finishing a jar of jam usually does not take too long. As an added bonus, acid also contributes to the flavor balance of the jam, preventing it from being too sweet.

Different fruits vary in acid and pectin content, so adjustments may be necessary to obtain the right texture and taste. You can also use a Brix test to measure the endogenous levels of sugars and dissolved nutrients. Basically if your jam contains riper and more nutrient dense fruits, the test will give a higher reading. Develop the right recipe, and you will want to eat out of the jam jar with a spoon!

References cited:

  1. The Science of Jam and Jelly Making. University of Kentucky.
  2. Jam Making: Why all the sugar? Iufost.org
  3. Why does jam go mouldy, even in the fridge? University of Liverpool
  4. Fishman, M.L. & Jen, J.J. Chemistry and Function of Pectins. June 1986.
  5. Jam Making 101. Seriouseats.
  6. Inversion of Sucrose. Colby College.


Catherine HuAbout the author: Catherine Hu is pursuing her B.S. in Psychobiology at UCLA. When she is not writing about food science, she enjoys exploring the city and can often be found enduring long wait times to try new mouthwatering dishes.

Read more by Catherine Hu


Science of Marinades

Chicken Tikka Masala, Beef Bulgogi, and Ceviche all have one thing in common: each protein is marinated, which contributes to the development of flavors and textures in the final dishes. The use of marinades is common across all cultures, and can provide a unique kick to food when done correctly.

Rostbrätel: marinated cutlet of pig neck. Photocredit: (Sebastian Wallroth/Wikimedia Commons)

What is marination?

Marination is the process of immersing foods in a liquid often made with oil, seasonings, and an acid or enzymatic component, to flavor and tenderize food. This liquid is called a marinade, and the term originally came from the use of seawater to preserve meat. The roots of the word are derived from the Latin word for sea (mare) [1].

Why is marination useful?

To understand the importance of marination, we must first address the components of raw meat. Consider tough, lean cuts of meat such as shank or flank. Meat toughness is related to the collagen and elastin fiber content in its connective tissues. One way to tenderize lean meat is with moist heat, as this breaks down stiff collagen proteins into soft, soluble gelatin [2]. Gelatin is responsible for that silky, falling-apart texture and mouthfeel [3]; this can be achieved with braising and stewing, where meat is simmered in liquid at a low temperature, allowing collagen to dissolve starting at 160º F. However, this conversion process can take some time, even up to 72 hours. Another reason to pre-tenderize meat before cooking is to prevent dried out meat: moisture is lost when heat is applied (despite being cooked in liquid).

This is where marination comes into play, as it provides another opportunity for protein breakdown. This method can thus shorten subsequent cooking time as well as minimize moisture loss as less heat is needed to “cook” the meat. Two types of marination include acidic and enzymatic marination, which both help break down the connective tissue in the meat.

Acidic marination

Acids, such as lemon juice or vinegar, work by denaturing proteins through disruption of hydrogen bonds in the collagen fibrils. Adding alcohol can also supplement the penetration of acid marination since fats present in meat are soluble in alcohol [4]. Beer and wine thus make great marinades, and they also confer their own tenderizing agents (tannins). Be careful not to overmarinate the meat, as prolonged exposure to acid can cause it to become tough. This occurs because after the proteins are denatured, they tighten as water content decreases [5]. Some marinades involve milk or yogurt since they have lower acid content.

Shrimp Ceviche, a dish that uses acidic marination. Photo Credit: Carlos Lopez (cloalpz/Flickr)

Enzymatic marination

Enzymes increase the rate at which cellular reactions occur, and certain enzymes help attack the protein networks of tough meat. Proteolytic enzymes such as fungal amylase (in legume seeds) and protease (in ginger) help break down muscle fiber protein into its constituent amino acids. Enzymes from tropical plants such as bromelain (in pineapple), papain (in papaya) and ficin (in latex of fig tree) break down collagen and elastin [4]. In fact, natives of pre-Columbian Mexico used to wrap their meat in papaya leaves before cooking since they found that it increased tenderness [6]. However, be sure to monitor the time of marination, for the enzymes can completely digest meats if they sit for too long.

Papaya contains papain, a proteolytic enzyme. Photo credit: Tatiana Gerus (Tatters/Flickr)


Another factor to take into account is the amount of contact the meat has with the marinade. Marination is a process of adsorption, where the marinade adheres to the outer surface of the meat rather than absorption, where it would penetrate all the way through [7]. This has resulted in some controversy over whether acidic and enzymatic marinades actually tenderize meat or not, but there are ways to alleviate this problem. It may be helpful to use thinner slices of meat to enhance the marinade penetration and reduce marination time. For thicker cuts, marinades can be injected to increase contact surfaces. Adding salt also helps, as it first draws out liquid by osmosis; then the resultant brine is reabsorbed into the meat while breaking down muscle structure. The brine draws flavors further down below the surface [8]. Fat such as oils are also useful to transfer fat-soluble flavors from the seasonings into the meat.

In general, tender cuts of meat should not require as much marination time as tougher cuts, and fish require even less time. Marinated meats should also be refrigerated to prevent harmful bacterial growth. Although it may seem a hassle to prepare ingredients for a marinade and remember to apply it to the meat for a certain time beforehand, the results can be well worth the wait.

References cited

  1. Some Surprising Facts About Marinades and the Origin of the Word. CulinaryLore.
  2. Collagen. About Food.
  3. Science of Slow Cooking. Science of Cooking.
  4. Juáres, M., Aldai, N., López-Campos, Ó., Dugan, M., Uttaro, B., Aalhus, J. Beef Texture and Juiciness. Handbook of Meat Processing. January 2012.
  5. Marinating Meats. Allrecipies.
  6. Alarcón-Rojo, A. Marination, Cooking, and Curing: Applications. Handbook of Poultry Science and Technology, Secondary Processing. February 2010.
  7. Saucy Science: Exploring the Science of Marinades. Scientific American.
  8. The Food Lab: More Tips For Perfect Steaks. Serious Eats.

Catherine HuAbout the author: Catherine Hu is pursuing her B.S. in Psychobiology at UCLA. When she is not writing about food science, she enjoys exploring the city and can often be found enduring long wait times to try new mouthwatering dishes.

Read more by Catherine Hu